Most recent additions/changes to GBA Dev FAQs (Newest first)
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GCC Error Message FAQs
Q: I get an "undefined reference to `IntrTable'". Why?
A: This is because the crt0.S file (compiled to crt0.o) that you are using
-has support for interrupts enabled but you have not supplied an interrupt pointer.
-You can fix this problem by either a) Editing your crt0.S file and disabling
-interrupts and then compiling it, or b) Supply an array of function pointers
-to interrupt service routines with the name of the array being 'IntrTable'.
Q: I get an "undefined reference to `InterruptProcess'". Why?
A: This is because the crt0.S file (compiled to crt0.o) that you are using
-has support for interrupts enabled but you have not supplied an interrupt function.
-You can fix this problem by either a) Editing your crt0.S file and disabling
-interrupts and then compiling it, or b) Supply your own function of the type
'void InterruptProcess (void)' that is used to service interrupts.
Interrupt FAQs
Q: What are the basics that I need to know to write GBA interrupt code?
A: 1) Interrupt entry point address is stored at 0x3007ffc. When
-an interrupt occurs, regardless of the type of interrupt, the GBA bios saves
-some registers and then it reads the 32 bit address value that is stored at
-0x3007ffc in IWRAM. It then branches to this address in ARM mode. (The
-actual code that the bios uses to do this included in the next question.)
-2) The address stored at 0x3007ffc must point to ARM code. Many people
-prefer to compile most or all of their GBA C code into Thumb code because it
-is smaller and runs faster out of GBA ROM or flash carts. In these cases,
- you can not put a pointer at 0x3007ffc directly to your C code. If you
-are using Jeff F's crt0.S file then you don't have to worry about this.
-Assembly code is included in that crt0.S file to automatically patch address
-0x3007ffc, at start up, with a pointer to ARM code in the crt0.S file. This ARM
-code then will branch to your ARM or Thumb code. If you enable FastInterrupts in
-that crt0.S file, then all that happens is that the ARM code loads a pointer to
-your ARM or Thumb C code and then jumps to it. For the FastInterrupts method,
-your C function needs the function name "void InterruptProcess (void)". There
-needs to be nothing special about this function. It can be treated basically
-like any other function in the sense that it requires no special needs when
-entering or exiting from the function. If you have a table of function
-pointers in your C code with the name "IntrTable" then you can use the
-SingleInterrupts or MultipleInterrupts options in the above mentioned crt0.S
-file. Each one of these function pointers would point to a specific interrupt
-type and allow you to call the specific type rather than having to figure this
-out yourself. (Ex. VBlankIntr(), SerialIntr(), etc) The MultipleInterrupts
-option allows multiple interrupts to occur at the same time. It switches to
-the main stack instead of the interrupt stack to minimize stack overflow
-problems due to too many interrupts and too little interrupt stack space.
-3) If writing interrupt assembly code, you must save registers R4-r11.
-This is because the bios doesn't save these for you. Read the next question
-for more info. However, C compilers automatically save these registers so
-this is not a worry if you are writing C code. If you are using the asm("");
-statement in your standard or interrupt C code then you must be very careful
-and follow the guidelines that are listed in the asm("") documentation.
GCC Error Message FAQs
Q: I get an "Error: invalid constant (1684) after fixup". Why?
A: This can occur if you do an assembly language "adr rX,MyLabel" where
-MyLabel is out of range. Try using "ldr rX,=MyLabel" instead. You may
-need to use a ".pool" directive afterwards in a non-code executing area.
DevKitAdvance FAQs
Q: How do I use interrupts with DevKitAdvance?
A: DKA does not support interrupts by default. To add this support you need
-to use a crt0.o file that has interrupt support enabled. Then you need to
-follow someone's example code for using interrupts in your own code.
Q: How do I provide my own crt0.o instead of using the default crt0.o?
A: You need to do all of the following:
-a) When linking use -nostartfiles so that the default crt0.o, crtbegin.o,
and crtend.o will be ignored.
-b) Compile your crt0.S file with something like: as -o crt0.o crt0.s
-(The S in crt0.S is capitalized so that the file will be preprocessed by gcc, i.e.
-#define, etc, can be used in .S files if you do that.)
-c) If you are using C++ then you need to copy DevKitAdv files crtbegin.o and
-crtend.o to the local project directory.
-d) Add crt0.o to the START of the list of .o files to link to to your project.
-(If it is not at the start of the list then the program entry point will be wrong.)
-Also add crtbegin.o and crtend.o to the list of .o files to link if you are using C++.
Q: How do I provide my own lnkscript instead of using the default lnkscript?
A: DevKitAdvance has a default link script file that is built-in to the GCC compiler
-and it is used by default unless you specify your own link script file. Add the
-following option at the linking stage to use your own link script: -T lnkscript
-(Where lnkscript is the name of your link script file.)
DMA FAQs
Q: Is there anything special I need to know about DMA operation?KEY=DMADelay
A: Some report that a DMA transfer isn't started until one instruction *after*
-the final write to the DMA control register to make it start. As a result,
-you need to delay 1 or 2 instructions (if writing assembly code... and make
-sure it doesn't get optimised out if it's trivial) after the write and
-before you try and access the DMA'ed memory.
Misc Hardware FAQs
Q: Can I hack the GameShark GBA (Action Replay GBX) to use the USB hardware for other stuff?
A: Probably, but realize that it uses the following chips: Cypress
-CY7C63743-SC Low-Speed USB chip, SST 39VF200A-90 2mbit flash, & Actel
-A54SX08A-FTQ100 FPGA. Since it contains a low-speed, rather than full-speed
-or high speed chip, it will not be able to transfer data very quickly over
-USB. Low-speed chips are designed mainly for low-speed mice, keyboards, etc.
-Here is an excerpt from page 31 of USB Complete: "Although the low-speed bus
-is 1.5 Megabits per second, the fastest guaranteed delivery for a single
-transfer is 8 bytes in 10 milliseconds, or 800 bytes per second."
-This particular USB chip has an embedded burned EPROM (that can't be erased)
-so you can't just upgrade it with a faster chip.
Visual C++ IDE FAQs
Q: I am getting the following error when I build the project: "fatal error
U1087: cannot have : and :: dependents for same target".
A: Your strings supplied for either the Devkit Advance directory or the
Project Directory have spaces in them. Choose a folder that has no spaces
in its path. (Info from Ian Stocker)
Q: I get the following error when I build: "undefined reference to [function
name]" and the build fails.
A: You are referring to a function from another source file, and this source
file is not getting compiled. Make sure you have included it in the list of
object files to build, which is the macro "O_FILES" in the makefile.
(Info from Ian Stocker)
Sprite FAQs
Q: Does the GBA support sprite collision detection in hardware?Key=SprColDet
A: Unfortunately, no. :-\
GCC Warning FAQs
Q: How do I fix "Warning: multi-line string literals are deprecated"?
A: You could do the following with GCC 2.9.x:
-asm("
- ...
- ...
- ");
-That will not work with GCC 3.x.x. Instead, you must do something like this:
-asm("\n"
-" ...\n"
-" ...\n"
-" \n");
-Or do this:
-asm("\n\
- ...\n\
- ...\n\
- \n");
DevKitAdvance FAQs
Q: Does malloc use internal or external WRAM?
A: It uses the label __eheap_start to indicate the start of malloc space
-which grows upward. This label is defined in the internal link script
-or in an external link script if you provide one. __eheap_start usually
-points to the start of unused external WRAM space.
Interrupt FAQs
Q: My interrupt routine works but upon exit the return location is not correct. Why?
A: If you are using DevKitAdvance, or Jeff F linker script for GCC, the interrupt
-stack is only 160 bytes long. If you are using extensive stack usage
-(i.e. many local variables or printf/sprintf, etc..) then this space
-can easily run out. Here are two options:
-
-a) In your lnkscript file, change this:
-__iheap_end = 0x3008000 - 0x400;
-__sp_usr = 0x3008000 - 0x100;
-to this instead:
-__iheap_end = 0x3008000 - 0x1000;
-__sp_usr = 0x3008000 - 0xd00;
-This will give the IRQ stack a total space of 3232 bytes.
-
-b) Modify the Jeff F crt0.S (from the crtls package) to use
-__MultipleInterrupts. This forces the interrupt to use the main
-stack, instead of the interrupt stack, thus allowing a much larger
-stack area.
GCC General FAQs
Q: What do various file extensions in the GBA dev world mean?
A: The following are the USUAL meanings:
-.bin - Raw cart ROM image. No extra headers. (Start location=0x8000000)
-.elf - Data-rich ROM image. This, usually GCC generated, image can not be
-used on flash carts but can be used by a few GBA emulators "as is". It can be
-converted to a raw ROM image by using "objcopy".
-.gba - Same as .bin
-.map - ROM image information file resulting from the -Map linker option.
-.mb - Raw multiboot ROM image. No extra headers. (Start location=0x2000000)
-(Note: Some .mb files operate the same as .mb.gba files.)
-.mb.gba - Raw multiboot/cart ROM image. No extra headers. This ROM can work
-as both a multiboot & a cart ROM image. If it is loaded at 0x8000000, it will copy
-itself to 0x2000000 and then start execution from there. If it is located at 0x2000000,
-then it will just immediately execute the rest of the image.
-.o - Object file resulting from compiling a .asm, .c, or .s file.
-.all.rodata.o - Used by DevKitAdvance & Jeff F linker script to put all sections
-in this file into the .rodata section.
-.s - Gnu Assembler (GAS) file
-.S - GAS file to be filtered through the GCC C pre-processor before GAS.
-(So that #define, #if, etc.. can be used.)
-.sav - Cart backup image. Contents of cart SRAM, EEPROM, or backup flash.
-.text.iwram.o - Used by DevKitAdvance & Jeff F linker script to put the text
-sections in this file into the .iwram section. Other sections are left alone.
GCC Warning Message FAQs
Q: How do I fix "Warning: XXX.o does not support interworking, whereas YYY.elf does"?
A: Try using the 'interflip' tool that comes with DevKitAdvance. Use
-the -mthumb-interwork command line option with it.
Q: How do I fix "Warning: architecture UNKNOWN"?
A: Objcopy will give this error when converting a binary file to
-an object file because it does not "recognize" the data type.
-There no current solution other than to just ignore this warning.
GCC General FAQs
Q: How can I look at the size of various sections or the linker output results?
A: Here are various methods. Use the one that you prefer:
-a) Add the following to your linker options: -Map filename.map
-b) nm --numeric-sort --demangle filename.elf
-c) objdump -h filename.elf
-(Please note that empty sections may incorrectly list the lma address as
-the vma address for some versions of objdump.)
GCC General FAQs
Q: How do I include ( or INCBIN ) raw data into a project?KEY=IncRaw
** Two extra methods have been added ***
GAS Assembly FAQs
Q: When I compile assembly code to go into section .iwram, only empty (0x00)
code is copied from ROM to IWRAM. Why?
A: It is not enough to just do the following:
-.SECTION .iwram
-you need to do the following instead:
-.SECTION .iwram,"ax",%progbits
-or otherwise the code that is put in ROM, to be copied to IWRAM,
-for some reason is composed only of the value 0x00.
-If you use the GCC command line option -save-temps when compiling
-a C file then you will notice similar directives in the resulting .s
-temporary files. Search this file on the keyword 'progbits' for
-other examples.
GCC General FAQs
Q: I would like to put a variable into a different section. How?
A: You can only do this for initialized, global variables. Here are some examples:
-int a[10000] __attribute__ ((section (".ewram"))) = { 0 };
-int init_data __attribute__ ((section (".iwram"))) = 0;
-Quoting from the GCC manual:
-"You may only use the section attribute with a fully initialized global
-definition because of the way linkers work. The linker requires each
-object be defined once, with the exception that uninitialized variables
-tentatively go in the common (or bss) section and can be multiply
-'defined'. You can force a variable to be initialized with the `-fno-common'
-flag or the nocommon attribute."
-
-Another method would be just to define a pointer and reference an array
-by using it:
-u8 *MyArray = (u8 *)0x2000000; // ewram start
-MyArray[0] = 1;
Cart FAQs
Q: What minimum information must all carts contain to boot & run?
A: As a minimum they must contain the following information:
-Nintendo Graphics Logo: ROM Locations $4 to $9f.
-96: $96 stored at ROM location $b2
-Complement Check: Correct complement value at ROM location $bd.
-
-To calculate the Complement check:
-1) Add all the values from ROM locations $a0 to (and including) $bc.
-2) Add $19 to this value.
-3) Exclusive OR with $ff.
-4) Save the 8 least significant bits of the result to ROM location $bd.
General GCC FAQs
Q: How do I do a long branch to a routine that is in Internal WRAM?
A: * Changed #define IN_IWRAM to the following two types to fix compile bugs: *
#define CODE_IN_IWRAM __attribute__ ((section (".iwram"), long_call))
#define VAR_IN_IWRAM __attribute__ ((section (".iwram"))) = {0}
DevKitAdvance FAQs
Q: How do I build a ROM image that will be multiboot (MBV2,etc) compatible?
A: #define MULTIBOOT const int __gba_multiboot;
-Then use it like this: MULTIBOOT
-(Putting the MULTIBOOT in your main project file, close to the top, might be useful.)
General C FAQs
Q: When I read a GBA hardware register the value never changes. Why?
A: Make sure you are using the volatile keyword when trying to read a hardware
-register that is modified by hardware. C compilers will often cache the value
-of a register if you don't declare it volatile. Use the following:
-
-typedef volatile unsigned short int vu16;
-i = *(vu16 *)REG_xxxx;
General C FAQs
Q: What are crt0.S/lnkscript (GCC) and start.s (SDT/ADS) used for in a project?
A: crt0.S and start.s (sometimes start.asm) both have similar functions.
-They are mainly designed to execute assembly code that occurs before
-user compiled code is executed; usually for initialization purposes.
-(Sometimes these files will contain other assembly code as well like
-interrupt support, etc.) The most common usage is to set uninitialized
-global variable RAM to 0 and to copy initialized global variables from
-ROM to RAM. A bare minimum crt0.S or start.s file must at least contain
-a branch to the start of compiled code, even if it contains nothing else.
-(The S of crt0.s is often capitalized because the GCC docs
-mention that it must be this way if you ever wish to run the GCC
-preprocessor on this file.)
-
-lnkscript is a linker script used by GCC that tells the linker where to
-put various sections (.text/.data/.rodata/.iwram/etc..) in the GBA
-memory map. Often a particular crt0.s and lnkscript may be designed
-to work together so they are often distributed together. Some GCC
-compilers (i.e. DevKitAdvance) may include a default lnkscript
-compiled into the compiler itself which will be used if the user
-doesn't specify a lnkscript file.
Flash Advance / Visoly Cart FAQs
Q: I'm having problems getting my FA linker to work. Ideas?
A: DON'T USE BATTERIES. Read more below. Also check out these FAQs:
-http://fafaq.gbaemu.com
General GCC FAQs
Q: I get an "undefined reference to '__EH_FRAME_BEGIN__'". Why?
A: You need to link in crtbegin.o and crtend.o.
Q: How do I do a long branch to a routine that is in Internal WRAM?
(Added GCC 3.0 & later specific info.)
Graphics FAQs
Q: How many cycles are used per various display events?
A: GB Clock (GBClk) = 4.194304 MHz
-GBC Clock = GBClk or 2xGBClk (Depending on mode)
-
-GBA Clock = 4 x GBClk = 16777216 Cycles/Second
-Display Size = 240 x 160
-Lines in VBlank = 68
-Total lines = 160 + 68 = 228
-Cycles/Line = 73.433us/Line = 1232
-Cycles/Frame = 16.743ms/Frame = 1232 * 228 = 280896
-Cycles/VBlank = 4.993ms/Frame = 1232 * 68 = 83776
-Cycles/HBlank = 16.212us/Line = 272
GCC Error FAQs
Q: I get an "undefined reference" to one of these: sin, cos. Why?
A: You need to add -lm option to your linker options to link in libm.a.
-This library (and all libraries) must be included AFTER the object files.
-So put it at the end of your link options.
Link Port FAQs
Q: Are there any things that should be avoided when writing comms code?
A: Make sure that if you do any DMA's that they are short enough not to
-disturb comms communications if you send more than one communication
-per frame. i.e. If you send one comms transfer followed by a second
-transfer, make sure that any DMA is not longer than the length of time
-it takes to send one comms transfer. The CPU is halted during DMA so
-an extensively long DMA may cause you to lose the next transfer due to
-the fact that GBA hardware can only buffer one comms transfer at a time.
GCC Error FAQs
Q: I get an "undefined reference to '__main'". Why?KEY=UndefUUMain
A: This is because you are not linking libgcc.a in your project.
-Normally, libgcc is linked without having to explicitly link it
-if you use GCC for linking. (If you use LD, directly, for linking
-then you need to link libgcc as well. Note: GCC calls LD to do
-linking.) The --nostdlib option will tell the compiler not to
-link libgcc. Function __main is used to setup c++ constructors
-if they are used.
Q: I get an "undefined reference to 'main'". Why?
A: This is because the file you are compiling contains no 'main'
-function. First, look at your C code and locate the name of the
-main function. Many use 'AgbMain' instead of 'main'. If you want
-a quick fix then rename 'AgbMain' (or whatever) to 'main'. (This
-will increase the size of your output binary file about 5500 bytes.)
-Or you can disable C++ support in your crt0.S file (if you have the
-one written by Jeff F.) and that will also fix the problem. For
-more info on why people sometimes use 'AgbMain' instead of 'main',
-refer to the following: http://www.devrs.com/gba/files/gbadevfaqs.php#CPPMain
Visoly Cart FAQs
Q: Is it possible to write to FA main program flash from GBA code?
A: Yes. Download FLinker from http://www.devrs.com/gba in the Apps/Misc section.
-It contains cartlib.c that allows you to do this. When you write to the flash
-chip you are actually setting bits to zero on the chip. If you do another write
-to the same flash location you can only clear additional bits. You can not set
-bits by using a write. In order to restore a memory location to all 1's you must
-erase it. Erasing can only be done by erasing a whole block. If you have an
-older full-size Flash Advance cart, a block contains 128k bytes. The newer
-short "Turbo" carts use two interleaved chips with an effective block size
-of 256k bytes. As mentioned above, you can erase each block a minimum of
-100,000 times but there are no guarantees above that point. The flash data
-sheet mentions that, on average, a block erase takes 1 second but it could
-have a worst case time of 5 seconds
Misc Hardware FAQs
Q: Where can I get a 3-point screwdriver to open the GBA case?
A:Try one of these links:
-http://www.panamericantool.com/triwing1.html
-http://www.starkelectronic.com/eclsd.htm
General GCC FAQs
Q: Are there any advantages to using GCC for linking rather than LD?
A: When you use GCC for linking it actually calls LD to do the actual linking.
-There is an advantage in that when you use GCC, it implicitly links in libgcc.a
-and libc.a. Libgcc mainly contains division code. Libc contains many different
-functions as noted here: http://www.devrs.com/gba/docs.php#docsmisc
General GCC Faqs
Q: How do I put my code in Internal WRAM so that it will run faster?
A: If you just have a few functions or procedures that needs to go in IWRAM
-then proceed to the next question for a solution.
-
-If you want to put all of the code in an object file into IWRAM then get
-v1.2 or later of the Crtls package from http://www.devrs.com/gba . Rename
-your object file to the following format: *.text.iwram.o . The linker script
-will now automatically put all code (.text section) in that object file(s)
-into IWRAM. The crt0.S (in the Crtls package) will make sure that that object
-file code gets copied from ROM to IWRAM upon startup of your GCC code. Proceed
-to the next question for methods of calling your functions or procedures in IWRAM.
Misc Hardware FAQs
Q: How do I conserve battery power when developing GBA software?
A: The main things would be to use 4/2 rom wait state timing and to leave
-the ROM prefetch turned off. 3/1 rom wait states uses about 20% more power.
-ROM prefetch increases power usage by about 10%. If using Visoly flash
-carts, keep in mind that Turbo 64M uses ~15% more and Turbo 128M uses ~35%
-more battery power than the older 64M carts. (Turbo carts are all small carts.)
ARM / Thumb Assembly FAQs
Q: ARM docs are confusing. How do I figure out how many clock cycles an instruction takes?
*** Updated for translating the ARM official docs ***
Interrupt FAQs
Q: What registers should I preserve in interrupt code?
A: The GBA BIOS was designed to allow passing interrupt handling directly
-to GCC or ARM SDT C procedures by following the APCS (Arm Procedure Call
-Standard) but. As a result, the BIOS saves registers r0-r3 & r12 for you.
-You should preserve all other registers yourself when writing custom
-assembly interrupt drivers. Here is the actual ARM IRQ handler code used
-by the BIOS. Due to the way it is written, it must be used to call an ARM
-routine which either processes the interrupt or passes it off to another
-routine (i.e. It can't directly call a Thumb routine since no 'bx'
-instruction is used):
-
- stmdb r13!,{r0-r3,r12,r14}
- mov r0,#0x4000000
- adr lr,IntRet
- ldr pc,[r0,#-4] @ pc = [0x3007ffc]
-IntRet:
- ldmia r13!,{r0-r3,r12,r14}
- subs pc,r14,#4
Misc Tools FAQs
Q: What disassemblers are available for ARM/Thumb?
A: IDA Pro - An expensive package from here: http://www.datarescue.com
-
-ARMSD - This comes in the 30 day trial version of the ARM SDT C compiler.
-
-OBJDUMP - This comes with the GCC binutils. Use option --disassemble.
-Note that it only works on .ELF files.
-
-Mappy - This GBA emulator has an export disassembly option.
General GCC FAQs
Q: Why do I see a repeating pattern of bytes when I view memory from 0 to 0x1ffffff?
A: With the exception of 0 to 0x3fff (GBA Bios which can be executed but
-is not externally readable), this is all memory space that is unavailable
-to user programs. What you are actually seeing is instruction prefetch
-data. More specifically, you are seeing repeating bytes of the second
-instruction after the instruction that you used to view this memory area.
-(Info from Mike Heckenbach.)
Q: Can the repeating bytes in memory from 0 to 0x1ffffff be useful?KEY=RepeatUses
A: This data repeats every 2 bytes in Thumb mode and every 4 bytes in ARM mode.
-All of this memory space is 0 wait state when you attempt to read or execute it.
-You can place a Thumb or ARM 'bx lr' instruction at 0x2000000 and then
-branch anywhere from 0x4000 to 0x1fffffe to execute a single assembly
-language instruction many times. (The repeating instruction would be the
-second instruction after the 'BX' instruction.) However, with the exception
-of carefully crafted 'ldm/stm' instructions, you probably can't do 'ldr/str'
-instructions due to the fact that they probably put new data on the bus,
-thus replacing the repeating instruction. This effect doesn't seem to
-occur in any address space except for 0 - 0x1ffffff. ('ldm/stm' idea from
-Mike Heckenbach.)
-The drawbacks to doing this is that your code might break on emulators or
-future hardware (GBA2?) since this is an undocumented effect.
General GCC FAQs
Q: How do I include ( or INCBIN ) raw data into a project?
-5) *** Convert your data to .o (gcc object) file #2 ***
-
-If you don't want to use the hacked program "ObjCopyRoda" in the last
-step then here is another, slightly more complicated, approach:
-objcopy -I binary -O elf32-little in_file temp.o
-(options -> ...G H I J K L M N O P Q...)
-(Ignore the "architecture UNKNOWN" warning message.)
-ld -T convert.ls temp.o -o outname.ext
-The convert.ls linker script file should contain the following text:
-
-SECTIONS { .rodata : { *(.data) } }
-
-Info from: http://sources.redhat.com/ml/binutils/2001-05/msg00513.html
GCC Optimization FAQs
Q: What operations are particularly slow on the ARM processor?
A: Division and modulus. The ARM has no division instruction so these are
-painfully slow. Whenever possible, use >> (right shift) or multiply by
-the reciprocal instead. If you absolutely need to use division then use
-the routines in the GBA BIOS. One report has the BIOS divison code
-operating ~2.5x faster than GCC division.
Visoly Cart FAQs
Q: Should I get a tall (65mm) or a short (44mm) Visoly cart?
A: The short (known as "Turbo") Visoly carts are rated for 10 nanosecond faster
-read access timings so they are, on average, more reliable than the taller carts.
-(i.e. A tall 64M cart uses 1 Intel 28F640J3A flash chip where the
-Turbo 64M cart uses 2 Intel 28F320J3A flash chips which are slightly faster.)
-
-The Turbo 64M carts are rated at 10 nanoseconds faster than Turbo 128M carts.
-As a result, on average, Turbo 64M carts are slightly more reliable than any
-of the other Visoly flash carts. (Turbo 128 uses 2 Intel 28F640J3A flash chips.)
Interrupt FAQs
Q: Are there cautions to clearing the IE or IME flags?
A: Interrupts may occur during execution of commands to clear the IE or IME
-flags. If you would like to clear the IE flag, clear the IME flag first, so
-that problems will not occur. When using multiple interrupts and an interrupt
-coincides with clearing the IME flag, the interrupt will still occur, even
-though interrupts are disabled (IME flag is cleared). This will cause the
-interrupt routines to be "out of sync" with the IME flag. Write interrupt
-routines that save the IME flag and IE/IF registers and then re-enable the
-IME flag so that multiple interrupts can occur. This condition is rare but
-must be considered in your project.
Sprite FAQs
Q: Is there a simple way of turning a sprite off?
A: The preferred method is to set rotation double size mode with
-rotation/scaling switched off. You can also set the sprite Y
-coordinate to 160.
Visoly Cart FAQs
Q: What are the specs of the Visoly flash carts?
A: There is a 64mbit version that contains 128kbytes of
-game save SRAM. You can put up to two games on this cart with each
-game having access to 64kbytes of game save SRAM.
-
-The 128mbit version contains 256kbytes of game save SRAM
-and allows up to 4 games to be stored on a cart. Each game
-has access to 64kbytes of game save SRAM.
General GCC FAQs
Q: Where can find other related FAQs?
A: Cross GCC FAQs: http://www.objsw.com/CrossGCC/
Q: Is there anything I should know to compile a GCC c++ program?
A: Your main program function should have the name: int main (void);
-The reason is that GCC will insert a call to __main right at the beginning
-of this function that is used to setup constructors. One reason that many
-do not commonly use this function name, when using C only, is because this
-causes the function __main to be linked with your program and ~5500 bytes
-are used up in the process.
Q: Why does GCC put global & static variables in ROM when you
simultaneously declare AND initialize it?
A: This is because your linker script and/or crt0.s is wrong. Get
-the ones from here: http://www.devrs.com/gba/ccode.php#cmisc
-Check the next question for more info.
Q: Why does sprintf, malloc, & other library functions not work right?
A: This is because your linker script and/or crt0.s is wrong. Get
-the ones from here: http://www.devrs.com/gba/ccode.php#cmisc
-Check the next question for more info.
Q: What are sections and how are they used?
A: GCC puts different code or data into different sections so as to
-organize the data according to its type. It is then the job of a
-linker script to put the various sections into either RAM or ROM.
-Here is what GCC does with code & global variables:
-
-code ---> text section
-u8 foo; ---> bss (uninitialized data, set to 0x00 ) section
-u8 foo [ ] = {0,0,...}; ---> data (initialized data) section
-const u8 foo [ ] = {0,0,...}; ---> rodata section
-
-However, you can force code or data to other sections by using
-the following:
-
-// do this only when declaring, not defining
-void myfunc(void) __attribute__ ((section(".mysection")));
-
-// vars must be initialized
-int myvar __attribute ((section(".mysection"))) = 0; (Info from Jason Wilkins)
-
-A correct linker script should do the following:
-
-text ---> ROM
-bss ---> RAM
-data ---> RAM (Loaded from ROM to RAM at startup of ROM.)
-rodata ---> ROM
-
-A correct crt0.s should clear all of bss to 0 and copy the data
-section initialized data from ROM to RAM. For a correct crt0.s
-& linker script get crtls from here:
-http://www.devrs.com/gba/ccode.php#cmisc
GCC Known Problems
Q: What can cause problems without reporting an error?
A: "On windows using gcc [editors note: some versions anyway] checks for
-the existence of include files case insensitivly, but it only includes
-the file if the case is exact. Getting it wrong results in the compiler
-reporting no error but also not including he file causing all sorts of
-problems downstream." (Info from Neil Graham)
-
-"I used objcopyroda [editors note: this should also apply to objcopy]
-to make a binary into an object file. It broke everything. I tracked it
-down to the fact that my file wasn't a multiple of 4 bytes long.
-Appending two bytes to the binary fed to objcopyroda fixed the problem."
-(Info from Neil Graham)
Sprite FAQs
Q: Why do my sprites not appear in graphics modes 3,4, & 5?
A: You need to put them at starting address 0x06014000 and the starting
-sprite number will be 512. You only have 512 sprites available in these
-modes. Sprites 0-511 are not available due to the memory that is
-consumed by the bitmap plane.
Cart FAQs
Q: Has anyone tried using CompactFlash to design a GBA flash cart?
A: These cards are too slow to be attached to the GBA cart interface.
Visoly Cart FAQs
Q: How many times can you reprogram the Visoly 64M flash cart?
A: The flash chip in that cart (Intel 28F640J3A) has an Erase/Reprogram
-lifetime of a minimum of 100,000 per block. So, you will probably wear
-out your cart connector long before you wear out the flash chip. :)
Link Port FAQs
Q: I want to make a PC<->GBA link cable. Where do I start?
A: The GBA has several different link port communications modes and there
-currently are no general purpose PC<->GBA link cables that can handle
-several different comms modes (except the MBV2.) Here are the 3 most
-popular PC<->GBA style link cables:
-
-1) Multiboot - This would be a cable that supports "multiboot" so that
-you can boot the GBA just using the link port (i.e. no cart installed.)
-Check the links below for more info on what hurdle there are in making
-this type of cable.
-2) UART Comms - This type of cable allows you to connect the GBA
-to a PC serial port. You need voltage conversion logic (max3222, max3232,
-etc) for this to work. With this type of cable you can use GNU GDB to
-do remote debugging of GBA software (i.e. debug in-circuit GBA code using a PC.)
-Link port pin SC is a handshake input in UART mode, however, on power up
-this pin is an output pin. As a result, you should put a resistor (1k ohm will work)
-between this pin and any voltage conversion logic to prevent 2 outputs shorting
-each other.
-3) Normal Comms - This is probably the simpliest of any of the PC<->GBA
-cable ideas, using to a PC parallel port. You *might* be able to use
-just wires and resistors (1k ohm - to do voltage conversion from 3.3v<->5v)
-with no other parts needed. Make the PC the clock master for this setup. (If
-you make the GBA clock master then any PC interrupt might cause loss of data.)
Graphics FAQs
Q: Where can I get more info on Mode 7 graphics mode?
A: UPDATED: The GBA only has graphics modes 0-5. Mode 7 refers to a graphics method
-rather than a GBA graphics mode. The term dates back to the SNES/Super Famicon
-days. The original SNES mode 7 was a graphics mode that supported rotation /
-scaling (zooming). The current (even if not exactly correct) use of the term
-"mode 7" often refers to using a rotation / scaling graphics mode on the GBA
-along with rewriting zoom/scaling values every scan line on a graphics tile
-map so that tiles toward the top of the screen appear farther away and tiles
-toward the bottom appear closer to the player. Use of DMA0 (set to trigger
-on H-blank interval) to copy scaling register manipulation data each scan
-line is one method of achieving this effect. You can also use an H-Blank
-interrupt to modify scaling registers but any DMA occuring while the screen
-is redrawing can delay an H-Blank interrupt and thus cause it to occur on
-the wrong line or at the wrong time.
Cart FAQs
Q: Is is possible to use a PC parallel port or a PC I/O card to emulate a GBA cart?
A: No. GBA bus speeds are just too fast for pc parallel port or PC I/O cards to keep up.
Misc Tools FAQs
Q: How do I pad a ROM or raw data to a given size?
A: There are at least 2 different methods of doing this:
-1) Get one of the standard GBA ROM "fixer" programs. (i.e. GBAFix, etc.)
-These programs often not only fix header area info but also pad to the next
-larger 2^X file size.
-2) Use the following DOS commands to pad to any size you want:
-copy /b filein.bin+largefile.bin temp.bin
-subset 0 filesize temp.bin fileout.bin
-largefile.bin is just any large file that probably should contain
-all 0xff's. (All 0xff's allow the file to compress well and write to
-flash carts a little faster.) The "copy" appends this file to the end
-of your data file and the "subset" trims the file size to exactly
-"filesize" bytes in size.
-You can generate your own largefile.bin by taking any small size file
-(preferably containing only 0xff data) and replicating it's size by
-using several "copy /b" commands. (Get subset [for DOS & linux]
-from http://www.devrs.com/gba/ . Look in the Apps / Misc section.)
Graphics FAQs
Q: Why do my sprites not appear in graphics modes 3,4, & 5?
A: Only sprites 512 and up can be used in the bitmap modes due to the
-fact that these modes use up excessive amounts of video ram.
Q: What is the difference between hardware sprites and software sprites?
A: A hardware sprite is where you use the game system sprite/object
-hardware to display a sprite.
-
-A software sprite is where you draw a sprite into background tiles
-or into a mode 3/4/5 bitmap plane. Most people don't usually use software
-sprites unless they are trying to do something that can't be performed
-with hardware sprites. The reason software sprites aren't often used
-is because it consumes lots of CPU time to move a software sprite.
-The term software sprite is confusing & sometimes misleading to some.
-A "software sprite" is nothing more that a bunch of writes (and possibly
-reads) directly to tile/bitmap VRAM.
Misc Hardware FAQs
Q: There is a small adjustment screw below the serial # of US GBA's. What's it for?
A: This is a voltage adjustment for the LCD display. DO NOT TOUCH IT.
-Messing with it can greatly shorten the lifetime of your LCD display.
-LCD displays must have an average of 0 volts DC on them (i.e. The positive
-and negative voltages put on them must nearly perfectly cancel out when
-you average the voltages over time.) If they don't average out to 0 volts
-then, over time, the LCD plating materials will start to come off which
-slowly destroys the display over time.
General GCC FAQs
Q: Does GCC GDB include a type of ARMulator for simulating arm code?
A: Yes. It is based on a version of the ARMulator that ARM released under GPL.
-It also allows remote debugging over a serial port (with a proper ARM/Thumb
-GBA-side stub) which is ideal for embedded systems such as the GBA.
-eCos comes with GDB stub ROMs for ARM targets that can be modified for GBA
-use. You can get eCos from here: http://sources.redhat.com/ecos/
Link Port FAQs
Q: What are the pin signals on the link interface?
A: NOTE: When looking at the top of the GBA with the battery-side of the
-GBA towards your feet (link port center notch is up), pin 1 is on the
-top row to the far right. Pin 2 is on the bottom row below pin 1. etc.
-
-Pin# - Signal - FullName - PositionOnConnector
-1 - PWR - +3.3Volts - UpperRight-1 - PWR - +3.3Volts - UpperRight (+5Volts when using GB/GBC cart.)
-2 - SO - SerialOut - LowerRight
-3 - SI - SerialIn - UpperMiddle
-4 - SD - SerialData - LowerMiddle
-5 - SC - SerialClock - UpperLeft
-6 - GND - Ground - LowerLeft
General GCC FAQs
Q: How do you force GCC to put a variable into a specific CPU register?
A: register u32 remainder asm ("r5"); will force "remainder" into regster R5, for example.
Visoly Cart FAQs
Q: Why does the Visoly flash cart sometimes crash when testing code?KEY=VisolyCrash
A: Some people experience this problem and others don't. Sometimes this will even
-only occur after the cart warms up. This appears to be due to the cart flash &/or
-logic sometimes being too slow for reliable operation. (i.e. The flash cart sometimes
-can't handle 3 wait/1 wait cart access timing.) To greatly improve reliability
-for problem flash carts, set the cart timing to 4 wait/2 wait. (i.e. Write 0 to
-0x4000204 [WAITCNT]) Most commercial carts are designed for 3 wait/1 wait
-cart access. As a result, those testing commercial code are most likely to
-experience these problems.
General Cart FAQs
*CORRECTION: "Address increment occurs on the low-to-high edge"*
Q: How does the GBA ROM cart interface work?
A: GBA ROMs are special chips that contain a standard ROM, address latches,
-and address counters all on one chip. Cart accesses can be either sequential
-or non-sequential. The first access to a random cart ROM location must be
-non-sequential. This type of access is done by putting the lower 16 bits
-of the ROM address on cart lines AD0-AD15 and setting /CS low to latch
-address lines A0-A15. Then /RD is strobed low to read 16 bits of data from
-that ROM location. (Data is valid on the rising edge of /RD.) The following
-sequential ROM location(s) can be read by again strobing /RD low. Sequential
-ROM access does not require doing another /CS high-to-low transitions because
-there are count up registers in the cart ROM chip that keep track of the next
-ROM location to read. Address increment occurs on the low-to-high edge of all
-/RD. In theory, you can read
-an entire GBA ROM with just one non-sequential read (address 0) and all of
-the other reads as sequential so address counters must be used on most address
-lines to exactly emulate a GBA ROM. However, you only need to use address latch /
-counters on A0-A15 in order to satisfy the GBA since A16-A23 are always
-accurate. (Thanks to Christian Grenier for Logic Analyzer data posted to
-gbadev mailing list ( http://groups.yahoo.com/group/gbadev ) Apr 11,2001.)
Q: What type of power supply does the Visoly flash cart programmer require?
A: It uses 6 AAA batteries or a power adapter. (There have been two reports
-that it doesn't work reliably even with brand new batteries so a power
-adapter was used instead.) The power adapter needs to meet the following
-requirements: 8 to 12 volts DC, 150 milliAmps(mA) or greater, center is negative.
Link Port FAQS
Q: There appear to be many link communication modes. Which are realistic?
A: The GBA supports many game link communication (comms) modes but
-most are not realistic for commercial games because they require special
-cables. So far, there is one standard GBP/GBC cable (GB cable has same
-wiring but physically larger connectors) and one common GBA cable. This
-multiplayer GBA cable is specifically designed to support multiplayer
-comms mode. It also supports low-speed (256kbits/sec) normal comms mode
-(2mbits/sec high-speed normal comms mode is currently only supported by
-wireless-style devices) but ONLY from higher priority to lower priority.
-(i.e. Master can talk to slave #1 but slave #1 can't talk to master, slave
-#1 can talk to slave #2 but slave #2 can't talk to slave #1, etc...) This
-is due to the way the cable is wired. To fully take advantage of GBA normal
-comms mode (di-directional comms) you need to use a standard GB/GBC cable.
-Unfortunately, it gets confusing when there are multiple cable
-choices for the GBA so it's probably highly likely that the currently
-common GBA multiplayer cable will become a standard for commercial
-games. In theory, the GBA multiplayer cable can support up to 256k bits/sec
-for one-way comms when used in low-speed normal comms mode and the GB/GBC
-cable can not (according to Nintendo anyway.) This would probably be due to
-the quality of the cables themselves. General purpose comms mode is
-unique in that it, in some form, supports all possible cable combinations.
-However, due to it's simplicity and low-level nature, it can be very
-software intensive to use this mode. Serial transfer algorithms that are
-done in hardware in other modes must strictly be handled in software in
-this mode.
Q: Are there any things that should be avoided when writing comms code?KEY=CommsCode
A: Make sure that if you do any DMA's that they are short enough not to
-disturb comms communications if you send more than one communication
-per frame. i.e. If you send one comms transfer followed by a second
-transfer, make sure that any DMA is not longer than the length of time
-it takes to send one comms transfer. The CPU is halted during DMA so
-an extensively long DMA may cause you to lose the next transfer due to
-the fact that GBA hardware can only buffer one comms transfer at a time.
GCC General FAQs
Q: How do I include raw data (such as a font) into a project?
( A fourth method was added to this question. Check the FAQs for
more info.)
Cart FAQs
Q: What type of backup methods are currently used in GBA carts?
A: SRAM - Battery backed up static RAM. It can be up to 64K bytes but it's usually
-32K bytes in size. It is located in the memory map from 0xe000000 to 0xe00ffff.
-Flash ROM - This is memory which can be up to 64K bytes and it is usually that
-exact size. It is located in the memory map from 0xe000000 to 0xe00ffff.
-Some of the manufacturers guarantee a minimum of 10,000 rewrites per 1 sector
-so these devices have a limited life compared to SRAM.
-EEPROM - This uses the /CS, A23, and D0 of the cart bus and is a serial connection.
-It usually is found in 4kbit or 64kbit size, but because of it's serial configuration,
-it could theoretically be any size. The maximum useable ROM size is 128MBits when using
-this device. Some of the manufacturers guarantee a minimum of 100,000 rewrites per
-address so these devices have a limited life compared to SRAM.
Q: Why do official Nintendo flash carts sometimes crash when you write to them?
A: Standard Nintendo flash carts have to be modified before you can actually
-write to them. This is done by removing resistor R7 and putting it at empty
-location R8. But, you can temporarily cause the ROM contents to disappear on
-unmodified or modified carts by changing modes in the flash chip. (The common way
-to fix this is by power off/on.) The first write that you do to flash sets up a
-particular mode or is ignored, depending on it's value. For more information
-on the available flash modes then check out the datasheet for LH28F320BJE
-on http://www.sharpmeg.com.
Link Port FAQs
Q: What data transfer rate can be expected in multiboot mode?
A: The GBA uses multiplayer communications mode fixed at 115200 bps to send
-multiboot data. 18 bits (start bit + 16 data bits + stop bit) are sent from
-master to slave and then the master waits for 18 bits from the slave. This
-translates to a transfer rate of 115200/18 = 6400 words or 12800 bytes/sec.
-Since communications occur in only one direction at any one given time, this
-means the amount of data that you can send in any one direction, per given
-amount of time, is cut in half. So effectively in this mode, the theoretical
-limit is 6.4k bytes/sec transfer in one direction.
-While the theoretical limit is 6.4k bytes/sec for multiplayer comms mode,
-multiboot effective maximum transfer rate appears to be around 5k bytes/sec
-from tests. This less than ideal rate might be due to the fact that data
-tranferred during multiboot is key encrypted and must be decrypted on the
-receive end.
Q: Is there encryption in multiboot transfer mode?
A: Yes. During transfer negotiations, the slave GBA sends a random number
-to the master GBA. The master then uses this number or key to encrypt
-data that is sent to the slave. As a result, if you did a data capture
-of the transferred data it would appear different each time you do
-a transfer for a given multiboot image.
GCC Optimization FAQs
Q: Are there any dangers to using -O3 optimization level?
A: Yes. Subroutines that contain or do nothing will be optimized out.
-GCC assumes that subroutines at this optimization level that contain
-nothing but inline assembly code [asm("....");] are basically empty
-so these subroutines are ignored and not included during compile.
GCC General FAQs
Q: When I try to compile it complains that it can't find cc1plus.exe. Why?
A: You need to download the the "C++ support files" from the
-C Compilers section on http://www.devrs.com/gba and put these
-files in the same directory as the other GNU compiler binaries.
Link Port FAQs
Q: Can you modify a GBA link cable to connect to a COM serial port to support multiboot?
A: There are many reasons you can't easily do this. The first reason is
-that a standard serial COM port uses 1/8/1 format but GBA multiplayer /
-multiboot link mode uses 1/16/1 format. (format = Start bit/Data Bits/Stop Bit)
Graphics FAQs
Q: How can you increase horizontal resolution 3x for high contrast images?
A: Yes. Use Sub-Pixel Rendering. The GBA hardware uses BGR (left-to-right) ordering.
-Check this site for the theory: http://grc.com/cleartype.htm
-and for a freeware windows program for generating text.
General GCC FAQs
Q: How do I include raw data (such as a font) into a project?
A:There are three different ways to do this:
-1) *** Convert your data to .h (gcc compiler) file ***
-2) *** Convert your data to .s (gas compiler) file ***
-3) *** Convert your data to .o (gcc object) file ***
(Read this question in the FAQs for the full how-to.)
General FAQs
Q: What is put into RAM on power up or reset?
A: Nothing. In fact, RAM may contain purely random values. It is totally
-up to the programmer to copy data from ROM to RAM or to initialize RAM
-to known values. Sometimes start up files crt0.s (for GCC) and start.s
-/ start.asm (for SDT) may initialize areas of RAM to some known values
-but this may not always be the case. Some programmers may prefer to
-leave these out of their ROM code to speed up ROM start up.
General FAQs
Q: What is multiboot mode?
A: That is where you can play a multiplayer game with several GBA's
-even if only one of the GBA's has a cart installed. FZero is an example
-of a commercial ROM that uses multiboot.
General FAQs
Q: How do you activate multiboot on a GBA?
A: If no cart is installed on the slave GBA then all you have to do is
-turn on the slave GBA. The master GBA will automatically boot the slave
-GBA if a GBA link cable (A GBC link cable won't work for GBA mode games.
-It only works for GBC mode games on the GBA.) is installed and a cart
-that has been designed for multiboot support is installed on the master
-GBA. If the slave GBA has some form of GBA cart installed, (regardless
-of the title) then you will need to hold down Start+Select while turning
-on the slave GBA. An extra jingle will then be heard that indicates that
-the installed cart is being ignored and the GBA is waiting for a multiboot
-from a master GBA. A slave unit waiting in multiboot mode (assuming a cart
-is installed) can cancel this mode by the user pressing A,B, or one of
-the directions buttons. At which time the installed cart will execute.
C Compiler FAQs
Q: What does the term "volatile" mean?
A: This means a value that can automatically change at any given instance.
-This is usually in reference to a hardware register that often has
-a different value ever time you read it. Specifically, it is useful
-to define some values as volatile so that a C compiler will not cache
-that value, but rather, get a fresh copy every time you request it's
-value. Otherwise, the compiler may return values which are inaccurate
-because the it doesn't realized the value can change.
Misc Hardware FAQs
Q: Is there any way to add a reset button to the GBA?
A: Yes, and it can be quite a handy feature if multiboot mode is
-figured out for development purposes. Just press the reset button
-to return to multiboot mode.
-To add a reset button you'll need to be handy with a soldering iron and
-happy with the fact that you are voiding your warranty by messing with
-your GBA. Remove the back cover of your GBA using a very small flat head
-screwdriver in the tri-wing screws. (Looking at the board you may
-notice the word Reset to the left of the board. Ignore this. This is
-not a useful signal and using it will short out a signal since it's a
-GBC mode output, not an input.) Next, loosen the clamp that holds
-the old LCD flat cable to the top of the board. To do this you need
-to very gently pry the plastic clip out on each side of the cable.
-THIS CLIP IS NOT DESIGNED TO COME OFF, JUST TO BE LOOSENED. SO DO NOT
-TRY TO PRY IT COMPLETELY OFF. Once this clip is loosened you should
-be able to remove this cable with NO force. Next unscrew the board
-and remove it. Flip it over. Just to the right of where the direction
-pad buttons are located is a small hole labeled Reset. Solder a wire
-from this hole to a switch. Solder a wire from the other side of this
-switch to the last pin on the cart connector that is closest to the
-direction pad buttons area. Put the board back in the GBA. Install the
-LCD cable. Press the LCD cable clip back in place so that the LCD cable
-is snug again. Put everything else back together. Do a short prayer
-that your GBA still works. Test.
GCC General FAQs
Q: I have a GCC compiler for GBA. Can I generate ARM or Thumb using a command line option?
A: This depends on the version of GCC that you have. Up until mid-year
-2000, GCC had to be built to support ARM or Thumb mode. You could not
-support both in the same compiler. Around mid-year 2000, ARM contributed
-a new backend to the GCC project that allowed one compiler build to
-support many of the ARM platforms including ARM & Thumb. Type the
-following from a DOS prompt: gcc -dumpmachine
-If it reports "arm-thumb-elf" then it supports both. If it reports
-"thumb-elf" or "arm-elf" then it only supports one mode.
Cart FAQs
Q: What is the largest size ROM supported by GBA?
A: The GBA cart interface has 24 address lines. Also there are
-16 data lines. 24 address lines = 2^24 = 16.7 million locations
-which means 16M x 16bit or 32M x 8bit. Which is a total of
-32MBytes or 256Mbits of memory space. To put more memory
-than this in a cart would require some form of bankswitching.
C Compiler FAQs
Q: Should I use a C complier that generates ARM or Thumb instructions?
A: Thumb code is able to provide up to 65% of the code size of ARM,
-and 160% of the performance of ARM when accessing 16 bit memory.
-(Source: ARM DDI 0029E) Since most code is usually run out of ROM
-(which is 16 bit) it makes sense to use a Thumb compiler. For the
-times when you need the extra performance of ARM, you can always
-code these subroutines in ARM assembly language and place them in
-internal WRAM.
GCC Optimization FAQs
Q: What operations are particularly slow on the ARM processor?
A: Division and modulus. The ARM has no division instruction so these are
-painfully slow. Whenever possible, use >> (right shift) instead.
-If you need to use division then use the routines in the GBA BIOS.
-One report has the BIOS divison code operating ~2.5x faster than GCC division.
Cart FAQs
Q: How much modifications to an existing GBA cart would need to be
done to make a GBA flash cart?
A: It's not as simple as replacing a ROM chip with a flash ROM.
-GBA ROMs are not even remotely similar, interface wise, to standard ROMS
-or flash ROMS. Read "How does the GBA ROM cart interface work?" below.
General FAQs
Q: Is GBC mode emulated on the GBA?
A: No. GBA contains two CPUs. One for GBA mode. One for GBC mode.
-Only one can be used at a time. The GBA decides which CPU to use
-on power up, depending on if there is a notch on the back of the
-cart or not. There is no currently known method for switching
-CPUs after powerup or for using both at the same time.
Link Port FAQs
Q: Is the GBA game link port physically different from the GBC link port?
A: The only difference is that there is an extra notch on the GBA
-link port. This is to prevent GBA-Only link cables from being used
-on a GBC. GBC link cables will work fine to connect two GBA's that
-wish to link to play a GBC game. A GBC link cable probably won't
-work for GBA link games due to link electrical differences.
Link Port FAQs
Q: What is the voltage on the GBA game link port?
A: It is 3.3 volts when using a GBA cart. It is 5 volts when using
-a GB/GBC cart.
GAS (Gnu ASsembler) FAQs
Q: How do I access a global C array or global C variables from assembly code?
A: All you have to do is just use it. You don't need to import
-or export anything. Let's assume you have a C array MyArray
-that you want to use. The following code will put a pointer
-to the start of this array in r0:
- ldr r0,pMyArray
- ...
- pMyArray: .word MyArray
-You can use the above code to access a C variable as well. In the
-case of a u32 variable, to read the value of that variable into r1
-you need the following in addition to the above code:
- ldr r1,[r0]
Graphics FAQs
Q: Is there a simple way of turning a sprite off?
A: Yes, set rotation double size mode with rotation/scaling switched off.
CART FAQs
Q: Is there a datasheet available for the GBA MX23L3206-12B Mask Rom?
A: This chip is made by http://www.macronix.com but they do not provide
-documentation for this chip online nor do they provide it if you contact them.
CART FAQs
Q: Can NeoGeo Pocket flash carts/copier be used with the GBA?
A: No. All versions of the NeoGeo Pocket use an 8 bit non-multiplexed
-data bus but the GBA uses a 16 bit multiplexed data bus.
CART FAQs
Q: How does the GBA cart interface work?
A: GBA ROMs are special chips that contain a standard ROM, address latches,
-and address counters all on one chip. Cart accesses can be either sequential
-or non-sequential. The first access to a random cart ROM location must be
-non-sequential. This type of access is done by putting the lower 16 bits
-of the ROM address on cart lines AD0-AD15 and strobing /WR low.
-Then /ROMCS and /RD are strobed low to read 16bits of data from that ROM
-location. The following sequential ROM location(s) can be read by again strobing
-/ROMCS and /RD low. Sequential ROM access does not require doing another
-/WR strobe because there are count up registers in the cart ROM
-chip that keep track of the next ROM location to read. In theory, you
-can read an entire GBA ROM with just one non-sequential read (address 0)
-and all of the other reads as sequential so address counters must be used
-on ALL address lines.
CART FAQs
Q: What type of eeprom memory is in the 8-pin chip in Mario Advance?
A: This appears to be a MPS-bus type eeprom. http://www.xicor.com has similar chips.
-(Thanks to Dark Fader.)
CART FAQs
Removed cart encryption info. Carts are not encrypted. They just
appeared that way due to a bogus header in a file used for comparison.
Graphics Tools FAQs
Q: I plan to put a JPEG & JPEG viewer into ROM. Should I down sample to save space?
A: No. A 256 color image won't JPEG compress/view very well and you won't
-save any ROM space by doing so. Leave the image at 16 million colors
-when you convert it to JPEG and put it in ROM.
DMA FAQs
Q: How long does a DMA transfer take?
A: It depends on the width of the DMA (16 or 32), the width of the
-source & destination memory, and the wait type of source & destination.
-Below is table of the CPU clock cycles required by each DMA
-transfer (Thanks to exoticorn):
-DMA16 / SRC16 = (fetch: 1+SrcWait) + (write:1+DestWait) = 2 + SrcWait + DestWait
-DMA32 / SRC16 / DEST16 = (fetch:2*(1+SrcWait)) + (write:2*(1+DestWait)) = 4 + 2*SrcWait + 2*DestWait
-DMA32 / SRC16 / DEST32 = (fetch:2*(1+SrcWait)) + (write:1+DestWait) = 3 + 2*SrcWait + DestWait
-DMA32 / SRC32 / DEST32 = (fetch:1+SrcWait) + (write:1+DestWait) = 2 + SrcWait + DestWait
-Example: 32 bit DMA from external WRAM(2:16) to internal WRAM(0:32) = 7 clocks per 32 bit word transfer
ARM / Thumb Assembly FAQs
Q: ARM docs are confusing. How do I figure out how many clock cycles an instruction takes?
*** updated to reflect prefetch usage ***
DMA FAQs
Q: What happens if an interrupt occurs during a DMA?KEY=DMAIntr
A: The interrupt will be processed upon completion of DMA.
C Complier FAQs
Q: Should I use a C complier that generates ARM or Thumb instructions?
A: There are no real advantages to be gained by running ARM code out
-of ROM. In some cases, ARM code will be even slower out of ROM and take
-up around 40% more ROM space. Since most code is usually run out of ROM
-it makes sense to use a Thumb compiler. For the times when you need the
-extra performance of ARM, you can always code these subroutines in ARM
-and place them in internal WRAM.
CART FAQs
Q: What is the prefetch buffer?
A: The prefetch buffer can be used to get bytes from game ROM before you
-actually need them. It only works for ROM and not other forms of memory
-and it is off by default. The advantage of turning it on is that fewer
-non-sequential (usually wait 3) ROM accesses will occur. You will have
-more sequential data (usually wait 1) ROM accesses instead. The
-disadvantage of turning on prefetch is that it uses more battery power.
DMA FAQs
Q: Does DMA halt the CPU or operate in the background?
A: DMA halts the CPU until completion. This would seem a tremendous
-disadvantage and it probably is in some cases. However, DMA can still
-be fast enough, in some cases, to be more efficient than using
-other methods of data transfer.
DMA FAQs
Q: If DMA halts the CPU, how can high priority DMA interrupt low priority DMA?
A: Because you can setup hblank, vblank, or sound DMA to occur automatically
-at some future point in time. When an automatic blank or sound interrupt
-occurs, it will halt any lower priority DMA and resume later upon completion
-of the higher priority DMA. Only One DMA can occur at any one time.
Graphics Tools FAQs
Q: Why are JPEGs produced by Photoshop so large?
A: That is because they sometimes put up to 7k bytes worth of comments
-into their file. Use JPEGTRAN.EXE to remove these comments
-and to compress the huffman tables. You can get it from devrs.com/gba
-(Look in the the Apps/Misc section.)
GAS (Gnu ASsembler) FAQs
Q: Why use ARM instructions at all since they often seem to take longer and take
up more memory than Thumb instructions?
A: Many ARM instructions are more powerful than Thumb and can often
-do twice as many things, or more, as the same number of Thumb instructions.
-If you use ARM in internal WRAM then they run at the same speed as Thumb
-so all of the advantages of ARM over Thumb become available.
GAS (Gnu ASsembler) FAQs
Q: ARM docs are confusing. How do I figure out how many clock cycles an instruction takes?
A: It depends on if it's an ARM instruction, Thumb instruction, operating
-in 16 bit or 32 bit memory, and the number of wait states of the memory.
-ARM instructions are 32 bit. Thumb are 16 bit. An ARM instruction in
-32 bit memory takes 1 fetch but 2 fetches in 16 bit memory. Wait
-states from ROM depend on if it's the first access (non sequential/usually 3 wait)
-or the next access (sequential/usually 1 wait). If you have the prefetch
-buffer disabled then you will hit the first access many times during ROM fetch.
-(Some info from exoticorn) Here are some example timings for the 'mov r0,#0' instruction:
-CPU Clocks = Mode / Memory Type (wait states:width)
-______________________________
-1 = Thumb / Internal WRAM (0:32)
-3 = Thumb / External WRAM (2:16)
-2 = Thumb / ROM (1:16)
-4 = Thumb / ROM (3:16)
-1 = ARM / Internal WRAM (0:32)
-6 = ARM / External WRAM (2:16)
-4 = ARM / ROM (1:16)
-8 = ARM / ROM (3:16)