Prex/Game Boy Advance - Documentation

Table of Contents

• How to run Prex on the Game Boy Advance?
• How to run Prex with VisualBoy Advance?
• How to compile Prex for Game Boy Advance?
• How to use On-screen Keyboard?

• Memory Map
• System Call

How to run Prex on the Game Boy Advance?

There exist some hardware to run Prex on the Game Boy Advance. Currently, I am using "Boot Cable USB" which is sold by Optimize. And, its control software is available for download from this page. Unfortunately, these are only japanese page... :-(
You may find other useful information to run your program on GBA at http://www.gbadev.org.

How to run Prex with the VisualBoy Advance?

VBA - VisualBoy Advance is most popular GBA emulator. You can find the latest release at http://www.vba.ngemu.com. To run Prex with VBA, simply select file->open and open the prex image. There is no need to prepare the original GBA BIOS.

The default keyboard setting of VBA is as follow:

How to compile Prex for Game Boy Advance?

Step 1. Unpack Archives

$ cd /usr/src
$ tar zxvf binitils-2.15.tar.gz
$ tar zxvf gcc-core-3.4.3.tar.gz
$ tar zxvf newlib-1.13.0.tar.gz

Step 2. Build Binutils

Configure and build binutils.

$ mkdir binutils-2.15-arm-elf
$ cd binutils-2.15-arm-elf
$ ../binutils-2.15/configure --prefix=/usr/local/arm --target=arm-elf --disable-nls \
 --with-cpu=arm7tdmi
$ make
$ make install

You must set the path to this binutils.

$ export PATH=/usr/local/arm/bin:$PATH
$ cd /usr/src

Step 3. Build gcc

$ mkdir gcc-3.4.3-arm-elf
$ cd gcc-3.4.3-arm-elf
$ ../gcc-3.4.3/configure --prefix=/usr/local/arm --target=arm-elf --with-gnu-as \
 --with-gnu-ld --with-cpu=arm7tdmi --disable-nls --enable-languages=c --with-newlib \
 --enable-interwork --with-headers=../newlib-1.13.0/newlib/libc/include/
$ make LANGUAGES="c"
$ make LANGUAGES="c" install
$ cd /usr/src

Step 4. Unpack Sources

$ cd /usr/src
$ tar zxvf prex-X.X.X.tar.gz
$ cd prex-X.X.X

Step 5. Configure

$ ./configure --target=arm-gba --cross-compile=arm-elf-

Step 6. Make

$ make

Note: There is a cross-compiling kit for GBA named DevKit Advance. It may be usable for compiling Prex.

How to use On-Screen Keyboard?

The driver for on-screen keyboard is provided to emulate a generic keyboard by using the GBA keypad. You can input any key by moving the cursor on the on-screen keyboard. The user can show/hide the on-screen keyboard on the GBA screen by pressing select button. The key assign is different for each case.

Memory Map

General Memory Usage

• 00000000 - 00003FFF BIOS - System ROM
• 02000000 - 0203FFFF EWRAM (256K)
• 03000000 - 03007FFF EWRAM (32K)
• 04000000 - 040003FF I/O Registers
• 05000000 - 040003FF Palette RAM
• 06000000 - 06017fFF VRAM
• 07000000 - 070003FF Object Attributes

VRAM Usage

• 06000000 - 06003FFF Tile 1 - Font
• 06004000 - 06005FFF Tile 2 - Keyboard Image (non shift)
• 06006000 - 06007FFF Tile 2 - Keyboard Image (shift)
• 06008000 - 06008FFF Background 3 Map (Console text)
• 06009000 - 06009FFF Background 1 Map (Keyboard non-shift)
• 0600A000 - 0600AFFF Background 2 Map (Keyboard shift)
• 0600B000 - 0600FFFF Not used
• 06010000 - 0601FFFF Cursor Data

System Call

Since the GBA BIOS does not allow to handle the SWI event by the program, we need some trick to handle the system call. The system call will be handled by the following steps.

• The application invoke the system call.
• The system call stub in the library will jump to the SWI exception vector (0x0202007C) in the kernel code.

#define SYSCALL0(name) \
        .global name; .align; \
name##: \
        stmfd   sp!, {r4, r5, lr}; \
        mov     r4, #SYS_##name; \
        ldr     r5, =0x202007c; \
        add     lr, pc, #2; \
        mov     pc, r5; \
        ldmfd   sp!, {r4, r5, pc};

• The kernel SWI handler will switch the processor mode to SVC mode.

ENTRY(syscall_entry)
        mov     r5, lr
        mrs     ip, cpsr
        bic     ip, ip, #PSR_MODE
        orr     ip, ip, #PSR_SVC_MODE
        msr     cpsr_c, ip
        mov     lr, r5