Chapter 8


The BIOS is a set of pre-coded routines that are within the GBA. We can call them at will with an instruction called SWI.


SWI stands for SoftWare Interrupt. What SWI does is call one of the builtin routines. Just give it a number and off it goes :). Example:
mov r0, #0x10
swi 0x80000 - SWI number 8 is SQRT (SQuare RooT). It will take the square root of r0 (and put the result in r0).
Notice that even though the SWI number is 8, with the ARM version of SWI, you need to do the SWI number * 0x10000 (or <<16, same thing). After running the above code, r0 should be 4.

There are several other BIOS routines, see GBAtek.htm for a list and how they work.

Now let's do two things, see the Division BIOS routine. And then we'll make a rotating Mode 3 background using the BGAffineTransform BIOS routines.

Division is SWI number 6, it works like so:
r0 = signed number
r1 = signed number to divide r0 by.
r0 = r0 / r1
r1 = r0 MOD r1 (this means remainder)
r3 = ABSOLUTE VALUE(r0 / r1) (will be the positive version of the number in r0)
I don't think you need a code example there, pretty self-explanatory.

Our second little program:

.arm .data @ anything called .data will be put into RAM by the GCC startup routine. @ what we want is the struct BGAffineSrc. .align 4 BG_affine_src: .word 100<<8 @ I got these values by trial and error. .word 100<<8 .short 0x6F .short 0x45 .short 0x280 .short 0x280 .zero 2 .text .global main main: mov r0, #0x4000000 mov r1, #0x400 add r1, r1, #3 strh r1, [r0] @ this is the load code from Chapter 4 mov r0, #0x6000000 ldr r1, =pic mov r2, #0x960 loop1: ldmia r1!, { r3,r4,r5,r6,r7,r8,r9,r10 } stmia r0!, { r3,r4,r5,r6,r7,r8,r9,r10 } subs r2, r2, #1 bne loop1 mov r4, #0x4000000 @ I'll use indexed addressing modes to access @ the I/O. ldr r0,=BG_affine_src @ r0 will be a pointer to the start of the struct. mov r1, #0x4000000 add r1, r1, #0x20 @ r1 will be a pointer to where the BIOS should @ write the result of the calculations, I'm going @ to have it write directly to the hardware registers. mov r2, #1 eor r3, r3, r3 infin: wait1: ldrh r5, [r4, #6] cmp r5, #161 @ wait for vblank. bne wait1 strh r3, [r0, #16] @ write the angle to the struct add r3, r3, #40 @ the angle is a 8:8 fixed point number. stmia sp, { r0,r1,r2,r3,r4,r5 } @ save some registers swi 0xE0000 @ call the BIOS routine. ldmia sp, { r0,r1,r2,r3,r4,r5 } @ restore registers b infin @ you'll see a lot of infinite loops .ltorg pic: .incbin "pic.bin" @ let's reuse that mode 3 picture data

I don't think that's too hard for you to be able to understand after some work at this point. A word of caution, when you use BIOS routines use a BIOS dump with VisualBoyAdvance, or just test with no$gba. It would seem that no$gba's BIOS emulation is somewhat better than VBA's. The program above works on the real GBA as do all programs I give you, unless otherwise noted. :)

In the next chapter, we'll have some fun learning a couple new instructions, including a discussion of THUMB mode.

That's all for now, How about ? Or the GBA ASM index?

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