#19858 - dagamer34 - Mon Apr 26, 2004 11:26 pm
I have decided to learn some ARM assebly so the first thing I wrote was a mode 3 pixel plotter (since it's supposed to be simple) but it doesn't seem to work exactly like it's supposed to. Somewhere my multiplication is off, and I get a pixel in the totally wrong place. Here's the code:
| Code: |
@Asm file
@ As declared in a C++ file
@ extern "C" { void func (u32 x, u32 y, u32 color); }
@ Plots a pixel in mode 3
.global func
.section .iwram
.arm
.align 2
func:
mov r1, r1, lsl #8 @ Multiply the Y address by 240
sub r1, r1, r1, lsr #8
mov r3, #0x6000000 @ Load the VRAM address
add r1, r3, r1 @ Add the VRAM address to the Y address
add r1, r0, r1 @ Add the X offset to the Y address
strh r2, [r1] @ Store the pixel
bx lr
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Little kids and Playstation 2's don't mix. :(
#19862 - sajiimori - Tue Apr 27, 2004 12:27 am
The VRAM offset you computed is measured in half-words. Before you add it to the VRAM address, shift it left a bit to get the offset in bytes.
#19865 - dagamer34 - Tue Apr 27, 2004 12:45 am
No difference.
| Code: |
@ Plots a pixel in mode 3
.global func
.section .iwram
.arm
.align 2
func:
mov r1, r1, lsl #8 @ Multiply the Y address by 240
sub r1, r1, r1, lsr #8
mov r3, #0x6000000 @ Load the VRAM address
mov r12, r3, lsl #1 @ Shift left the VRAM address to get it measured in bytes
add r1, r12, r1 @ Add the VRAM address to the Y address
add r1, r0, r1 @ Add the X offset to the Y address
strh r2, [r1] @ Store the pixel
bx lr
|
_________________
Little kids and Playstation 2's don't mix. :(
#19868 - Miked0801 - Tue Apr 27, 2004 1:29 am
| Code: |
func:
mov r3, r1, lsl #9 @ Multiply the Y address by 512
sub r3, r1, r1, lsl #5 @ - 32 * Y == 480 * y (240 * 2)
add r3, r3, r0, lsl #1 @ Add the X offset (*2) to the Y address
add r3, r3, #0x6000000 @ Add the VRAM address
strh r2, [r3] @ Store the pixel
bx lr
|
Same idea, but making sure to mul both X and Y components by 2. I believe the original problem is with your mul by 240 though I may be incorrect. Untested as I'm at home, but I think mine will do the job quickly. Too bad we're in half-word mode or we could get rid of 1 more instruction by using addressing to do an add or shift. A double pixel plotter might be in order to do that and be a bit faster perhaps.
Mike
#19869 - dagamer34 - Tue Apr 27, 2004 1:45 am
Still nothing...
I'm not sure what's the problem.
_________________
Little kids and Playstation 2's don't mix. :(
#19873 - sajiimori - Tue Apr 27, 2004 2:42 am
You multiplied the VRAM base address by 2, which makes no sense -- VRAM really is at 0x6000000, not 0xC000000. I was talking about the offset.
#19883 - FluBBa - Tue Apr 27, 2004 10:57 am
I think this thread has it right.
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I probably suck, my not is a programmer.