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Loads of comments to explain what all the steps are doing

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Gwilym Kuiper 2021-10-26 22:25:35 +01:00
parent 050ee184b0
commit 9524b4b3c3
1 changed files with 19 additions and 5 deletions
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24
agb/src/sound/mixer/mixer.s
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@ -101,17 +101,31 @@ agb_arm_func agb_rs__mixer_add_stereo
@ The sound buffer must be SOUND_BUFFER_SIZE * 2 in size = 176 * 2 @ The sound buffer must be SOUND_BUFFER_SIZE * 2 in size = 176 * 2
push {r4-r8} push {r4-r8}
ldr r5, =0x0000FFFF ldr r5, =0x00000FFF
.macro mixer_add_loop_simple_stereo .macro mixer_add_loop_simple_stereo
ldrsh r6, [r0], #2 @ load the current sound sample to r6 ldrsh r6, [r0], #2 @ load the current sound sample to r6
ldr r4, [r1] @ read the current value ldr r4, [r1] @ read the current value
mov r7, r6, asr #8 @ This is slightly convoluted, but is mainly done for performance reasons. It is better
and r7, r7, r5 @ to hit ROM just once and then do 4 really simple instructions then do 2 ldrsbs however annoying
lsl r6, r6, #24 @ this is. Also, since all this code is in IWRAM and we never hit ROM otherwise, all accesses
orr r6, r7, r6, asr #8 @ are sequential and exactly the size of the bus to ROM (16 bits), so hopefully this will be super fast.
@
@ The next 4 instructions set up the current value in r6 to be in the expected format
@ 1 = 2s complement marks (so if negative, these are all 1s, if positive these are 0s)
@ L = the left sample
@ R = the right sample
@ 0 = all zeros
@ Split into bytes
@
@ At this point
@ r6 = | 1 | 1 | L | R | where the upper bytes are 1s if L is negative. No care about R
mov r7, r6, asr #8 @ r7 = | 1 | 1 | 1 | L | drop R off the right hand side
and r7, r7, r5 @ r7 = | 0 | 0 | 1 | L | exactly what we want this to be. The mask puts the 1 as 00001111 ready for the shift later
lsl r6, r6, #24 @ r6 = | R | 0 | 0 | 0 | drop everything except the right sample
orr r6, r7, r6, asr #8 @ r6 = | 1 | R | 1 | L | now we have it perfectly set up
add r4, r4, r6, lsl #4 @ r4 += r6 << 4 (calculating both the left and right samples together) add r4, r4, r6, lsl #4 @ r4 += r6 << 4 (calculating both the left and right samples together)