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Gwilym Kuiper 2022-05-04 20:56:39 +01:00
parent 4a06acba6b
commit 4d2ad8859b
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18
agb/src/sound/mixer/mixer.s
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@ -156,6 +156,24 @@ SWAP_SIGN .req r11
ldr r2, agb_rs__buffer_size @ loop counter
mov r4, r2
@ The idea for this solution came from pimpmobile:
@ https://github.com/kusma/pimpmobile/blob/f2b2be49e806ca2a0d99cf91b3838d6d10f86b7d/src/pimp_mixer_clip_arm.S
@
@ The register should be 127 bigger then what you actually want, and we'll correct for that later. Hence the
@ add instructions in `load_sample`.
@
@ The idea behind this is in the bit patters of -128 and 127 which are 10000000 and 01111111 respectively,
@ and we want to clamp the value between them.
@
@ The first instruction calculates `-((sample + 128) >> 8)`. If sample is between -128 and 127, then
@ 0 <= sample + 128 <= 255 which means that shifting that right by 8 is 0. Hence the zero flag will be set, so
@ the `andne` instruction won't execute.
@
@ If the sample is outside of a signed 8 bit value, then `sample >> 8` will either be -1 or 1 (we assume that samples)
@ don't go too high, but the idea still works, so you can generalise this further if you want. This value is stored in TEMP
@
@ -1 has binary expansion (as a 32-bit integer) of all 1s and 1 of all zeros and then a 1.
@ So (-1 logical >> 24) gives 11111111 and (1 logical >> 24) gives 00000000 so register is clamped between these two values.
.macro clamp_s8 reg:req
subs TEMP, CONST_0, \reg, asr #8
andne \reg, CONST_FF, TEMP, lsr #24