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Fix signed overflow in SA-1 clock frequency calculation #4

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Fix signed overflow in SA-1 clock frequency calculation #4

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44 changes: 35 additions & 9 deletions speed_test.asm
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Original file line number Diff line number Diff line change
Expand Up @@ -1844,35 +1844,61 @@ sa1_clock_finish_16:
sta $2251
asl
lda.w #63018 ; 8.16 fixed point. ==> 315.0 / 88.0 * 1000000 * 6 / (262 * 1364) * 16 / 1000 * 65536
sta $4a ; save multiplier
bra sa1_clock_finish_continue

sa1_clock_finish:
stz $2250
inc
sta $46
sta $2251
asl
asl ; save sign bit for later, adjustment needed for unsigned multiplication
lda.w #59079 ; 8.16 fixed point. ==> 315.0 / 88.0 * 1000000 * 6 / (262 * 1364) * 15 / 1000 * 65536
sta $4a ; save multiplier
.continue ; +/- 0.9 kHz resolution (or 0.00090 MHz)
sta $2253
ldy #$01
bcs +
lda $46
+ ldx $2307
ldx $2307
stx $48
ldx $2306
clc
adc $2308
lda $2308
bcc + ; if loop counter was "signed negative" (>32767), add the other factor
clc ; to the upper 16 bits of the result to turn the signed multiplication
adc $4a ; into an unsigned one => add the fixed multiplier.
+ clc ; fixed multiplier (59079 or 63018) is always "signed negative"
adc $46 ; so always adjust => always add the loop counter.
sty $2250
pha
lsr ; halve the dividend to make sure it is signed positive
sta $2251
lda.w #10000
lda.w #10000 ; divisor is known signed positive (10000)
sta $2253
nop
xba
lda $2306
sta $46
asl
sta $46 ; quotient
lda $2308
sta $2251
asl
sta $4a ; remainder
pla ; get the original dividend
ror ; check lsb
bcc +
inc $4a ; adjust remainder if lsb was set in dividend
+ lda $4a ; it might now be equal to the divisor
cmp.w #10000
bcc + ; if remainder is smaller than divisor, we're done.
; otherwise adjust results:
; e.g. 50001/10000 => 25000/10000 => q=2, r=5000
; double both => q=4, r=10000.
; original lsb of dividend was 1 => add to remainder => r=10001.
; since the remainder is >= 10000 increase the quotient by one and
; subtract the divisor from the remainder (modulo operation):
; q=5; r=(10001-10000)=1.
inc $46 ; increase quotient
sec
sbc.w #10000 ; adjust remainder to be less than divisor
+ sta $2251
lda.w #100
sta $2253
nop
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