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mult54.a
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164 lines (148 loc) · 6.46 KB
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; mult54.a
; from The Merlin 128 Macro Assembler disk, via 'The Fridge': http://www.ffd2.com/fridge/math/mult-div.s
; with an optimisation for speed (changing pha/pla to tax/txa), then unrolled a bit more
; (see mult2)
;
; 16 bit x 16 bit unsigned multiply, 32 bit result
; Average cycles: 497.00
; 192 bytes
; acc*aux -> [acc,acc+1,ext,ext+1] (low,hi) 32 bit result
aux = $02 ; 2 bytes input1
acc = $04 ; 2 bytes input2 } result
ext = $06 ; 2 bytes }
* = $0200
; (acc, acc+1, ext, ext+1) = (aux, aux+1) * (acc, acc+1)
mult
lda #0 ; A holds the low byte of ext (zero for now)
sta ext+1 ; high byte of ext = 0
ldy #2 ; loop counter
lsr acc+1 ; } acc_ext >> 1
ror acc ; }
bcc + ; skip if carry clear
clc ; }
adc aux ; }
tax ; remember A }
lda aux+1 ; } ext += aux
adc ext+1 ; }
sta ext+1 ; }
txa ; recall A
+
loop
; loop step 1
ror ext+1 ; }
ror ; }
ror acc+1 ; } acc_ext >> 1
ror acc ; }
bcc + ; skip if carry clear
clc ; }
adc aux ; }
tax ; remember A }
lda aux+1 ; } ext += aux
adc ext+1 ; }
sta ext+1 ; }
txa ; recall A
+
; loop step 2
ror ext+1 ; }
ror ; }
ror acc+1 ; } acc_ext >> 1
ror acc ; }
bcc + ; skip if carry clear
clc ; }
adc aux ; }
tax ; remember A }
lda aux+1 ; } ext += aux
adc ext+1 ; }
sta ext+1 ; }
txa ; recall A
+
; loop step 3
ror ext+1 ; }
ror ; }
ror acc+1 ; } acc_ext >> 1
ror acc ; }
bcc + ; skip if carry clear
clc ; }
adc aux ; }
tax ; remember A }
lda aux+1 ; } ext += aux
adc ext+1 ; }
sta ext+1 ; }
txa ; recall A
+
; loop step 4
ror ext+1 ; }
ror ; }
ror acc+1 ; } acc_ext >> 1
ror acc ; }
bcc + ; skip if carry clear
clc ; }
adc aux ; }
tax ; remember A }
lda aux+1 ; } ext += aux
adc ext+1 ; }
sta ext+1 ; }
txa ; recall A
+
; loop step 5
ror ext+1 ; }
ror ; }
ror acc+1 ; } acc_ext >> 1
ror acc ; }
bcc + ; skip if carry clear
clc ; }
adc aux ; }
tax ; remember A }
lda aux+1 ; } ext += aux
adc ext+1 ; }
sta ext+1 ; }
txa ; recall A
+
; loop step 6
ror ext+1 ; }
ror ; }
ror acc+1 ; } acc_ext >> 1
ror acc ; }
bcc + ; skip if carry clear
clc ; }
adc aux ; }
tax ; remember A }
lda aux+1 ; } ext += aux
adc ext+1 ; }
sta ext+1 ; }
txa ; recall A
+
; loop step 7
ror ext+1 ; }
ror ; }
ror acc+1 ; } acc_ext >> 1
ror acc ; }
bcc + ; skip if carry clear
clc ; }
adc aux ; }
tax ; remember A }
lda aux+1 ; } ext += aux
adc ext+1 ; }
sta ext+1 ; }
txa ; recall A
+
; loop step 8
ror ext+1 ; }
ror ; }
ror acc+1 ; } acc_ext >> 1
ror acc ; }
bcc + ; skip if carry clear
clc ; }
adc aux ; }
tax ; remember A }
lda aux+1 ; } ext += aux
adc ext+1 ; }
sta ext+1 ; }
txa ; recall A
+
dey ; decrement loop counter
beq exit ; loop back if not done yet
jmp loop ;
exit
sta ext ;
rts ;