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;;; Author: Leon Bottou
;;; Public Domain.
cpu 6502
; 16 bit calculus functions.
; Two pseudoregisters: ACC, ARG
; Four temporary bytes: TEMP
dummy
* = $20
acc ds 2
arg ds 2
temp ds 4
movi macro
lda #(\1) & $ff
sta \2
lda #(\1) >> 8
sta (\2)+1
endm
movm macro
lda \1
sta \2
lda (\1)+1
sta (\2)+2
endm
* = $8000
code
add16 lda acc ; ACC <- ACC + ARG
clc
adc arg
sta acc
lda acc+1
adc arg+1
sta acc+1
rts
clr16 lda #0 ; clear ACC
sta acc
sta acc+1
.1 rts
abs16 bit acc+1 ; absolute value of ACC
bpl .1
neg16 lda #0 ; negate ACC
sec
sbc acc
sta acc
lda #0
sbc acc+1
sta acc+1
rts
neg16a lda #0 ; negate ARG
sec
sbc arg
sta arg
lda #0
sbc arg+1
sta arg+1
rts
sub16 lda acc ; ACC <- ACC - ARG
sec
sbc arg
sta acc
lda acc+1
sbc arg+1
sta acc+1
rts
code
ovfl bit .ov1
.ov1 rts
code
muls16 lda acc+1 ; ACC <- ACC * ARG
eor arg+1
sta temp+3
bit arg+1
bpl .0
jsr neg16a
.0 jsr abs16
lda #$80
sta temp+1
lda #0
sta temp
sta temp+2
.1 lsr acc+1
lsr acc
bcc .2
pha
lda temp+2
clc
adc arg
sta temp+2
pla
adc arg+1
.2 ror
ror temp+2
ror temp+1
ror temp
bcc .1
ora temp+2
php
movm temp,acc
plp
bne ovfl
bit acc+1
bmi ovfl
bit temp+3
bpl .3
jsr neg16
.3 rts
code
divmod movm acc,temp ; ACC <- ACC / ARG (unsigned)
lda #$01 ; TEMP+2 <- ACC % ARG (unsigned)
sta acc
asl
sta temp+2
sta temp+3
sta acc+1
lda arg
ora arg
beq ovfl
.1 asl temp
rol temp+1
rol temp+2
rol temp+3
lda temp+2
cmp arg
lda temp+3
sbc arg+1
bcc .2
sta temp+3
lda temp+2
sbc arg
sta temp+2
sec
.2 rol acc
rol acc+1
bcc .1
rts
code
divs16 lda acc+1 ; ACC <- ACC / ARG
eor arg+1
pha
bit arg+1
bpl .0
jsr neg16a
.0 jsr abs16
jsr divmod
pla
bpl .1
jmp neg16
.1 rts
code
mod16 lda acc+1 ; ACC <- ACC % ARG
pha
bit arg+1
bpl .0
jsr neg16a
.0 jsr abs16
jsr divmod
movm temp+2,acc
pla
bpl .1
jmp neg16
.1 rts
|
