collapseos/apps/zasm/literal.asm

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NASM
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2019-05-01 11:26:41 -04:00
; Parse the decimal char at A and extract it's 0-9 numerical value. Put the
; result in A.
;
; On success, the carry flag is reset. On error, it is set.
parseDecimal:
; First, let's see if we have an easy 0-9 case
cp '0'
ret c ; if < '0', we have a problem
cp '9'+1
; We are in the 0-9 range
sub a, '0' ; C is clear
ret
; Parses the string at (HL) and returns the 16-bit value in IX.
; As soon as the number doesn't fit 16-bit any more, parsing stops and the
; number is invalid. If the number is valid, Z is set, otherwise, unset.
parseNumber:
push hl
push de
push bc
ld ix, 0
.loop:
ld a, (hl)
cp 0
jr z, .end ; success!
call parseDecimal
jr c, .error
; Now, let's add A to IX. First, multiply by 10.
ld d, ixh ; we need a copy of the initial copy for later
ld e, ixl
add ix, ix ; x2
add ix, ix ; x4
add ix, ix ; x8
add ix, de ; x9
add ix, de ; x10
add a, ixl
jr nc, .nocarry
inc ixh
.nocarry:
ld ixl, a
; We didn't bother checking for the C flag at each step because we
; check for overflow afterwards. If ixh < d, we overflowed
ld a, ixh
cp d
jr c, .error ; carry is set? overflow
inc hl
jr .loop
.error:
call JUMP_UNSETZ
.end:
pop bc
pop de
pop hl
ret