mirror of https://github.com/hsoft/collapseos.git
78 lines
1.7 KiB
NASM
78 lines
1.7 KiB
NASM
; *** Requirements ***
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; None
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;
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; *** Code ***
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; Parse the decimal char at A and extract it's 0-9 numerical value. Put the
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; result in A.
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;
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; On success, the carry flag is reset. On error, it is set.
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; Also, zero flag set if '0'
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; parseDecimalDigit has been replaced with the following code inline:
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; add a, 0xff-'9' ; maps '0'-'9' onto 0xf6-0xff
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; sub 0xff-9 ; maps to 0-9 and carries if not a digit
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; Parse string at (HL) as a decimal value and return value in IX under the
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; same conditions as parseLiteral.
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; Sets Z on success, unset on error.
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parseDecimal:
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push hl
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ld a, (hl)
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add a, 0xff-'9' ; maps '0'-'9' onto 0xf6-0xff
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sub 0xff-9 ; maps to 0-9 and carries if not a digit
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exx ; preserve bc, hl, de
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ld h, 0
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ld l, a ; load first digit in without multiplying
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ld b, 3 ; Carries can only occur for decimals >=5 in length
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jr c, .end
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.loop:
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exx
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inc hl
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ld a, (hl)
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exx
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; inline parseDecimalDigit
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add a, 0xff-'9' ; maps '0'-'9' onto 0xf6-0xff
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sub 0xff-9 ; maps to 0-9 and carries if not a digit
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jr c, .end
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add hl, hl ; x2
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ld d, h
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ld e, l ; de is x2
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add hl, hl ; x4
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add hl, hl ; x8
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add hl, de ; x10
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ld d, 0
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ld e, a
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add hl, de
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jr c, .end ; if hl was 0x1999, it may carry here
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djnz .loop
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inc b ; so loop only executes once more
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; only numbers >0x1999 can carry when multiplied by 10.
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ld de, 0xE666
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ex de, hl
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add hl, de
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ex de, hl
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jr nc, .loop ; if it doesn't carry, it's small enough
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exx
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inc hl
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ld a, (hl)
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exx
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add a, 0xd0 ; the next line expects a null to be mapped to 0xd0
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.end:
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; Because of the add and sub in parseDecimalDigit, null is mapped
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; to 0x00+(0xff-'9')-(0xff-9)=-0x30=0xd0
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sub 0xd0 ; if a is null, set Z
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; a is checked for null before any errors
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push hl \ pop ix
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exx ; restore original de and bc
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pop hl
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ret
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