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64 lines
1.5 KiB
NASM
64 lines
1.5 KiB
NASM
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; Parse the hex char at A and extract it's 0-15 numerical value. Put the result
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; 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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parseHex:
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; First, let's see if we have an easy 0-9 case
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cp '0'
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jr c, .error ; if < '0', we have a problem
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cp '9'+1
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jr nc, .alpha ; if >= '9'+1, we might have alpha
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; We are in the 0-9 range
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sub '0' ; C is clear
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ret
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.alpha:
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call upcase
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cp 'A'
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jr c, .error ; if < 'A', we have a problem
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cp 'F'+1
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jr nc, .error ; if >= 'F', we have a problem
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; We have alpha.
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sub 'A'-10 ; C is clear
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ret
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.error:
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scf
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ret
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; Parses 2 characters of the string pointed to by HL and returns the numerical
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; value in A. If the second character is a "special" character (<0x21) we don't
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; error out: the result will be the one from the first char only.
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; HL is set to point to the last char of the pair.
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;
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; On success, the carry flag is reset. On error, it is set.
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parseHexPair:
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push bc
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ld a, (hl)
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call parseHex
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jr c, .end ; error? goto end, keeping the C flag on
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rla \ rla \ rla \ rla ; let's push this in MSB
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ld b, a
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inc hl
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ld a, (hl)
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cp 0x21
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jr c, .single ; special char? single digit
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call parseHex
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jr c, .end ; error?
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or b ; join left-shifted + new. we're done!
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; C flag was set on parseHex and is necessarily clear at this point
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jr .end
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.single:
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; If we have a single digit, our result is already stored in B, but
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; we have to right-shift it back.
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ld a, b
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and 0xf0
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rra \ rra \ rra \ rra
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dec hl
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.end:
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pop bc
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ret
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