collapseos/kernel/parse.asm
Virgil Dupras 7fad3b0c90 Move /parts/z80 to /kernel
Let go of that "meta os" thing. it's not as meta as I made it sound
like. It's a kernel.
2019-05-19 11:19:41 -04:00

64 lines
1.5 KiB
NASM

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