mirror of https://github.com/hsoft/collapseos.git
126 lines
2.9 KiB
NASM
126 lines
2.9 KiB
NASM
; Fill B bytes at (HL) with A
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fill:
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push bc
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push hl
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.loop:
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ld (hl), a
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inc hl
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djnz .loop
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pop hl
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pop bc
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ret
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; Increase HL until the memory address it points to is equal to A for a maximum
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; of 0xff bytes. Returns the new HL value as well as the number of bytes
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; iterated in A.
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; If a null char is encountered before we find A, processing is stopped in the
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; same way as if we found our char (so, we look for A *or* 0)
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; Set Z if the character is found. Unsets it if not
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findchar:
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push bc
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ld c, a ; let's use C as our cp target
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ld a, 0xff
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ld b, a
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.loop: ld a, (hl)
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cp c
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jr z, .match
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or a ; cp 0
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jr z, .nomatch
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inc hl
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djnz .loop
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.nomatch:
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call unsetZ
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jr .end
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.match:
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; We ran 0xff-B loops. That's the result that goes in A.
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ld a, 0xff
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sub b
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cp a ; ensure Z
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.end:
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pop bc
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ret
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; Format the lower nibble of A into a hex char and stores the result in A.
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fmtHex:
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; The idea here is that there's 7 characters between '9' and 'A'
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; in the ASCII table, and so we add 7 if the digit is >9.
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; daa is designed for using Binary Coded Decimal format, where each
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; nibble represents a single base 10 digit. If a nibble has a value >9,
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; it adds 6 to that nibble, carrying to the next nibble and bringing the
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; value back between 0-9. This gives us 6 of that 7 we needed to add, so
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; then we just condtionally set the carry and add that carry, along with
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; a number that maps 0 to '0'. We also need the upper nibble to be a
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; set value, and have the N, C and H flags clear.
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or 0xf0
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daa ; now a =0x50 + the original value + 0x06 if >= 0xfa
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add a, 0xa0 ; cause a carry for the values that were >=0x0a
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adc a, 0x40
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ret
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; Formats value in A into a string hex pair. Stores it in the memory location
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; that HL points to. Does *not* add a null char at the end.
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fmtHexPair:
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push af
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; let's start with the rightmost char
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inc hl
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call fmtHex
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ld (hl), a
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; and now with the leftmost
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dec hl
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pop af
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push af
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rra \ rra \ rra \ rra
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call fmtHex
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ld (hl), a
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pop af
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ret
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; Compares strings pointed to by HL and DE up to A count of characters. If
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; equal, Z is set. If not equal, Z is reset.
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strncmp:
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push bc
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push hl
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push de
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ld b, a
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.loop:
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ld a, (de)
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cp (hl)
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jr nz, .end ; not equal? break early. NZ is carried out
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; to the called
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cp 0 ; If our chars are null, stop the cmp
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jr z, .end ; The positive result will be carried to the
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; caller
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inc hl
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inc de
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djnz .loop
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; We went through all chars with success, but our current Z flag is
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; unset because of the cp 0. Let's do a dummy CP to set the Z flag.
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cp a
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.end:
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pop de
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pop hl
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pop bc
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; Because we don't call anything else than CP that modify the Z flag,
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; our Z value will be that of the last cp (reset if we broke the loop
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; early, set otherwise)
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ret
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; Transforms the character in A, if it's in the a-z range, into its upcase
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; version.
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upcase:
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cp 'a'
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ret c ; A < 'a'. nothing to do
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cp 'z'+1
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ret nc ; A >= 'z'+1. nothing to do
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; 'a' - 'A' == 0x20
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sub 0x20
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ret
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