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https://github.com/hsoft/collapseos.git
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71 lines
1.6 KiB
NASM
71 lines
1.6 KiB
NASM
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; *** Requirements ***
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; printnstr
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;
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; *** Variables ***
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; Used to store formatted hex values just before printing it.
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.equ STDIO_HEX_FMT STDIO_RAMSTART
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.equ STDIO_RAMEND @+2
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; *** Code ***
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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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; Print the hex char in A
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printHex:
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push bc
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push hl
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ld hl, STDIO_HEX_FMT
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call fmtHexPair
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ld b, 2
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call printnstr
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pop hl
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pop bc
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ret
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; Print the hex pair in HL
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printHexPair:
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push af
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ld a, h
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call printHex
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ld a, l
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call printHex
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pop af
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ret
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