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Commented the use of daa

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I made the comments surrounding my use of daa much clearer, so it isn't quite so mystical what's being done here.
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Clanmaster21 2019-10-19 17:24:50 +01:00 committed by GitHub
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1 changed files with 8 additions and 4 deletions
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kernel/core.asm
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@ -183,10 +183,14 @@ findchar:
; Format the lower nibble of A into a hex char and stores the result in A.
; daa does the following operation if we were working in a high level language:
; a += (2*N-1)*(0x60*(a >= 0xa0) + 0x06*((a & 0x0f) >= 0x0a))
; N is the Negative flag. The C and H flags are also taken into account, but we
; clear both so this is a good enough description for our purposes.
; The idea here is that in the ASCII table, there's 7 characters between
; '9' and 'A', and so we add 7 if the digit is >9.
; daa is designed for using Binary Coded Decimal format, where each
; nibble represents a single base 10 digit. If a nibble has a value
; greater than 9, it adds 6 to that nibble, carrying a 1 to the next
; nibble and bringing the value back between 0-9. This gives us 6 of that
; 7 we needed to add, so then we just condtionally set the carry and
; add that carry, along with a number that maps 0 to '0'.
fmtHex:
; we need to make the upper nibble a known value
; also clears the N, C and H flags for daa