converting ascii hex to binary (was Re: [sdiy] TMS7000 disassembler ? )

[The Old Crow]

On Mon, 25 Feb 2002 CHoaglin at aol.com wrote:

> while we're on the subject of disassemblers, does anybody know how to go
> about converting an ASCII hex
> 
> ie: <ADDRESS>-----<HEX>------<ASCII REPRESENTATION>

> file into a raw binary file containing only the binary representation of
> the hex?

  Hm..you want to convery hex-ascii to binary?  In what language?

  Since each ascii digit takes up a byte, you want to collapse every digit 
pair into one byte.  The usual way to do this is (hope you can follow PIC 
code):

  Note, I am assuming the input ascii digits are known to be 0 to 9 or A 
to F.

; W has asciihex digit to convert
;
    addlw   -'A'	;W-'A', is W between '0' and '9' or 'A' and 'F'?
    skpc		;C flag=1 means W-'A' where W between 'A' and 'F'
     addlw  'A'-10+'0'	;If W is 0 to 9, W=W-'A'-10+'0' gets 0x00 to 0x09
    addlw   10		;If W is A to F, W=W-'A'+10 yields 0x0A to 0x0F

  That converts one of the 2 digits of a byte.  Do the same thing on the 
other.  Then simply take the first digitx16 (left shift x4) and add it to 
the second for the final result.

  Sort of like:

; do MSD
    movf   MSD,W	;MSD is a variable name I made up = some ram loc.
    addlw  -'A'
    skpc
     addlw 'A'-10+'0'
    addlw  10
    movwf  MSBD		;Another made-up varname

; do LSD (druuuug humor)
    movf   LSD,W        ;LSD is a var...blahblah
    addlw  -'A'
    skpc
     addlw 'A'-10+'0'
    addlw  10
    movwf  LSBD         ;LSBD blah

;form final byte
    rlcf   MSBD,f	;MSBDx2
    rlcf   MSBD,f	;x4
    rlcf   MSBD,f	;x8
    rlcf   MSBD,w	;x16

    andlw  0xF0		;make sure low nyb clean
    iorwf  LSBD,W	;form the byte
    movwf  BINBYTE	;hooray, another varname which is result

  If I was less lazy, I could shrink that code scrap a little more, but 
then it wouldn't be as obvious.

Crow
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