*****************************************

* Hexadecimal string to BINARY conversion

* OTHER MODULES REQUIRED: TO_UPPER, IS_TERMIN, IS_XDIGIT

* ENTRY: X=start of a hex string terminated by a space,
*          comma, CR, or NULL.

* EXIT: D=binary number
*       CC carry set iferror (too large, non-numeric)
*       Y=terminator position or error char.


 nam Convert Hex String to Binary
 ttl Assembler Library Module

 section .text


HEX_BIN:
 clra init number
 clrb
 pshs d,x
 tfr d,y digit counter

loop
 ldb ,x+ get next digit
 lbsr TO_UPPER convert to uppercase
 lbsr IS_TERMIN end of string?
 beq exit yes, go home
 lbsr IS_XDIGIT make sure it's valid digit
 bne error not 0..9, a..f
 cmpb #'9 convert to binary value
 bls notAtoF
 subb #7 fix a..f
notAtoF
 subb #'0 convert to binary 0..15

* now shift the digit to bits 7..4

 lslb
 lslb
 lslb
 lslb

* now shift the value in to the result

 lda #4 number of bits
l1
 lslb digit bit to carry
 rol 1,s carry bit to result
 rol 0,s
 deca done 4?
 bne l1 no, loop

 leay 1,y number of digits done
 cmpy #4 
 bhi error more than 4
 bra loop keep going


exit
 clrb clear carry=no error
 sty -2,s test y (count)
 bne done no digits?

error
 clr 0,s
 clr 1,s
 orcc #1 set carry

done
 leay -1,x terminator/error pos
 puls d,x,pc

 endsect