6502 code (not very good) to read the two-char ASCII representation of a byte and convert it to a binary byte

read_ascii_byte.a65

;---------------------------------------------------------------------
;   Read (using bsread) a pair of ASCII chars representing a printable
;   byte value, and convert it to that byte value, returning it in A.

asc_0       .equ    30
asc_9       .equ    39
asc_A       .equ    41
asc_F       .equ    46

            .area   ZP
rab_temp:   .ds     1
            .area   CODE

rd_ascii_byte:                  ; XXX test framework has 14 char limit
            jsr     bsread
            jsr     rab_decode
            asl                 ; store top nybble
            asl
            asl
            asl
            sta     rab_temp
            jsr     bsread
            jsr     rab_decode
            ora     rab_temp
            rts

;   Decode an ASCII char to a binary digit
rab_decode: cmp     #asc_0
            bmi     rab_error
            cmp     #asc_9+1
            bpl     rab_hex
            sec
            sbc     #asc_0      ; to numeric value
            rts
rab_hex:    cmp     #asc_A
            bmi     rab_error
            cmp     #asc_F+1
            bpl     rab_error
            sec
            sbc     #asc_A-0A   ; to numeric value
            rts

rab_error:  brk                 ; let the test framework catch this