read ASCII hexidecimal

readhex.s

; read hex value from ASCII

ASC     equ $fa                         ; ASCII char passed in
RESULT  equ $fb                         ; result location

    org     $300                        ; Apple II small program area

start:
    lda     ASC                         ; mov char -> accumulator
    cmp     #$3a                        ; >= ':'?
    bcs     isalpha                     ; could be 0xa - 0xf
    cmp     #$30                        ; '0'
    bcc     err                         ; <?
    and     #$f                         ; A - $30
    sta     RESULT                      ; ready
    rts                                 ; done
isalpha:
    cmp     #$67                        ; 'g'
    bcs     err                         ; >=?
    sbc     #0                          ; carry should be set: - 1
    and     #$1f                        ; mask high bits
    cmp     #6                          ; 'G' or 'g'
    bcs     err                         ; >=?
    adc     #10                         ; align to value
    sta     RESULT                      ; return answer
    rts
err:                                    ; error known
    lda     #$ff                        ; return 0xff
    sta     RESULT
    rts
    brk
; BRUN READHEX

    ldx     #0
test:
    stx     ASC
    jsr     start
    sta     $2000, x
    inx
    bne     test
    brk