m68k: ByteToASCII

; --------------------------------------------------------------
; INPUTS:
;   d0.b - Byte value
;
; RETURNS:
;   d0.w - ASCII characters to represent byte
;   (high byte is high nybble, low byte is low nybble)
;
; TRASHES:
;   d1
;
; NOTES:
;   The 68k stack, whenever pushed or popped by one byte in an
;   instruction, increments or decrements by two bytes in order
;   to prevent subsequent 16/32-bit reads or writes from odd
;   addresses. The lines
;
;       move.b  .ascii_table(pc,d0.w),-(sp)
;       move.w  (sp)+,d0
;
;   take advantage of this as a shortcut for bit-shifting
;   by 8 (multiplication by $100), saving a few cycles. This
;   gain is increased by the following line
;
;       move.b  .ascii_table(pc,d1.w),d0
;
;   doing away with the need to clear the lower byte of the
;   resulting word (as the stack may contain any data at
;   the time of operation).
;
;
;   This function follows the convention of using d0-d3 and
;   a0-a3 as "scratch" registers (able to be overwritten at
;   any time). If retaining the contents of d1 is desired,
;   add the lines
;
;       move.l  d1,-(sp)
;
;   and
;
;       move.l  (sp)+,d1
;
;   to the beginning and end of the function respectively.
;   Alternatively, do this (or another method of copying)
;   before and after calling the function.
; --------------------------------------------------------------

ByteToASCII:
    moveq   #$0F,d1
    and.b   d0,d1
    lsr.b   #4,d0
    andi.w  #$000F,d0
    move.b  .ascii_table(pc,d0.w),-(sp)
    move.w  (sp)+,d0
    move.b  .ascii_table(pc,d1.w),d0
    rts                 ; return ((ascii_table[(byte & 0xF0) >> 4] << 8) | ascii_table[byte & 0x0F]);

.ascii_table:
    dc.b    "0"
    dc.b    "1"
    dc.b    "2"
    dc.b    "3"
    dc.b    "4"
    dc.b    "5"
    dc.b    "6"
    dc.b    "7"
    dc.b    "8"
    dc.b    "9"
    dc.b    "A"
    dc.b    "B"
    dc.b    "C"
    dc.b    "D"
    dc.b    "E"
    dc.b    "F"