Nice! But it can be done faster if original data is in RAM:
;------------------------------------------------------
; Input: hl = Pointer to data
; Output: d = high nibble, e = low nibble
; Modifies: a, d, e
;------------------------------------------------------
GET_NIBBLES:
xor a
rrd
ld e,a
rrd
ld d,a
rrd
ret
Ideal to use HL as a pointer to unpack the byte-packed data and BC as a counter. What do you think?
Of course if data is not in RAM you can add this lines
;------------------------------------------------------
; Input: a = data
; Output: d = high nibble, e = low nibble
; Modifies: a, de, hl
;------------------------------------------------------
GET_NIBBLESROM:
ld hl,FREEBYTEINRAM ; you need to reserve a byte in ram to do this
ld [hl],a
jp GET_NIBBLES ; or place the appropriate code here
