From the The Unofficial Amstrad WWW Resource:
This routine uses Firmware
;; A useful procedure to display a 8-bit number in binary
;;
.txt_output equ &bb5a
;;------------------------------
;; DISPLAY A BYTE IN BINARY FORM
;;
;; Enter:
;; A = 8-bit value
;;
;; Exit:
;; AF corrupt.
;; B corrupt.
;; All other registers preserved
.display_binary
ld b,8 ;number of bits to display
.get_bit
rlca ;rotate current top bit into carry
;if the bit was 1, the carry is set. (i.e. carry=1)
;if the bit was 0, the carry is reset. (i.e carry=0)
push af
ld a,"0" ;ASCII character for bit value
adc a,0 ;add carry, if carry is set A="1", otherwise A="0"
call txt_output ;display character onscreen
pop af
djnz get_bit ;loop, until all bits have been displayed
ret