From the The Unofficial Amstrad WWW Resource:
This routine uses Firmware
;; A procedure to display a 8-bit number in hex form
;;
.txt_output equ &bb5a
;;----------------------------
;; DISPLAY A BYTE IN HEX FORM
;;
;; Enter:
;; A = 8-bit value
;;
;; Exit:
;; AF corrupt, All other registers preserved
.print_hex_byte
push af ;store original byte value
;; move upper nibble into lower nibble
rrca
rrca
rrca
rrca
;; display nibble as ASCII
call print_byte_digit
pop af ;retrieve original byte value
;;-------------------------------------------
;; display number in lower nibble (bits 3..0)
.print_byte_digit
and %00001111 ;isolate lower nibble. (This contains the digit value 0...15)
add a,"0" ;add ASCII for 0. Digits 0...9 become "0"..."9", digits 10..15
;become ":"...."A"
cp "9"+1
jr c,number ;if number is in digit range 0...9, display digit
add a,"A"-"9"-1 ;modify ASCII value so that digits in the range 10...15
;;become "A"...."F"
.number
jp txt_output ;display digit