Programming:Display a 8-bit number in hex

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

The .print_byte_digit routine can be further optimised by devious use of DAA

;;-------------------------------------------
;; display number in lower nibble (bits 3..0) 
.print_byte_digit
and %00001111   ;isolate lower nibble. (This contains the digit value 0...15)
cp &a           ;Less than 10?  Set carry if so
sbc a,&69
daa             ;Because previous instruction was a subtract operation this
                ;is known as 'das' in x86
jp txt_output   ;display digit