cpu 8086
org 0x3000

;Store a keycode
;Read a keypress
mov ah, 0x0
int 0x16
;Store the keycode
mov [scan], ah
mov [ascii], al

;Print the prefix
mov si, prefix
mov ah, 0x0
int 0x21

;Convert the keycode to a hex string
;Convert the scancode
mov al, [scan]
mov di, keycode
call byte2hex
;Convert the ascii value
mov al, [ascii]
mov di, keycode
add di, 0x2
call byte2hex

;Print the keycode
mov si, keycode
mov ah, 0x2
int 0x21

;Return
int 0x20

;Data
prefix  db "0x", 0x0
scan    db 0x0
ascii   db 0x0
keycode times 0x5 db 0x0

;***

;Convert the value of a byte to a hex string
byte2hex:

;Store AX, BX, CX, and SI in the stack
push ax
push bx
push cx
push si

;Setup
;Move the byte to AH
mov ah, al
;Set SI to the hex digit key
mov si, key
;Set a counter for the two hex digits
mov cx, 0x2

;Convert the byte
;Read a nibble
loop:
times 0x4 rol ax, 0x1
mov bx, ax
;Convert the nibble to a hex digit
and bx, 0xf
mov bl, [si + bx]
;Store the hex digit
mov [di], bl
;Convert the next nibble
inc di
dec cx
jnz loop

;Load SI, CX, BX, and AX from the stack
pop si
pop cx
pop bx
pop ax

;Return
ret

;Data
key db "0123456789abcdef"