Printing hex values in x86 assembly

Question

I need to make a routine that will convert a memory address into a string of bytes. That string would then be the input for a function that prints null-terminated strings (which I was already able to make). For example, if I have an address 0x1bf9 I need to print the text "1bf9" to the screen. The book hasn't gone into 32 bit mode yet, but it kind of hinted that we would need it for that too. This is what I have so far:

TABLE:
db "0123456789ABCDEF", 0

STRING:
db 0

hex_to_char:
    lea bx, TABLE
    mov ax, dx

    mov ah, al ;make al and ah equal so we can isolate each half of the byte
    shr ah, 4 ;ah now has the high nibble
    and al, 0x0F ;al now has the low nibble
    xlat ;lookup al's contents in our table
    xchg ah, al ;flip around the bytes so now we can get the higher nibble 
    xlat ;look up what we just flipped
    inc STRING
    mov [STRING], ah ;append the new character to a string of bytes
    inc STRING
    mov [STRING], al ;append the new character to the string of bytes

    ret

possible duplicate of Printing Hexadecimal Digits with Assembly — Ciro Santilli 新疆再教育营六四事件法轮功郝海东
How to convert a binary integer number to a hex string? - simple and somewhat efficient 32-bit loop converting a whole 32-bit number, and very efficient SIMD versions. — Peter Cordes

lurker lurker · Accepted Answer · 2013-09-18T18:41:03

This attempts to increment a literal label, which isn't correct. Also, your STRING memory location only allocates one byte (char) instead of a larger number to accommodate the size of string you intend.

STRING:
    db 0

    inc STRING   ;THIS WON'T WORK
    mov [STRING], ah ;append the new character to a string of bytes
    inc STRING   ;THIS WON'T WORK
    mov [STRING], al ;append the new character to the string of bytes

A neutral comment: the character table used for xlat does not need to be zero terminated.

Also I'd recommend saving and restoring some registers as good asm programming practice. That way, the calling function doesn't need to worry about registers being changed "behind its back". Ultimately then, you probably want something like this:

TABLE:
    db "0123456789ABCDEF", 0

hex_to_char:
    push bx

    mov   bx, TABLE
    mov   ax, dx

    mov   ah, al            ;make al and ah equal so we can isolate each half of the byte
    shr   ah, 4             ;ah now has the high nibble
    and   al, 0x0F          ;al now has the low nibble
    xlat                    ;lookup al's contents in our table
    xchg  ah, al            ;flip around the bytes so now we can get the higher nibble 
    xlat                    ;look up what we just flipped

    mov   bx, STRING
    xchg  ah, al
    mov   [bx], ax          ;append the new character to the string of bytes

    pop bx
    ret

    section .bss

STRING:
    resb  50                ; reserve 50 bytes for the string

EDIT: some desirable tweaks based upon Peter Cordes' input.

Printing hex values in x86 assembly

3 Answers