Printing hex values in x86 assembly

问题

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

回答1:

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 ax
    push bx

    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

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

    pop bx
    pop ax
    ret

    section .bss

STRING:
    resb  50                ; reserve 50 bytes for the string

If you want the function to return the character it just saved out, you can skip the push/pop of ax.

回答2:

Please take a look at my answer on this page for to convert a 32 bit value in EAX to 8 hexadecimal ASCII bytes: Printing out a number in assembly language?

来源：https://stackoverflow.com/questions/18879672/printing-hex-values-in-x86-assembly

标签

assembly

printing

x86

hex