The XLAT (Translate) instruction is an elegant and efficient tool in the x86 assembly language, used to perform table lookups and translations. It's particularly useful for tasks like character encoding conversions, where you need to map input values to output values using a predefined translation table.

What is XLAT?

The XLAT instruction translates a byte in the AL register using a table pointed to by the BX register. Essentially, it replaces the value in AL with the value at the memory address formed by adding the value in AL to the address in BX.

Syntax

XLAT

How Does XLAT Work?

Here’s a step-by-step breakdown of how the XLAT instruction operates:

1. Setup: You need to have a table of values stored in memory, and the base address of this table should be loaded into the BX register.
2. Indexing: The AL register should contain the index of the value you want to look up in the table.
3. Translation: When you execute the XLAT instruction, it will add the value in AL to the address in BX and load the byte from the resulting memory address into AL.

Example Usage

Consider an example where you want to translate uppercase ASCII letters ('A' to 'Z') to their lowercase counterparts ('a' to 'z'). Here’s how you can do it with XLAT:

section .data
    translation_table db 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'

section .text
    global _start

_start:
    ; Set up the translation table base address
    mov bx, translation_table

    ; Example input: Translate 'C' (ASCII 67)
    mov al, 'C'         ; AL = 67
    sub al, 65          ; AL = 2 (index into the table for 'C')

    ; Perform the translation
    xlat                ; AL = translation_table[AL]

    ; Now AL should contain 'c' (ASCII 99)

    ; Exit program (Linux system call)
    mov eax, 60         ; syscall number for exit
    xor edi, edi        ; status 0
    syscall

Explanation of the Code:

1. Translation Table: The translation_table in the .data section contains the lowercase letters corresponding to the uppercase letters 'A' to 'Z'.
2. Setting Base Address: The base address of the translation_table is loaded into the BX register.
3. Index Preparation: The ASCII value of 'C' (67) is loaded into AL. Since the table starts at 'A', we subtract 65 to get the correct index (2).
4. Executing XLAT: The XLAT instruction translates the value at the calculated address (BX + AL) and stores it back in AL. For 'C', this would be 'c' (ASCII 99).
5. Program Exit: The program exits using a Linux system call.

When to Use XLAT

The XLAT instruction is particularly useful in scenarios such as:

• Character Encoding Conversion: Converting characters from one encoding scheme to another.
• Lookup Table Operations: Efficiently performing lookups for various data transformation tasks.
• Data Mapping: Quickly mapping input values to corresponding output values using predefined tables.