The SCASX instructions in x86 assembly language are powerful tools for scanning strings, enabling programmers to efficiently search for specific values within blocks of memory. The SCASX family consists of scasb, scasw, scasd, and scasq, each designed to handle different data sizes: bytes, words, and doublewords respectively.

Understanding scasX Instructions

The scasX instructions are used to scan a string or array to find a specific value. They compare the value in the accumulator register (AL, AX, or EAX) with the value at the memory location pointed to by the Destination Index (DI) register. The comparison is performed on a byte, word, or doubleword basis, depending on the specific scasX instruction used.

Variants of scasX Instructions

scasb (Scan String Byte):

• Compares the byte in AL with the byte at [DI].
• AL is the 8-bit accumulator register.
• Example: scasb

scasw (Scan String Word):

• Compares the word in AX with the word at [DI].
• AX is the 16-bit accumulator register.
• Example: scasw

scasd (Scan String Doubleword):

• Compares the doubleword in EAX with the doubleword at [DI].
• EAX is the 32-bit accumulator register.
• Example: scasd

scasq (Scan String Quadword):

• Compares the quadword in RAX with the quadword at [RDI].
• RAX is the 64-bit accumulator register.
• Example: scasq

Syntax

The general syntax for scasX instructions is as follows:

SCASB  ; Scan byte string

SCASW  ; Scan word string

SCASD  ; Scan doubleword string

SCASQ	: Scan quadword string

Instruction Operation

The general operation of scasX instructions can be summarized as follows:

1. The value in the accumulator register (AL, AX, EAX, or RAX) is compared with the value at the memory address pointed to by DI.
2. The Direction Flag (DF) determines whether the DI register is incremented or decremented after the comparison.
   • If DF is cleared (cld), DI is incremented.
   • If DF is set (std), DI is decremented.
3. The result of the comparison affects the CPU flags (Zero Flag, Sign Flag, Carry Flag, etc.), which can be used for conditional branching.

Practical Usage

The scasX instructions are often used in loops to search for a specific value in a string or array. Below are some practical examples demonstrating their usage.

Example 1: Searching for a Byte in a String

section .data
    msg db 'Hello, World!', 0

section .text
    org 0x7C00

start:
    xor ax, ax
    mov ss, ax
    mov sp, 0x7C00

    mov ax, 0x07C0
    mov ds, ax

    ; Load the address of the string and the character to search for
    mov di, msg
    mov al, 'W'
    mov cx, 13 ; Length of the string

    ; Clear the Direction Flag to increment DI
    cld

search_loop:
    repe scasb  ; Repeat while CX is not zero and [DI] != AL
    jz found    ; Jump if zero flag is set (found the character)

not_found:
    ; Hang if not found
    cli
    hlt