8086-ASSEMBLY-LANGUAGE-PROGRAMS/Source Code/Memory Operations/memory_scan.asm at main · Amey-Thakur/8086-ASSEMBLY-LANGUAGE-PROGRAMS · GitHub

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; =============================================================================
; TITLE: Memory Search (Scan)
; DESCRIPTION: Search for the first occurrence of a specific byte within
;              a memory block using the SCASB (Scan String Byte) instruction.
; AUTHOR: Amey Thakur (https://github.com/Amey-Thakur)
; REPOSITORY: https://github.com/Amey-Thakur/8086-ASSEMBLY-LANGUAGE-PROGRAMS
; LICENSE: MIT License
; =============================================================================

.MODEL SMALL
.STACK 100H

; -----------------------------------------------------------------------------
; DATA SEGMENT
; -----------------------------------------------------------------------------
.DATA
    BUFFER      DB 10, 20, 30, 40, 50, 60, 70, 80, 90, 100
    B_LEN       EQU 10
    TARGET      DB 50                    ; Value to look for

    MSG_FOUND   DB 'Value located! 1-based index position: $'
    MSG_FAIL    DB 'Value not found in the provided memory block.$'

; -----------------------------------------------------------------------------
; CODE SEGMENT
; -----------------------------------------------------------------------------
.CODE
MAIN PROC
    ; Segment registers
    MOV AX, @DATA
    MOV DS, AX
    MOV ES, AX                          ; ES is required for SCASB

    ; -------------------------------------------------------------------------
    ; STRING SCAN SETUP
    ; Pointer: ES:DI
    ; Search Criteria: AL
    ; -------------------------------------------------------------------------
    LEA DI, BUFFER
    MOV AL, TARGET
    MOV CX, B_LEN                       ; Max bytes to scan

    CLD                                 ; Increment DI

    ; REPNE SCASB: Repeat 'Scan String Byte' While Not Equal AND CX > 0
    ; Compares AL with ES:[DI] and updates flags.
    REPNE SCASB

    ; Logic check: If ZF is set, it means a match was found.
    JNE NOT_FOUND

    ; Calculate the 1-based position:
    ; (Original Length - Remaining CX)
    MOV AX, B_LEN
    SUB AX, CX                          ; Correct index calculation

    ; Print success label
    PUSH AX                             ; Save index
    LEA DX, MSG_FOUND
    MOV AH, 09H
    INT 21H
    POP AX                              ; Restore index

    ; Convert numeric position in AL to ASCII and print (assumes index < 10)
    ADD AL, '0'
    MOV DL, AL
    MOV AH, 02H                         ; DOS: Print char
    INT 21H
    JMP EXIT_SUCCESS

NOT_FOUND:
    LEA DX, MSG_FAIL
    MOV AH, 09H
    INT 21H

EXIT_SUCCESS:
    ; Terminate
    MOV AH, 4CH
    INT 21H
MAIN ENDP
END MAIN

; =============================================================================
; TECHNICAL NOTES
; =============================================================================
; 1. ANALOGY:
;    - SCASB is the basis for the 'strchr' or 'memchr' functions in high-level
;      languages.
; 2. INDEXING:
;    - Since SCASB increments DI *after* a match is found, the CX register
;      contains (Remaining - 1). The formula (Limit - CX) gives the match position.
; = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =