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

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; =============================================================================
; TITLE: Memory Block Copy
; DESCRIPTION: Demonstrate efficient memory-to-memory data transfer using
;              the 8086 specialized string instruction MOVSB.
; 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
    SOURCE      DB 'Hello, World!', 0    ; Null-terminated source string
    SOURCE_LEN  EQU $ - SOURCE           ; Automatically calculate length

    DEST        DB 20 DUP(0)             ; Destination buffer (initialized to 0)
    MSG         DB 'Memory block transfer successfully completed!$'

; -----------------------------------------------------------------------------
; CODE SEGMENT
; -----------------------------------------------------------------------------
.CODE
MAIN PROC
    ; Initialize Segment Registers
    MOV AX, @DATA
    MOV DS, AX                          ; DS points to Source segment
    MOV ES, AX                          ; ES points to Destination segment (same here)

    ; -------------------------------------------------------------------------
    ; STRING INSTRUCTION SETUP
    ; Source Pointer: DS:SI
    ; Destination Pointer: ES:DI
    ; Count: CX
    ; -------------------------------------------------------------------------
    LEA SI, SOURCE                      ; Source offset
    LEA DI, DEST                        ; Destination offset
    MOV CX, SOURCE_LEN                  ; Number of bytes to copy

    ; CLD (Clear Direction Flag): Ensures SI and DI increment (forward)
    CLD

    ; REP MOVSB: Repeat 'Move String Byte' while CX > 0
    ; Automatically handles: ES:[DI] = DS:[SI]; SI++; DI++; CX--;
    REP MOVSB

    ; Verify by printing success message
    LEA DX, MSG
    MOV AH, 09H                         ; DOS: Print string
    INT 21H

    ; Return to DOS
    MOV AH, 4CH
    INT 21H
MAIN ENDP
END MAIN

; =============================================================================
; TECHNICAL NOTES
; =============================================================================
; 1. PERFORMANCE:
;    - 'REP MOVSB' is significantly faster than a manual loop and MOV instruction.
;    - Always ensure ES is correctly initialized as many string instructions
;      implicitly use ES for the destination.
; 2. FLAGS:
;    - Direction Flag (DF): 0 (CLD) for forward, 1 (STD) for backward.
; = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =