In this blog post, we’ll explore how to perform a block transfer using an 8086 assembly language program. The following code snippet demonstrates this process:

DATA SEGMENT
STRING1 DB 01H,02H,03H,04H,05H
STRING2 DB 4 DUP(0)
DATA ENDS

CODE SEGMENT
ASSUME CS:CODE,DS:DATA
START: MOV AX,DATA
MOV DS,AX
MOV ES,AX
LEA SI,STRING1
LEA DI,STRING2
MOV CX,05H
CLD
REP MOVSB
INT 3
CODE ENDS
END START

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Overall Process:

• The program initializes registers and sets up pointers to the source array (STRING1) and the destination array (STRING2).
• It sets up the CX register as a counter for the number of bytes to transfer.
• The CLD (Clear Direction Flag) instruction ensures that data is copied in a forward direction.
• The REP MOVSB instruction is used to transfer bytes from STRING1 to STRING2 efficiently.
• The process continues until all bytes in STRING1 are copied to STRING2.
• The program terminates after successfully transferring the data.

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Step-by-Step Explanation of Block Transfer Process:

Data Segment:

• STRING1: Defines a source array of hexadecimal numbers (01H, 02H, 03H, 04H, 05H).
• STRING2: Defines a destination array initialized with zeroes using 4 DUP(0), reserving space.

Code Segment: Initialization

1. MOV AX, DATA: Loads the address of the data segment into the AX register.
2. MOV DS, AX: Sets the DS register to point to the data segment, allowing access to source data.
3. MOV ES, AX: Sets the ES register to the same data segment for destination storage.
4. LEA SI, STRING1: Loads the address of the source array STRING1 into the SI register.
5. LEA DI, STRING2: Loads the address of the destination array STRING2 into the DI register.
6. MOV CX, 05H: Sets the loop counter CX to 5, corresponding to the number of elements to transfer.

Performing the Block Transfer:

1. CLD: Clears the direction flag to ensure auto-increment mode.
2. REP MOVSB: Executes MOVSB instruction CX times, copying bytes from STRING1 to STRING2.

Program Termination:

• INT 3: Generates an interrupt 3, which typically halts the program execution.

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Output

C:\TASM>masm bt.asm
Microsoft (R) Macro Assembler Version 5.00
Copyright (C) Microsoft Corp 1981-1985, 1987.  All rights reserved.
 
Object filename [bt.OBJ]:
Source listing  [NUL.LST]:
Cross-reference [NUL.CRF]:
bt.asm(1): warning A4001: Extra characters on line
 
  50422 + 450234 Bytes symbol space free
 
      1 Warning Errors
      0 Severe  Errors
 
C:\TASM>link bt.obj
Microsoft (R) Overlay Linker  Version 3.60
Copyright (C) Microsoft Corp 1983-1987.  All rights reserved.
 
Run File [BT.EXE]:
List File [NUL.MAP]:
Libraries [.LIB]:
LINK : warning L4021: no stack segment
 
C:\TASM>debug bt.exe
-g
 
AX=0B97  BX=0000  CX=0000  DX=0000  SP=0000  BP=0000  SI=0005  DI=000A
DS=0B97  ES=0B97  SS=0B97  CS=0B98  IP=0015   NV UP EI PL NZ NA PO NC
0B98:0015 CC            INT     3
-d 0B97:0000
0B97:0000  01 02 03 04 05 01 02 03-04 05 00 00 00 00 00 00   ................
0B97:0010  B8 97 0B 8E D8 8E C0 8D-36 00 00 8D 3E 05 00 B9   ........6...>...
0B97:0020  05 00 FC F3 A4 CC 15 8A-86 70 FF 2A E4 50 B8 FD   .........p.*.P..
0B97:0030  05 50 FF 36 24 21 E8 77-63 83 C4 06 FF 36 24 21   .P.6$!.wc....6$!
0B97:0040  B8 0A 00 50 E8 47 5E 83-C4 04 5E 8B E5 5D C3 90   ...P.G^...^..]..
0B97:0050  55 8B EC 81 EC 84 00 C4-5E 04 26 80 7F 0A 00 74   U.......^.&....t
0B97:0060  3E 8B 46 08 8B 56 0A 89-46 FC 89 56 FE C4 5E FC   >.F..V..F..V..^.
0B97:0070  26 8A 47 0C 2A E4 40 50-8B C3 05 0C 00 52 50 E8   &.G.*[email protected].
-q
 
C:\TASM>

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Understanding the Memory Dump

The memory dump displayed in the DEBUG session shows the contents of memory after running the program. Let’s analyze the key parts.

Address	Data (Hex)	ASCII Representation	Description
0B97:0000	01 02 03 04 05	.....	First five bytes from STRING1
0B97:0005	01 02 03 04 05	.....	Copied data in STRING2
0B97:000A	00 00 00 00 00 00	......	Unused/initialized memory

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Final Thoughts:

This simple assembly program demonstrates how to perform a block transfer efficiently using REP MOVSB. It’s a powerful way to copy data in assembly language, often used in low-level programming and system operations.

“Block transfer in assembly—because moving bytes is just another form of digital teleportation!” 😆