Topic – string – Ch. 11 [Marut] Ch. 4 [Brey]

• String Data Transfer Instructions– The Direction Flag– LODS Instructions– STOS Instructions– MOVS Instructions

• Few Examples

String Data Transfers

• 8086 – string a byte or word array • What is array?• Need – register indirect addressing mode• Allow mem2mem operations• Five Instructions– LODS, STOS, MOVS, INS and OUTS

• Each instruction allows data transfer either a single byte, word or double word

Flags - recap

• 6 status flags

• 3 control flags – to control Processor’s ops.Direction flag [DF]: to determine the direction

in which string operations will proceed. By 2 index registers – Si and DI

Recap – ch. 3write on board

• SP, BP, SI and DI – point to [contain the offset addresses of] mem locations.

• SP – stack pointer • BP – base pointer • SI – source index register is used to point to mem

locations in the data segment addressed by DS [DS is one of the Segment registers – CS-code/DS-data/SS-stack/ES-extra segment]

• DI – Destination index – same as DI. Esp. for string ops that use DI to access memory locations addressed by ES.

The Direction Flag, DF

• DF = 0, auto-increment mode of SI, DI• DF = 1, auto-decrement mode of SI, DI

• CLD instruction clears the D flag (D = 0) – clears direction flag• STD instruction sets the D flag (D = 1) – sets DF• CLD, STD have no effect on other flags

• SI (Source Index) points to DS (Data Segment) i.e., DS:[SI]• DI (Destination Index) points to ES (Extra Segment) i.e., ES:[DI]

LODS Instructions• LODS instructions loads AL [with a Byte], AX [with a Word] or EAX

[with a Boubleword] with data indexed by SI register [EAX for 80386/+]

• LODSB – load string byte• LODSW – load string doubleword

Table 4-10: from Brey’s book

Example

STRING1 DB ‘ABC’

MOV AX, @DATAMOV DS, AXLEA SI, STRING1CLDLODSBLODSB

Read

STOS Instructions

• STOS instructions stores data form AL, AX or EAX to memory indexed by DI register

• STOSB – store string byte

Table 4-11: from Brey’s book

Example

STRING1 DB ‘HELLO’

MOV AX, @DATAMOV ES, AXLEA DI, STRING1CLDMOV AL, ‘A’STOSBSTOSB

Read

MOVS Instructions

• MOVS – move string from one memory location to other

Table 4-13 : From Brey’s Book

Example

.DATASTRING1 DB ‘HELLO’STRING1 DB 5 DUP (?)

MOV AX, @DATAMOV DS, AXMOV ES, AX

LEA SI, STRING1LEA DI, STRING2CLDMOVSBMOVSB

Read

Agenda

• String Data Transfer Instructions– The Direction Flag– LODS Instructions– STOS Instructions– MOVS Instructions

• More Examples Read

Concatenate Two Input StringsThen Display

Input String 1:Hello

Input String 2:World!

Concatenated String:Hello World!

Display message 1

Read first string

Display message 2

Read second string

Concatenate the two strings

Display the result

Display message 1Read first stringDisplay message 2Read second stringConcatenate the two stringsDisplay the result

Display message 1Read first string

Display message 1Read first stringDisplay message 2Read second stringConcatenate the two stringsDisplay the result

Display message 2Read second string

Display message 1Read first stringDisplay message 2Read second stringConcatenate the two stringsDisplay the result

Concatenate the two stringsDisplay the result

References

materials are from Dr. Sazzad, NSU• Ch 11, Assembly Language Programming – by Charls

Marut• Section 4-4, Intel Microprocessors – by Brey