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Lecture 6 String Operations

Lecture 6 String Operations. Modified and Presented By Dr. Rajesh Palit Asst. Professor, EECS, NSU Originally Prepared By Dr. Shazzad Hosain , EECS, NSU. Agenda. String Data Transfer Instructions The Direction Flag LODS Instructions STOS Instructions MOVS Instructions

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Lecture 6 String Operations

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  1. Lecture 6String Operations Modified and Presented By Dr. Rajesh Palit Asst. Professor, EECS, NSU Originally Prepared By Dr. ShazzadHosain, EECS, NSU

  2. Agenda • String Data Transfer Instructions • The Direction Flag • LODS Instructions • STOS Instructions • MOVS Instructions • INS and OUTS Instructions • More Examples

  3. String Data Transfers • Five Instructions • LODS, STOS, MOVS, INS and OUTS • Each instruction allows data transfer either a single byte, word or double word

  4. 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) • STD instruction sets the D flag (D = 1) • SI (Source Index) points to DS (Data Segment) i.e. DS:[SI] • DI (Destination Index) points to ES (Extra Segment) i.e. ES:[DI]

  5. LODS Instructions • LODS instructions loads AL, AX or EAX with data indexed by SI register • LODSB – load string byte Table 4-10: from Brey’s book

  6. Example STRING1 DB ‘ABC’ MOV AX, @DATA MOV DS, AX LEA SI, STRING1 CLD LODSB LODSB

  7. 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

  8. Example STRING1 DB ‘HELLO’ MOV AX, @DATA MOV ES, AX LEA DI, STRING1 CLD MOV AL, ‘A’ STOSB STOSB

  9. REP (Repeat Prefix)) • REP is used to execute string instructions repeatedly by CX times. • REP automatically decrements CX by 1 • REP works for any string instructions except LODS instruction

  10. Example : Clear the video text display 1 2 3 80 **** a b c 1 2 * * * 25 Video display 5 4 3 2 1 0 c b a 20H 20H 20H 07H 07H 07H Example 4-5: From Brey’s Book B800H Text memory

  11. MOVS Instructions • MOVSB – move string byte from one memory location to other Table 4-13 : From Brey’s Book

  12. Example .DATA STRING1 DB ‘HELLO’ STRING1 DB 5 DUP (?) MOV AX, @DATA MOV DS, AX MOV ES, AX LEA SI, STRING1 LEA DI, STRING2 CLD MOVSB MOVSB

  13. Example: Scroll Up One Line 1 2 3 80 **** 1 2 * * * 25 Video display * * SI 160 * * 5 4 3 2 1 0 Example 4-6: From Brey’s Book DI B800H Text memory

  14. INS Instructions • INSB – Input String Byte, from I/O device to memory location Table 4-14: From Brey’s Book

  15. Example • Read 50 bytes of data from an I/O device whose address in 03ACH and store the data in LISTS array Example 4-7: From Brey’s Book

  16. OUTS Instructions • OUTSB – Output String Byte, from string memory location to I/O device Table 4-15: From Brey’s Book

  17. Example • Transfer data form memory array (ARRAY) to an I/O device at I/O address 3ACH Example 4-8: From Brey’s Book

  18. Agenda • String Data Transfer Instructions • The Direction Flag • LODS Instructions • STOS Instructions • MOVS Instructions • INS and OUTS Instructions • More Examples

  19. Concatenate Two Input StringsThen Display Display message 1 Read first string Display message 2 Read second string Concatenate the two strings Display the result Input String 1: Hello Input String 2: World! Concatenated String: Hello World!

  20. Display message 1 Read first string Display message 2 Read second string Concatenate the two strings Display the result Display message 1 Read first string

  21. Display message 1 Read first string Display message 2 Read second string Concatenate the two strings Display the result Display message 2 Read second string

  22. Display message 1 Read first string Display message 2 Read second string Concatenate the two strings Display the result Concatenate the two strings Display the result

  23. References • Ch 11, Assembly Language Programming – by Charles Marut • Section 4-4, Intel Microprocessors – by Brey

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