INSTRUCTOR: ABDULMUTTALIB A. H. ALDOURI

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Note: PUSHF / POPF have no operands The figure below shows that if (SS) = 3000H, (SP) = 0042H, so the execution of POP CX loads CX by the word 4050H form the stack segment. The SP is incremented by 2.

1 Note: PUSHF / POPF have no operands The figure below shows that if (SS) = 3000H, (SP) = 0042H, so the execution of POP CX loads CX by the word 4050H form the stack segment. The SP is incremented by 2. The figure below shows the Execution of PUSH CX if (SS) = 3000H, (SP) = 0034H and (CX)=2030H. This instruction copies the contents of CX in locations 30033H and 30032H in stack segment. The SP is decremented by 2. 41

2 String Instructions 1. MOVS ( MOVSB / MOVSW ) MOVES STRING BYTE OR STRING WORD: This instruction copies a byte or a word from location in the data segment to a location in the extra segment. The offset of the source in the data segment must be in the SI register. The offset of the destination in the extra segment must be in the DI register. For multiple-byte or multiple-word moves, the number of elements to be moved is put in the CX register(counter). If DF is 0, then SI and DI will incremented by 1 after a byte move and by 2 after a word move. If DF is 1, then SI and DI will be decremented by 1 after a byte move and by 2 after a word move. MOVS does not affect any flag. MOVSB MOVSW (ES:DI) (DS:SI) (ES:DI) (DS:SI) SI SI - 1 SI SI + 2 DI DI - 1 DI DI + 2 (ES:DI) (DS:SI) SI SI + 1 DI DI + 1 (ES:DI) (DS:SI) SI SI - 2 DI DI LODS ( LODSB / LODSW ) LOAD STRING BYTE INTO AL OR STRING WORD INTO AX :This instruction copies a byte or a word from a string location pointed to by SI to AL or to AX. If DF is 0, SI will be automatically incremented (by 1 for a byte string, and 2 for a word string) to point to the next element of the string. If DF is 1, SI will be automatically decremented (by 1 for a byte string, and 2 for a word string) to point to the previous element of the string. LODS does not affect any flag. LODSB LODSW (AL) (DS:SI) (AX) (DS:SI) SI SI - 1 SI SI + 2 (AL) (DS:SI) SI SI + 1 (AX) (DS:SI) SI SI STOS ( STOSB / STOSW ) STORE STRING BYTE OR STRING WORD: This instruction copies a byte from AL or a word from AX to a memory location in the extra segment pointed to by DI. If DF is 0, then DI will automatically incremented by 1 for a byte string and by 2 for a word string. If DI is 1, DI will be automatically decremented by 1 for a byte string and by 2 for a word string. STOS does not affect any flag. STOSB STOSW (ES:DI) (AL) (ES:DI) (AX) DI DI - 1 DI DI + 2 (ES:DI) (AL) DI DI + 1 (ES:DI) (AX) DI DI

4. CMPS ( CMPSB / CMPSW ) COMPARE STRING BYTE OR STRING WORD : This instruction can be used to compare a byte / word in the source string with a byte / word in the destination string.

3 4. CMPS ( CMPSB / CMPSW ) COMPARE STRING BYTE OR STRING WORD : This instruction can be used to compare a byte / word in the source string with a byte / word in the destination string. SI stores the offset of the byte or word in the source string, and DI stores the offset of the byte or word in the destination string. The AF, CF, OF, PF, SF, and ZF flags are affected by the comparison, but the two operands are not affected. If DF is 1, then SI and DI will automatically be decremented by 1 for a byte string and by 2 for a word string. If DF is 0, then SI and DI will automatically be incremented by 1 for byte strings and by 2 for word strings. CMPSB CMPSW (DS:SI) - (ES:DI) (DS:SI) - (ES:DI) SI SI - 1 SI SI + 2 DI DI - 1 DI DI + 2 (DS:SI) - (ES:DI) SI SI + 1 DI DI + 1 (DS:SI) - (ES:DI) SI SI - 2 DI DI SCAS ( SCASB / SCASW ) SCAN A STRING BYTE OR A STRING WORD : This instruction compares a byte in AL or a word in AX with a byte or a word in Extra-segment, DI must contain the effective address. If DF is 0, then DI will be incremented by 1 for byte strings and by 2 for word strings. If DF is 0, then DI will be decremented by 1 for byte strings and by 2 for word strings. SCAS affects AF, CF, OF, PF, SF, and ZF, but it does not change either the operand in AL / AX or the operand in the string. SCASB SCASW (AL) - (ES:DI) (AX) - (ES:DI) DI DI - 1 DI DI + 2 (AL) - (ES:DI) DI DI + 1 The following table summarizes all the string instructions: (AX) - (ES:DI) DI DI

4 Ex: Write a piece of code that transfers a block of 256 bytes stored at locations starting at 34000H to locations starting at 36000H. MOV AX, 3000H MOV DS, AX MOV ES, AX MOV SI, 4000H MOV DI, 6000H H.W : Repeat by using MOVSW MOV CX, 100H NEXT: MOVSB LOOP NEXT HLT Ex: Write an ALP to initialize a block of memory starting at 8A000H with English alphabet (A to Z) using string instructions. MOV AX, 8000H MOV ES, AX MOV AL, 'A' MOV DI, A000H MOV CX, 18H AGAIN: STOSB INC AL LOOP AGAIN HLT Ex: Write an ALP that scans the byte 05H in a block of memory of size 16 starting at 2A000H, if is found loads BL with 01H otherwise load BL with 00H. MOV AX, 2000H MOV ES, AX MOV AL, 05 MOV DI, 0A000H MOV CL, 10H MOV BL, 00H AGAIN: SCASB LOOPNE AGAIN JNE END MOV BL, 01H END: HLT 44

6. REP & (REPE / REPZ) & (REPNE / REPNZ) REPEAT STRING INSTRUCTION UNTIL SPECIFIED CONDITIONS EXIST: REP (Repeat) is written before one of the string instructions.

5 6. REP & (REPE / REPZ) & (REPNE / REPNZ) REPEAT STRING INSTRUCTION UNTIL SPECIFIED CONDITIONS EXIST: REP (Repeat) is written before one of the string instructions. It will cause the CX register to be decremented and the string instruction to be repeated until CX = 0. The instruction REP MOVSB, for example, will continue to copy string bytes until the number of bytes into CX has been copied. REPE and REPZ (Repeat if Equal and Repeat if Zero) : They will cause the string instruction to be repeated as long as the compared bytes or words are equal (ZF = 1) and CX is not equal to zero. In other words, there are two conditions that will stop the repetition: CX = 0 or string bytes or words not equal. REPNE and REPNZ (Repeat if Not Equal and Repeat if Not Zero) respectively. They are often used with the Compare String instruction or with the Scan String instruction. They will cause the string instruction to be repeated as long as the compared bytes or words are not equal (ZF = 0) and CX is not yet counted down to zero. Ex: Write an ALP that loads a block of memory of size 256 starting at 7A000H with a value 55H by repeating the STOSB instruction. MOV AX, 7000H MOV ES, AX MOV DI, 0A000H H.W Repeat by using MOV instruction MOV AL, 55H MOV CX, 100H REP STOSB HLT 45

6 Ex : Two arrays of size 1Kbyte stored at 21000H and 52000H respectively. Write a piece of code to store 01H in AH if they are equal or 00H if they are not equal. MOV AX, 2000H MOV DS, AX MOV AX, 5000H MOV ES, AX MOV SI, 1000H MOV DI, 2000H H.W. : Repeat using CMPSW MOV AH, 00H MOV CX, 400H REPE CMPSB JNE END MOV AH, 01H END: HLT Ex: Describe what happen as the following sequence of instruction is executed. MOV AX, 5000H MOV DS, AX MOV AX, 5000H MOV ES, AX MOV CX, 20H MOV SI, 2000H MOV DI, 4000H REP MOVSB HLT This program is used to transfer 32 bytes form locations starting at 52000H to another locations starting at 54000H. 46

Subroutines (مهمت او وظيفت معينت) A subroutine is a special program that performs a certain task and is called for execution from any point in the main program.

CALL Instruction (call a procedure) The CALL instruction is used to transfer execution to a subprogram or a procedure. There two basic type of calls : 1.

7 Subroutines (مهمت او وظيفت معينت) A subroutine is a special program that performs a certain task and is called for execution from any point in the main program. A subroutine is also known as a procedure. A return instruction must be included at the end of the subroutine to return execution to the main program. CALL Instruction (call a procedure) The CALL instruction is used to transfer execution to a subprogram or a procedure. There two basic type of calls : 1. near call is a call to a procedure, which is in the same code segment as the CALL instruction. When the 8086 executes a near CALL instruction, it decrements the stack pointer by 2 and copies the offset of the next instruction after the CALL into the stack. 2. far call is a call to a procedure, which is in a different segment. When the 8086 executes a far call, it decrements the SP by 2 and copies the content of the CS register to the stack. It then decrements the SP by 2 again and copies the offset of the instruction after the CALL instruction to the stack. A RET instruction at the end of the procedure will return execution to the next instruction after the CALL instruction by restoring the saved values IP (in the case of near call) or CS and IP (in the case of far call) from the stack. 48

8 Ex: Write an ALP to calculate the combinations from the following formula, store the result in AX. MOV CL, n MOV CH, m CMP CL, CH JC STOP MOV BL, CL CALL FACT MOV SI, AX MOV BL, CH CALL FACT MOV DI, AX SUB CL, CH MOV BL, CL CALL FACT MUL DI MOV CX, AX MOV AX, SI XOR DX, DX DIV CX STOP: HLT ; n is positive integer less than or equal to 08H ; m is positive integer less than or equal to 08H ; Subroutine finds the factorial FACT: MOV AX, 0001H CMP BL, 00H JE END MOV BH, 00H AGAIN: MUL BX DEC BX JNZ AGAIN END: RET If n = 06H and m = 04H then after execution (AX) = 000FH If n = 08H and m = 04H then after execution (AX) = 0046H If n = 06H and m = 06H then after execution (AX) = 0001H 40

Experiment N o 8. String Handling Instructions

Experiment N o 8. String Handling Instructions Experiment N o 8 String Handling Instructions Introduction: In this experiment you will deal with string handling instructions, such as reading a string, moving a string from one memory location to another,