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Questions and Answers
What is the primary purpose of string manipulation instructions in a program?
What is the primary purpose of string manipulation instructions in a program?
- To only work with signed numbers
- To increase the size of a program
- To only work with alphanumeric characters
- To facilitate operations such as storing, moving, and comparing strings (correct)
Which register stores the number of iterations for string instructions?
Which register stores the number of iterations for string instructions?
- EDX
- ESI
- EDI
- ECX (correct)
What is the function of the MOVSB instruction?
What is the function of the MOVSB instruction?
- Compare two strings
- Store a character in a string
- Copy a byte from ESI to EDI (correct)
- Search for a particular byte or word
In which direction can string manipulation instructions operate on strings?
In which direction can string manipulation instructions operate on strings?
What is the result of the MOVSB instruction on the flags?
What is the result of the MOVSB instruction on the flags?
What is the purpose of combining string instructions with a REP instruction?
What is the purpose of combining string instructions with a REP instruction?
What type of information can be stored in a string?
What type of information can be stored in a string?
What is the effect of the direction flag (DF) on the MOVSB instruction?
What is the effect of the direction flag (DF) on the MOVSB instruction?
What does the SCASB instruction do?
What does the SCASB instruction do?
What is the purpose of the 'cld' instruction in this code snippet?
What is the purpose of the 'cld' instruction in this code snippet?
What happens to the SI register when the Direction Flag (DF) is 0?
What happens to the SI register when the Direction Flag (DF) is 0?
What does the REP prefix do?
What does the REP prefix do?
What is the purpose of the 'repe cmpsb' instruction?
What is the purpose of the 'repe cmpsb' instruction?
What does the MOVSB instruction do?
What does the MOVSB instruction do?
What is the purpose of the 'mov cx, 100' instruction?
What is the purpose of the 'mov cx, 100' instruction?
What happens to the DI register when the Direction Flag (DF) is 1?
What happens to the DI register when the Direction Flag (DF) is 1?
What is the purpose of the 'mov di, offset string' instruction?
What is the purpose of the 'mov di, offset string' instruction?
What is the purpose of the 'mov al, 0dh' instruction?
What is the purpose of the 'mov al, 0dh' instruction?
What does the CMPSB instruction do?
What does the CMPSB instruction do?
What does the LODSB instruction do?
What does the LODSB instruction do?
What is the Virtual Mode Flag used for in a processor?
What is the Virtual Mode Flag used for in a processor?
What does the STOSB instruction do?
What does the STOSB instruction do?
What is the purpose of the Nested Task Flag?
What is the purpose of the Nested Task Flag?
What is the purpose of the Virtual Interrupt Flag / Virtual Interrupt Pending Flag?
What is the purpose of the Virtual Interrupt Flag / Virtual Interrupt Pending Flag?
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Study Notes
Processor Control Instructions
- String manipulation instructions facilitate operations such as storing characters, moving data, comparing alphanumeric strings, and searching for particular bytes or words.
- In operating strings longer than a doubleword, string instructions can be combined with a REP instruction to create a repeating instruction.
String Instructions
- MOVSB: moves the string byte addressed by the ESI register to the location addressed by the EDI register.
- CMPSB: subtracts the destination string byte from the ESI and updates the status of the control flags according to results.
- SCASB: subtracts the destination string byte from the AL register and updates the status of the control flags according to results.
- LODSB: loads the source string byte identified by the ESI register into the AL register.
- STOSB: stores the source string byte from the AL register into the memory location identified with the EDI register.
- REP: repeats a given instruction while the ECX register is not zero.
REP Instruction
- In 64-bit mode, the ECX register is extended and referred to as the RFLAGS.
- The number of iterations is in the ECX register, corresponding to the number of string elements to be operated on.
Flags and Registers
- ECX register: stores the number of iterations for REP instruction.
- ESI register: addresses the source string.
- EDI register: addresses the destination string.
- RFLAGS: extended ECX register in 64-bit mode.
- DF (Direction Flag): indicates the direction of string operations (increment or decrement).
- C, Z, S, O, P, and A flags: updated according to the results of string instructions.
Example Usage
- MOV SI, offset first: points SI at the source string.
- MOV DI, offset second: points DI at the destination string.
- CLD: clears the DF, so SI and DI will auto-increment.
- MOV CX, 100: allocates the number of string elements in CX.
- REP cmpsb: repeats the comparison of string bytes until the end of the string or until compared bytes are not equal.
- MOV DI, offset string: puts the offset of the string into DI.
- MOV AL, 0dh: loads the byte to be scanned for into AL.
- MOV CX, 80: uses CX as an element counter.
Operating Mode Flags
- Nested Task Flag: indicates if the current task is nested within another task in protected mode.
- Virtual Mode Flag: enables or disables a virtual 8086 mode, determining whether the processor runs as an 8086 machine.
- Virtual Interrupt Flag / Virtual Interrupt Pending Flag: used in a multitasking environment.
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