Answered: 12) When the Direction flag is clear…

Computer Networking: A Top-Down Approach (7th Edition)

7th Edition

ISBN:9780133594140

Author:James Kurose, Keith Ross

Publisher:James Kurose, Keith Ross

Chapter1: Computer Networks And The Internet

Section: Chapter Questions

Problem R1RQ: What is the difference between a host and an end system? List several different types of end...

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Transcribed Image Text:### Understanding the Direction Flag and SCASB Instruction in Assembly Language **Question:** Where does EDI point when the Direction flag is clear and SCASB has found a matching character? **Explanation:** When writing assembly code, it is crucial to understand the behavior of various flags and instructions for precise control of the program flow and data manipulation. #### Key Concepts: 1. **Direction Flag (DF):** - The Direction flag in the FLAGS register determines the direction in which string operations process data. - If the Direction flag is **clear** (DF = 0), string operations proceed from lower memory addresses to higher memory addresses. - If the Direction flag is **set** (DF = 1), string operations proceed from higher memory addresses to lower memory addresses. 2. **SCASB Instruction:** - The SCASB (Scan String Byte) instruction compares the value in the AL register with each byte in the string addressed by the EDI register. - After each comparison, SCASB increments or decrements EDI based on the state of the Direction flag. #### Scenario: - **Direction flag is clear:** This means the DF is 0. - **SCASB finds a matching character:** EDI points to the address of the matching character. **Answer:** When the Direction flag is clear, and SCASB has found a matching character, EDI points to the byte after the matching character, because SCASB increments EDI after each comparison when the Direction flag is clear.

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