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Introduction to Computer Organization and Assembly Language

Introduction to Computer Organization and Assembly Language . Sheet 3. The FLAGS Register - Status Flags. Carry Flag (CF): CF = 1 if there is a carry out from the msb on addition. CF = 1 if there is a borrow into the msb on subtraction. CF = 0 otherwise.

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Introduction to Computer Organization and Assembly Language

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  1. Introduction to Computer Organization and Assembly Language Sheet 3 By: Nora Alaqeel

  2. The FLAGS Register - Status Flags • Carry Flag (CF): • CF = 1 if there is a carry out from the msb on addition. • CF = 1 if there is a borrow into the msb on subtraction. • CF = 0 otherwise. • CF is also affected by shift and rotate instructions. • Parity Flag (PF): • PF = 1 if the low byte of a result has an even number of one bits • PF = 0 if the low byte has odd parity • (ex. if a result is FFFE PF = 0). By: Nora Alaqeel

  3. The FLAGS Register - Status Flags • Zero Flag (ZF): • ZF = 1 for a zero result. • ZF = 0 for nonzero result. • Sign Flag (SF): • SF = 1 if the msb of a result is 1. • (i.e. Negative if signed interpretation is used). • SF = 0 if the msb is 0. By: Nora Alaqeel

  4. The FLAGS Register - Status Flags • Overflow Flag (OF): • OF = 1 if signed overflow occurred. • OF = 0 otherwise. By: Nora Alaqeel

  5. How Instructions Affect the Flags • In general, each time the processor executes an instruction, the flags • are altered to reflect the result. • However, some instructions don’t affect any of the flags, affects • only some of them, or may leave them undefined. Instruction Affects flags MOV/XCHG none ADD/SUB all INC/DEC all except CF By: Nora Alaqeel

  6. Question1 Q1:For each of the following instructions, give the new destination contents and the new settings of SF,ZF,PF, CF and OF. Suppose that the flags are initially 0 in each part of this question: By: Nora Alaqeel

  7. Question1 • a. ADD AX, BX ;AX= 4034h, BX=8051h 4034 0100 0000 0011 0100 8051 1000 0000 0101 0001 C085 1100 0000 1000 0101 SF=1 , ZF=0 , PF=0 , CF=0 , OF=0 By: Nora Alaqeel

  8. Question1 • b. NEG AX ;AX= 6431h FFFF 6431 9BCE 1 9BCF 1001 1011 1100 1111 SF=1 , ZF=0 , PF=1 , CF=1 , OF=0 By: Nora Alaqeel

  9. Question1 • c. SUB AL, BL ;AL= 34h, BL=51h • 0011 0100 • 51 0101 0001 E31110 0011 SF=1 , ZF=0 , PF=0 , CF=1 , OF=0 By: Nora Alaqeel

  10. Question1 • d. DEC BL ;BL=01h 01 0000 0001 1 0000 0001 00 0000 0000 SF=0 , ZF=1 , PF=1 , CF=not effected , OF=0 By: Nora Alaqeel

  11. Question2 Q2: Write a program to do the following: The program should prompts the user to enter an uppercase letter, then display that letter in lowercase at the same line. Sample Execution: ENTER UPPER CASE LETTER: Aa By: Nora Alaqeel

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