Introduction to Computer Organization and Assembly Language Sheet

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

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

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

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

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 MOV/XCHG ADD/SUB INC/DEC By: Nora Alaqeel Affects flags none all except CF

Question 1 Q 1: 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

Question 1 �a. ADD AX, BX ; AX= 4034 h, BX=8051 h 4034 8051 C 085 0100 0011 0100 1000 0101 0001 1100 0000 1000 0101 SF=1 , ZF=0 , PF=0 , CF=0 , OF=0 By: Nora Alaqeel

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

Question 1 �c. SUB AL, BL ; AL= 34 h, BL=51 h 34 51 0011 0100 0101 0001 E 3 1110 0011 SF=1 , ZF=0 , PF=0 , CF=1 , OF=0 By: Nora Alaqeel

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

Question 2 �Q 2: 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

Question 2 By: Nora Alaqeel