Lecture 4 Assembly Language 1 Lecture

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Lecture 4 ( Assembly Language) 1

Lecture Outline • Introduction • The FLAGS Register • Overflow • How instruction affect the flags 2

Introduction • One important feature that distinguishes a computer from other machines is the computer’s ability to make decisions. • Decision are made based on the current state of the processor. • In the 8086 processor, the processor state is implemented as nine individual bits called flags. • Each decision made by the 8086 is based on the values of these flags. • The flags are placed in the FLAGS register. 3

The FLAGS Register • Two kinds of flags: • Status flags: reflect the result of an instruction executed by the processor (ex. ZF , PF , SF , OF) • Control Flag: enable or disable certain operations of the processor (ex. If the IF “Interrupt Flag” is set to 0, inputs from the keyboard are ignored by the processor). 4

The FLAGS Register 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 OF DF IF TF SF ZF AF PF CF • Status Flags • Carry Flag (CF) • Parity Flag (PF) • Auxiliary carry Flag (AF) • Zero Flag (ZF) • Sign Flag (SF) • Overflow Flag (OF) • Control Flags • Trap Flag (TF) • Interrupt Flag (IF) • Direction Flag (DF) 5

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). 6

The FLAGS Register - Status Flags • Auxiliary carry Flag (AF): • AF = 1 if there is a carry out from bit 3 on addition. • AF = 1 if there is a borrow into bit 3 on subtraction. • AF = 0 otherwise. • AF is used in BCD operations. • Zero Flag (ZF): • ZF = 1 for a zero result. • ZF = 0 for nonzero result. 7

The FLAGS Register - Status Flags • 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. • Overflow Flag (OF): • OF = 1 if signed overflow occurred. • OF = 0 otherwise. 8

Overflow • The phenomenon of overflow is associated with the fact that the range of numbers that can be represented in a computer is limited. • If the result of an operation falls outside the range, overflow occurs and the truncated result that is saved will be incorrect. 9

How the Processor Determines that Overflow Occurred? Signed Overflow • On addition of numbers with the same sign, signed overflow occurs when the sum has a different sign. • Subtraction of numbers with different signs is like adding numbers of the same sign (ex. A - (-B) = A + B). Signed overflow occurs if the result has a different sign than expected. • In addition of numbers with different signs, overflow is impossible. • Subtraction of numbers with the same sign cannot give overflow. 10

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 NEG Affects flags none all except CF all (CF = 1 unless result is 0, OF = 1 if word operand is 8000 h or byte operand is 80 h) 11

How Instructions Affect the Flags Example: ADD AX, BX where AX contains FFFFh, BX contains FFFFh +FFFFh 1 FFFEh = 1111 1110 b SF = 1 because the msb is 1. PF = 0 because there are 7 (odd number) of 1 bits in the low byte. ZF = 0 because the result is nonzero. CF = 1 because there is a carry out of the msb on addition. OF = 0 because the sign of the stored result is the same as that of the numbers being added. 12

How Instructions Affect the Flags Example: ADD AL, BL where AL contains 80 h, BL contains 80 h +80 h 100 h SF = 0 because the msb is 0. PF = 1 because all the bits in the result are zero. ZF = 1 because the result is 0. CF = 1 because there is a carry out of the msb on addition. OF = 1 because the numbers being added are both negative, but the result is 0. 13

How Instructions Affect the Flags Example: SUB AX, BX where AX contains 8000 h, BX contains 0001 h 8000 h - 0001 h 7 FFFh = 0111 1111 b SF = 0 because the msb is 0. PF = 1 because there are 8 (even number) one bits in the low byte. ZF = 0 because the result is nonzero. CF = 0 because a smaller unsigned number is being subtracted from a larger one. OF = 1 because a positive number is being subtracted from a negative one (like adding two negatives), and the result is positive. 14

How Instructions Affect the Flags Example: MOV AX, -5 None of the flags are affected by MOV. 15

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