lecture 4 ( assembly language) 1. lecture outline introduction the flags register overflow how...

Lecture 4( Assembly Language)

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Lecture Outline

• Introduction• The FLAGS Register• Overflow• How instruction affect the flags

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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.

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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).

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The FLAGS Register

• 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)

OF DF IF TF SF ZF AF PF

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1CF

0

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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).

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• 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.

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• 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.

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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.

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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.

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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 noneADD/SUB allINC/DEC all except CFNEG all (CF = 1 unless result is 0,

OF = 1 if word operand is 8000h or byte operand is 80h)

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Example: ADD AX, BX where AX contains FFFFh, BX contains FFFFh

FFFFh +FFFFh 1FFFEh = 1111 1111 1111 1110b

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


Example: ADD AL, BL where AL contains 80h, BL contains 80h

80h +80h 100h

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


Example: SUB AX, BX where AX contains 8000h, BX contains 0001h

8000h - 0001h 7FFFh = 0111 1111 1111 1111b

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


Example: MOV AX, -5

None of the flags are affected by MOV.

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lecture 4 ( assembly language) 1. lecture outline introduction the flags register overflow how...

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