Loops and Code Reviews-Assembly Language Programming-Lab Slides, Slides of Assembly Language Programming

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• Lab

Summary

 CMP

 JCXZ

 LOOP

 CodeView

Comparison Instruction

 Comparison Instruction (CMP) is an

instruction that only changes the flag

bits

 Useful for checking the entire contents

of a register or a memory location

against another value

 Followed by a conditional jmp

instruction

CMP continued

 Conditional jumps that follow the comparison

are JA, JB …

 For example:

CMP AL, 10 H

JAE SUBER

Conditional jump instructions test the following

flag bits sign(S), zero(Z), carry(C), parity(P),

overflow(O)

JLE Z=1 or S<>0 Jump is less than or equal

JNC C=0 Jump if no carry

JNE or JNZ Z=0 Jump if not equal, jump if not zero

JNO O=0 Jump if no overflow

JNS S=0 Jump if no sign

JNP or JPO P=0 Jump if no parity, jump if parity odd

JO O=1 Jump if overflow set

JP or JPE P=1 Jump if parity set, jump if parity even

JS S=1 Jump is sign is set

JECXZ ECX = 0 Jump if ECX is zero

JCXZ CX=0 Jump if CX is zero

Singed and unsigned

comparisons

 When signed numbers are compared

use the JG, JL, JGE, JLE, JE, JNE

 The terms greater than or less than refer

to singed numbers

 When unsigned numbers are compared

use JA, JB, JAE, JBE, JE, JNE

 The terms below and above refer to

unsigned numbers

LOOP instruction

 Loop is a combination of decrement CX

and JNZ conditional jump

 Loop decrements CX if CX <> 0 it

jumps to the address indicated

otherwise continues with next

sequential instruction

Multiplication instructions

 80×86 architecture has two multiplication instruction mnemonics  imul instruction treats its operands as signed numbers; the sign of the product is determined by the usual rules for multiplying signed numbers  mul instruction treats its operands as unsigned binary numbers; the product is also unsigned  If only non-negative numbers are to be multiplied, mul should usually be chosen instead of imul since it is a little faster

Mul continued…

 source has byte length, then it is multiplied by the

byte in AL; the product is 16 bits long, with a destination of the AX register

 If source has word length, then it is multiplied by the

word in AX; the product is 32 bits long, with its low order 16 bits going to the AX register and its high order 16 bits going to the DX register

 If source is a doubleword, then it is multiplied by the

doubleword in EAX; the product is 64 bits long, with its low order 32 bits in the EAX register and its high order 32 bits in the EDX register

Mul continued…

 For byte multiplication, the original value in

AX is replaced

 For word multiplication, the original values in

AX and DX are both wiped out. Similarly, for

doubleword multiplication the values in EAX

and EDX are replaced by the product.

 In each case the source operand is

unchanged unless it is half of the destination

location.

MUL continued

 AX: 00:

 BX: 00:

 DX: ??:??

 Mul bx

 DX:00:

 AX: 00:0A

 AX:00:

 Byte at factor FF

 Mul factor

 AX: 04:FB

 CF, OF 1

Imul instructions

 imul register, source

 source operand can be in a register, in

memory, or immediate

 other factor is in the register, which also

serves as the destination

 Operands must be words or doublewords, not

bytes

 product must "fit" in same size as the factors;

if it does, CF and OF are cleared to 0, if not

they are set to 1

Imul continued…

 third imul format is

 imul register, source, immediate

 the first operand, a register, is only the

destination for the product

 the two factors are the contents of the register

or memory location given by source and the

immediate value

 Operands register and source are the same

size, both 16-bit or both 32-bit

 If the product will fit in the destination register,

then CF and OF are cleared to 0; if not, they

are set to 1 docsity.com