# Ch. 7 Logic, Shift and Rotate instr.

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Ch. 7 Logic, Shift and Rotate instr.
Logic Instructions AND, OR, XOR and NOT TEST Instruction Shift Instructions Left shift – doubles a number Right shift – halves a number … multiply or divide by powers of 2 MUL, DIV – much slower than shift instr… Rotate Stack Operations Introduction to Procedures

7.1 AND, OR and XOR Instructions
AND destination, source OR destination, source XOR destination, source Dest. – must be a Register or mem. Source – const, reg., mem Memory-to-memory operations are not allowed Effect on flags SF, ZF, PF reflect the result AF is undefined CF, OF = 0 SF – sign ZF – zero PF – parity AF – auxiliary carry OF – overflow CF – carry

Use of Logic Instructions
Selectively modify the bits of destination b AND 1 = b (b represents a bit, 0/1) b AND 0 = 0 b OR 0 = b b OR 1 = 1 b XOR 0 = b b XOR 1 = ~b (complement of b) So, AND can be used to clear specific destination bit OR can be used to set specific destination bit XOR can be used to complement specific destination bit

Examples mask Example 7.2: Clear the sign bit of AL while leaving the other bits unchanged. AND AL, 7Fh Example 7.3: Set the msb and lsb of AL while preserving the other bits. OR AL, 81h Example 7.4: Change the sign bit of DX XOR DX, 8000h = 7Fh = 81h See example from book

How to clear a register? MOV AX, 0 ; machine code 3 bytes
SUB AX, AX ; .. 2 bytes XOR AX, AX ; .. 2 bytes -- BUT mem2mem operations only for MOV is allowed here – so to clear a memory location, use MOV.

7.1.2 NOT Instruction 1’s Complement operation NOT destination
No effect on status flags Example 7.5: Complement the bits in AX NOT AX

7 Logic Instructions TEST Instruction Shift and Rotate Instructions
AND, OR, XOR and NOT TEST Instruction Shift and Rotate Instructions Stack Operations Introduction to Procedures

7.2 TEST Instruction TEST performs AND of the destination with source – but no change of the dest. contents TEST destination, source Effects on flags SF, ZF, PF reflect the result AF is undefined CF, OF = 0 TEST vs. CMP CMP is subtraction operation

JZ – jump if equal to zero
TEST Example Jump to label BELOW if AL contains an even number TESET AL, 1 ; is AL even? JZ BELOW ; yes, go to BELOW JZ – jump if equal to zero use to examine individual bits in an operand. mask contains 1’s in the bits positions to be tested & 0’s elsewhere Even numbers have a 0 in bit#0. So, mask = b = 1

Agenda Logic Instructions TEST Instruction
AND, OR, XOR and NOT TEST Instruction Shift and Rotate Instructions Stack Operations Introduction to Procedures

7.2 Shift and Rotate Instructions
Two types of shift and rotate instructions Logical Shift / Rotate Arithmetic Shift/Rotate Both logical and arithmetic left shift are identical But right shifts are different

opcode dest, 1 ; 1 bit change opcode dest, CL ; N bits change
; CL contians N Dest  reg., mem SHL  Multiplication by left shift by multiplies of 2 SAL  for numeric multiplication

Shift and Rotate Instructions
SHL DH, 3 ; DH = DH = C = 1 SHR DH, 3 ; DH = DH = C = 1 SAL DH, 2 ; DH = DH = C = 1 SAR DH, 2 ; DH = DH = C = 1

Rotate Instructions ROL – rotate left ROR – r right RCL – r carry left
RCR – r c right Let DH = 8Ah = CF = 1 After first RCR DH = CF = 0 After second RCR DH = CF = 1

Agenda Logic Instructions TEST Instruction
AND, OR, XOR and NOT TEST Instruction Shift and Rotate Instructions Stack Operations

Stack vs. Queue Stack Queue LIFO : Last In First Out
FIFO : First In First Out Queue Stack

PUSH vs. POP in Stack

PUSH Instructions

POP Instructions

References Some materials are from Dr. Sazzad, NSU
Ch 7, Assembly Language Programming – by Charls Marut Ch 4, Intel Microprocessors – by Brey