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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 OR XOR • • • • destination, source destination, source destination, source SF – sign ZF – zero PF – parity AF – auxiliary carry OF – overflow CF – carry 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 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 0111 1111 = 7Fh Example 7.3: Set the msb and lsb of AL while preserving the other bits. OR AL, 81h 1000 0001 = 81h Example 7.4: Change the sign bit of DX XOR DX, 8000h See example from book How to clear a register? 1. MOV AX, 0 ; machine code 3 bytes 2. SUB AX, AX ; .. 2 bytes 3. 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 – 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 TEST Example • Jump to label BELOW if AL contains an even number TESET JZ AL, 1 BELOW ; is AL even? ; 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 = 0000 0001b = 1 Agenda • Logic Instructions – 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 = 1110 1111 DH = 0111 1000 SAL DH, 2 C=1 ; DH = 1110 1111 DH = 1011 1100 SHR C=1 DH, 3 ; DH = 1110 1111 DH = 0001 1101 SAR DH, 2 C=1 ; DH = 1110 1111 DH = 1111 1011 C=1 Rotate Instructions ROL – rotate left ROR – r right RCL – r carry left RCR – r c right Let DH = 8Ah = 1000 1010 CF = 1 After first RCR DH = 1100 0101 CF = 0 After second RCR DH = 0110 0010 CF = 1 Agenda • Logic Instructions – AND, OR, XOR and NOT • TEST Instruction • Shift and Rotate Instructions • Stack Operations Stack vs. Queue • Stack – LIFO : Last In First Out • Queue – 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