Assembly Lang. – Intel 8086 Ch . 7: Logic , Shift and Rotate instr.

1. Assembly Lang. – Intel 8086Ch. 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

2. 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

3. 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

4. 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 0111 1111 = 7Fh 1000 0001 = 81h See example from book

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

6. 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. 7 • Logic Instructions • AND, OR, XOR and NOT • TEST Instruction • Shift and Rotate Instructions • Stack Operations • Introduction to Procedures

8. 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

9. 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 = 0000 0001b = 1

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

11. 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

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

13. Shift and Rotate Instructions SHL DH, 3 ; DH = 1110 1111 SHR DH, 3 ; DH = 1110 1111 DH = 0111 1000 DH = 1011 1100 DH = 0001 1101 DH = 1111 1011 C = 1 C = 1 C = 1 C = 1 SAL DH, 2 ; DH = 1110 1111 SAR DH, 2 ; DH = 1110 1111

14. 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

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

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

17. PUSH vs. POP in Stack

18. PUSH Instructions

19. POP Instructions

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