Logic Instructions. Basic Logic Instructions (AND, OR, XOR, TEST, NOT, NEG) Shift and Rotate instructions (SHL, SAL, SHR, SAR) Segment 4A

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1 Segment 4A Logic Instructions Basic Logic Instructions (AND, OR, XOR, TEST, NOT, NEG) Shift and Rotate instructions (SHL, SAL, SHR, SAR) Course Instructor Mohammed Abdul kader Lecturer, EEE, IIUC

2 Basic Logic Instructions AND OR Exclusive OR (XOR) NOT TEST NEG AND Instruction AND operation performs logical Multiplication. The AND operation clears bits of a binary number. The task of clearing a bit in a binary number is called masking. AND instruction uses any addressing mode except memory to memory and segment register addressing. Example: AND AL, BL AND AX, [DI] 2

3 OR Instruction Basic Logic Instructions (Continued) OR operation performs logical addition and is often called the Inclusive-OR function OR instruction is used to set bit of a unknown binary number. Example: OR AH, BL ; AH= AH or BL OR DX, [BX] ; DX is Ored with the word contents of data segment memory location addressed by BX 3

4 Exclusive-OR Instruction Basic Logic Instructions (Continued) The exclusive-or instruction produces 1 when input logics are different otherwise it produces 0. Because of this Exclusive-OR is sometimes called a comparator. The exclusive-or instruction is useful if some bits of a register or memory location must be inverted without changing the other bits. Example: XOR CH, DL ; CH= CH xor DL XOR DX, [SI] ; DX is exclusive-ored with the word content of the data segment memory location addressed by SI. 4

5 Problem: Basic Logic Instructions (Continued) Develop a short sequence of instructions that sets the rightmost five bits, invert leftmost three bits and clear middle four bits of DI without changing the remaining three bits of DI. Save the result in SI. Solution: invert clear set b15 b14 b13 b12 b11 b10 b9 b8 b7 b6 b5 b4 b3 b2 b2 B XOR with E000H to invert AND with FC3FH to clear OR with 001FH to set XOR DI, E000H ; invert leftmost three bits AND DI, FC3FH ; clear middle three bits OR DI, 001FH ; set rightmost five bits MOV SI, DI ; save result in SI 5

6 TEST Instruction Basic Logic Instructions (Continued) TEST instruction performs the AND operation. The difference is that the AND instruction changes the destination operand, whereas the TEST instruction does not. The TEST instruction functions in the manner as a CMP instruction. The difference is that the TEST instruction normally a single bit, whereas the CMP instruction tests the entire byte or, word. It only changes zero flag (Z) bit. The zero flag (Z) is a logic 1 (indicating a zero result) if the bit under test is a zero and Z=0 (indicating a nonzero result) if the bit under test is not zero. Usually the TEST instruction is followed by either the JZ (jump if result zero, Z=1) or, JNZ (jump if result not zero, Z=0) instruction. Example: TEST DL, DH ; DL is ANDed with DH. TEST AH, 4 ; AH is ANDed with 4. Problem: (a) Write an instruction set to test the rightmost and leftmost bit position of AL register. The program should jumps to the operand address RIGHT if right most bit of AL is set and jumps to the address LEFT if leftmost bit of AL is set. 6

7 NOT Basic Logic Instructions (Continued) NOT instruction performs logical inversion (invert all bits of a byte or word) or find the one s complement. Example: NOT CH ; CH is one s complemented NOT BYTE PTR[BX] ; The byte contents of data segment memory location addressed by BX are one s complemented. NEG NEG instruction performs arithmetic sign inversion or, the two s complement. The arithmetic sign of a signed number changes from positive to negative or from negative to positive by NEG instruction. Example: NEG CH ; CH is two s complemented. NEG AX; AX is two s complemented. 7

8 SHIFT and ROTATE instructions SHIFT and ROTATE instructions are used to shift or rotate any memory data or register. The instructions mostly use to control I/O devices. SHIFT Instruction Shift instructions position or move numbers to the left or right within a register or memory location. They also perform simple arithmetic such as multiplication by powers of 2 +n (left shift) and division by powers of 2 n right shift. The microprocessor instruction set contains four different shift instruction. Two logical shift : (a) Shift operand bits left (SHL) (b) Shift operand bits right (SHR) Two arithmetic shift: (c) Shift arithmetic Left (SAL) (d) Shift arithmetic Right (SAR) Logical shifts (SHL and SHR): The logical shifts move a zero into the rightmost bit position for a logical left shift and a 0 into the leftmost bit position for a logical right shift. MSB is shifted to CF in case of SHL and LSB shifted to CF in case of SHR. 8

9 SHIFT and ROTATE instructions (continued) Arithmetic shifts (SAL and SAR): The arithmetic left shift and logical left shift are identical. The arithmetic right shift and logical right shift are different because the arithmetic right shift copies the sign bit through the numbers, where as logical right shift copies a 0 through the numbers. Logical VS arithmetic shift: Logical shift operations function with unsigned numbers and arithmetic shifts function with signed numbers. Logical shifts multiply or divide unsigned data, and arithmetic shifts multiply or divide signed data. 9

10 SHIFT and ROTATE instructions (continued) Two different modes used in shift counting: One modes uses an immediate shift count and other uses register CL to hold the shift count. Note that CL must hold the shift count. Example: SHL AX, 1 ; AX is logically shifted left 1 place. SHR BX, 12 ; BX is logically shifted right 12 places (12 times) SAR SI, 2 ; SI is arithmetically shifted right 2 places. SAL DATA, CL ; The contents of data segment memory location DATA are arithmetically shifted left the number of spaces specified by CL. Division by SHIFT instructions: A shift right always divides by 2 for each bit position shifted. Two times right shifting divides by 4. For n times right shift the number is divided by 2 n. Multiplication by SHIFT instructions: A shift left always multiplies by 2 for each bit position shifted. Two times left shifting multiplies by 4. For n times left shift the number is multiplied by 2 n. 10

11 ROTATE instruction SHIFT and ROTATE instructions (continued) Rotate instructions position binary data by rotating the information in a register or memory location, either from one end to another or through the carry flag. There are four available rotate instructions: Rotate through carry left (RCL) Rotate out of carry left (ROL) Rotate through carry right (RCR) Rotate out of carry right (ROR) 11

12 ROTATE instruction (Continued) SHIFT and ROTATE instructions (continued) A rotate count can be immediate or located in register CL. Example: ROL SI, 14 ; SI rotates left (out of carry) 14 places. RCL BL, 6 ; BL rotates left through carry 6 places. RCR AH, CL ; AH rotates right through carry the number of places specified by CL. ROR WORD PTR[BP], 2 ; The word contents of the stack segment memory location addressed by BP rotate right (out of carry) 2 places. 12 Problem: (a) AX=0F07H and BX=6644H, find the value of AX and BX after the execution following instructions- MOV CL, 4; RCL AX, CL; ROR BX, CL (b) AX=0F07H and BX=6644H, find the value of AX and BX after the execution following instructions- SHL AX, 4 SAR BX, 3

Microprocessor & Computer Architecture / Lecture

Microprocessor & Computer Architecture / Lecture 4- Shift and Rotate Instructions Shift and rotate instructions manipulate binary numbers at the binary bit level, as did the AND, OR, Exclusive-OR, and NOT instructions. The microprocessor contains a complete