Shift & Rotate Instructions) Lecture 6 Program Logic and Control (Boolean Operation Shift & Rotate Instructions)

Lecture Outline Introduction Logic Instructions AND, OR, XOR Instructions NOT Instruction TEST Instruction Shift Instructions The SHL Instruction The SHR Instruction Rotate Instructions The ROL Instruction The ROR Instruction Logic Instructions 1

Introduction Logic, shift, and rotate instructions can be used to change the bit pattern in a byte or word. The ability to manipulate bits is generally absent in high-level languages (except C) and is an important reason for programming in assembly language. It is used to check bit in a register or memory location, clear or set a register contents Logic Instructions 2

Logic Instructions Logic instructions can be used to clear, set , and examine individual bits in a register or variable. Logic instructions: AND. OR. XOR. NOT. TEST. Logic Instructions 3

AND, OR, XOR Instructions a b a AND b a OR b a XOR b 0 0 0 0 0 0 1 0 1 1 1 0 0 1 1 1 1 1 1 0 10101010 10101010 10101010 AND 11110000 OR 11110000 XOR 11110000 = 10100000 = 11111010 = 01011010 Logic Instructions 4

AND, OR, XOR Instructions Syntax AND destination, source OR destination, source XOR destination, source The result of the operation is stored in the destination. Memory-to-memory operations are not allowed. Effect on flags: SF, ZF, PF reflect the result. AF is undefined. CF = OF = 0. register or memory location constant, register, or memory location Logic Instructions 5

AND, OR, XOR Instructions One use of AND, OR, and XOR is to selectively modify the bits in the destination. To do this, we construct a source bit pattern known as a mask. The mask bits are chosen so that the corresponding destination bits are modified in the desired manner when the instruction is executed. To choose the mask bits, we make use of the following properties: b AND 1 = b • b OR 0 = b • b XOR 0 = b b AND 0 = 0 • b OR 1 =1 • b XOR 1 = ~b where b represent a bit (0 or 1). Logic Instructions 6

AND, OR, XOR Instructions From these, we may conclude that: The AND instruction can be used to clear specific destination bits while preserving the others. (0 mask bit clears - 1 mask bit preserves). The OR instruction can be used to set specific destination bits while preserving the others. (1 mask bit sets - 0 mask bit preserves). The XOR instruction can be used to complement specific destination bits while preserving the others. (1 mask bit complements - 0 mask bit preserves). Logic Instructions 7

AND, OR, XOR Instructions Clear the sign bit of AL while leaving the other bits unchanged? AND AL, 7Fh Set the msb & lsb of AL while preserving the other bits? OR AL, 81h Change the sign bit of DX? XOR DX, 8000h Logic Instructions 8

AND, OR, XOR Instructions Convert an ASCII digit to a number? Character code 0 00110000 1 00110001 : : 9 00111001 AND AL, 0Fh Logic Instructions 9

AND, OR, XOR Instructions Convert the lowercase character in DL to uppercase? Character code character code a 01100001 A 01000001 b 01100010 B 01000010 : : : : z 01111010 C 01011010 AND DL, 0DFh Logic Instructions 10

AND, OR, XOR Instructions Clear AX? MOV AX, 0 or SUB AX, AX XOR AX,AX Three bytes machine code Two bytes machine code (efficient) Two bytes machine code (efficient) Clear A? MOV A, 0 Logic Instructions 11

AND, OR, XOR Instructions Test CX for zero? OR CX, CX CX is unchanged but if CX = 0 then ZF = 1. Logic Instructions 12

NOT Instruction a NOT a 0 1 1 0 NOT 10101010 = 01010101 0 1 1 0 NOT 10101010 = 01010101 Logic Instructions 13

NOT Instruction The NOT instruction performs the one’s complement operation on the destination. Syntax: NOT destination There is no effect on the status flags. Complement the bits in AX? NOT AX Logic Instructions 14

TEST Instruction The TEST instruction performs an AND operation of the destination with the source but does not change the destination contents. The purpose of the TEST instruction is to set the status flags. Syntax: TEST destination, source Effect on flags: SF, ZF, PF reflect the result. AF is undefined. CF = OF = 0. Logic Instructions 15

TEST Instruction The TEST instruction can be used to examine individual bits in an operand. The mask should contain: 1’s in the bit positions to be tested. 0’s elsewhere. Jump to BELOW if AL contains an even number? TEST AL,1 JZ BELOW Logic Instructions 16

Introduction to shift and rotate instructions The shift and rotate instructions shift the bits in the destination operand by one or more positions either to the left or right. For a shift instruction, the bits shifted out are lost. These instructions can be used to multiply and divide by powers of 2.(shift left to multiply and shift right to divide by 2) For a rotate instruction, bits shifted out from one end of the operand are put back into the other end. Shift & Rotate Instructions 17

SHIFT Instructions Shift instructions: SHL. SHR. Syntax: Opcode destination, 1 Opcode destination, CL ; where CL contains N Effect on flags: SF, PF, ZF reflect the result. AF is undefined. CF = last bit shifed out. OF = 1 if the result changes sign on last shift. register or memory location Shift & Rotate Instructions 18

The SHL Instruction The SHL (shift left) instruction shifts the bits in the destination to the left. SHL destination, 1 A 0 is shifted into the rightmost bit possition and the msb is shifted into CF. SHL destination, CL ; where CL contains N N single left shifts are made. The value of CL remains the same. CF 7 6 5 4 3 2 1 0 CF 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Shift & Rotate Instructions 19

The SHL Instruction Suppose DH contains 8Ah and CL contains 3. What are the values of DH and of CF after the instruction SHL DH, CL is executed? DH before SHL DH, CL CF & DH After 1 0 0 0 1 0 1 0 000 1 0 0 0 1 0 1 0 CF 7 6 5 4 3 2 1 0 CF 7 6 5 4 3 2 1 0 0 1 0 1 0 0 0 0 SAL (shift Arithmetic Left) instruction can be used instead of SHL. Both have same machine codes. Shift & Rotate Instructions 20

The SHL Instruction - Multiplication by SHL A left shift on a binary number multiplies it by 2. Ex. if AL contains 5d = 00000101b A left shift gives 10d = 00001010b Another left shift gives 20d = 00010100 When we treat left shifts as multiplication, overflow may occur. For 1 left shift, CF & OF accurately indicate unsigned and signed overflow, respectively. Overflow flags are not reliable indicators for a multiple left shift. Ex. SHL BL, CL ; where BL = 80h and CL = 2 CF = OF = 0 even though both signed & unsigned overflow occurred. Shift & Rotate Instructions 21

The SHR Instruction The SHR (shift right) instruction shifts the bits in the destination to the right SHR destination, 1 A 0 is shifted into the msb possition and the rightmost bit is shifted into CF. SHR destination, CL ; where CL contains N N single right shifts are made. The value of CL remains the same. 7 6 5 4 3 2 1 0 CF 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 CF Shift & Rotate Instructions 22

The SHR Instruction Suppose DH contains 8Ah and CL contains 2. What are the values of DH and of CF after the instruction SHR DH, CL is executed? DH before SHR DH, CL CF & DH After 1 0 0 0 1 0 1 0 00 1 0 0 0 1 0 1 0 7 6 5 4 3 2 1 0 CF 7 6 5 4 3 2 1 0 CF 0 0 1 0 0 0 1 0 1 SAR (shift Arithmetic Right) instruction can also used instead of SHR Both have same machine codes Shift & Rotate Instructions 23

The SHR Instruction - Division by SHR For even numbers, a right shift divides it by 2. Ex. if AL contains 6d = 00000110b A right shift gives 3d = 00000011b For odd numbers, a right shift halves it and rounds down to the nearest integer. Ex. if AL contains 5d = 00000101b A right shift gives 2d = 00000010b Use SHR to divide 65143 by 4, put the quotient in AX. MOV AX, 65143 ; AX has number MOV CL, 2 ; CL has number of right shifts SHR AX, CL ; divide by 4 Shift & Rotate Instructions 24

Rotate Instructions Rotate instructions: ROL. ROR. Syntax: Opcode destination, 1 Opcode destination, CL ; where CL contains N Effect on flags: SF, PF, ZF reflect the result. AF is undefined. CF = last bit shifted out. OF = 1 if the result changes sign on last shift. register or memory location Shift & Rotate Instructions 25

The ROL Instruction The ROL (rotate left) instruction shifts the bits in the destination to the left. The msb is shifted into the rightmost bit and also into the CF. CF 7 6 5 4 3 2 1 0 CF 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Shift & Rotate Instructions 26

The ROL Instruction Use ROL to count the number of 1 bits in BX, without changing BX. Put the answer in AX. XOR AX, AX ; AX counts bits MOV CX, 16 ; loop counter TOP: ROL BX, 1 ; CF = bit rotated out JNC NEXT ; 0 bit INC AX ; 1bit, increment total NEXT: LOOP TOP ; loop until done Shift & Rotate Instructions 27

The ROR Instruction The ROR (rotate right) instruction shifts the bits in the destination to the right. The rightmost is shifted into the msb bit and also into the CF. 7 6 5 4 3 2 1 0 CF 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 CF Shift & Rotate Instructions 28

APPLICATION shift and rotate instructions is used for binary and hex I/O. For binary input, we assume a program reads in a binary number from the keyboard, followed by a carriage return. Hex input consists of digits 0 to 9 and letters A to F followed by a carriage return. Shift & Rotate Instructions 29

APPLICATION ( Binary I/O) Algorithm for Binary Input Clear BX /* BX will hold binary value*/ Input a character /* ‘0’ or ‘1’ */ While character <> CR DO Convert character to binary value Left shift BX Insert value into 1sb of BX Input a character END_WHILE Shift & Rotate Instructions 30

APPLICATION Binary Input in Assembly code XOR BX,BX ; CLEAR BX MOV AH,1 INT 21H ; Input character WHILE_: CMP AL, 0DH ; CR? JE END_WHILE ; YES, DONE AND AL, 0FH ; CONVERT TO BINARY VALUE SHL BX,1 ; MAKE ROOM FOR NEW VALUE OR BL,AL ; PUT VALUE IN BX INT 21H ; READ A CHARACTER JMP WHILE_ ; LOOP BACK END_WHILE: Shift & Rotate Instructions 31

APPLICATION ( Binary I/O) Algorithm for Binary Output For 16 times do Rotate Left BX ;BX holds output value put msb in to CF If CF= 1 THEN output ‘1’ ELSE Output ‘0’ END IF END FOR Shift & Rotate Instructions 32

APPLICATION Binary Output in Assembly code MOV CX,16 MOV AH,2 TOP_: ROL BX,1 ;content in BX JC display_one ; print bit ‘1’ MOV DL,’0’ ; else Print bit ‘0’ JMP display display_one : MOV DL,’1’ Display: INT 21H ; Print bit either ‘0’ or ‘1’ LOOP TOP_ ; LOOP BACK Shift & Rotate Instructions 33