Files 3 Handouts Lecture 8

8/13/2019 Files 3 Handouts Lecture 8

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Lecture on Logical & Shift InstructionsLogical instructions: The 88/86 processors has instructions to perform bit by bit logic

operation on the specified source and destination operands.

Mnemonic Meaning Format Operation Flags Affected

AND

OR

XOR

NOT

Logical AND

Logical Inclusive OR

Logical Exclusive OR

LOGICAL NOT

AND D,S

OR D,S

XOR D,S

NOT D

(S) (D) (D)

(S)+(D) (D)

(S) (D) (D) _

(D)

(D)

OF, SF, ZF, PF, CFAF undefined



None

+

Destination Source

Register

Register MemoryRegister Memory

Accumulator

Register

MemoryRegister

ImmediateImmediate

Immediate

Destination

RegisterMemory

Allowed operand

for NOT Operation Allowed operands for AND,OR, and XOR operations


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Logical Instructions

AND Uses any addressing mode except memory-to-memory and

segment registers

Especially used to clear certain bits (masking)xxxx xxxx AND 0000 1111 = 0000 xxxx

(clear the first four bits)

Examples: AND BL, 0FHAND AL, [345H]

OR Used to set certain bits

xxxx xxxx OR 0000 1111 = xxxx 1111(Set the upper four bits)


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XOR Used to invert certain bitsxxxx xxxx XOR 0000 1111 = xxxxxxxx

-Example: Clear bits 0 and 1, set bits 6 and 7, invert bit 5 ofregister CL:

AND CL, 1111 1100B ; clear bits 0 and 1OR CL, 1100 0000B; set bits 6 and 7XOR CL, 0010 0000B; invert bit 5.


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Shift InstructionsMnemonic Meaning Format Operation Flags Affected

SAL/SHL

SHR

SAR

Shift arithmeticLeft/shiftLogical left

Shift logicalright

Shift arithmeticright

SAL/SHL D, Count

SHR D, Count

SAR D, Count

Shift the (D) left by the numberof bit positions equal to countand fill the vacated bits

positions on the right with zeros

Shift the (D) right by thenumber of bit positions equal tocount and fill the vacated bits

positions on the left with zeros

Shift the (D) right by thenumber of bit positions equal tocount and fill the vacated bits

positions on the left with theoriginal most significant bit

CF,PF,SF,ZFAF undefinedOF undefined if count 1



Destination Count

Register

Register MemoryMemory

1

CL1CL


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SHL

SAL

SHR

CF

0

SAR

0

CF

CF

Sign bit

0

CF

Target register or memory


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Note that the amount of shift specified in the source operand can be definedExplicitly if it is one bit or should be stored in CL if more than 1.

EX: SHL AX, 1causes the 16-bit register to be shifted 1-bit position to the leftWhere the vacated LSB is filled with zero and the bit shifted out of the MSBis saved in CF

0 0 0 1 0 0 1 0 0 0 1 1 0 1 0 0xCF

Register AX before instruction SHL AX, 1

0 0 1 0 0 1 0 0 0 1 1 0 1 0 0 00 0

Shift arithmetic left SAL is exactly the same as SHL


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EX: MOV CL, 2HSHR AX, CL

The two MSBs are filled with zeros and the LSB is thrown away while theSecond LSB is saved in CF.

0 0 0 1 0 0 1 0 0 0 1 1 0 1 0 0 xCF

0 0 1 0 0 1 0 0 0 1 1 0 1 0 0 00 0


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In Arithmetic shift to the right SAR the vacated bits at the left are filled withThe value of the original MSB of the operand. Thus the original sign of theNumber is maintained.

Ex: Assume (CL)= 2 and (AX)= 091AH. Determine the new contents of AX And CF after the instruction SAR AX, CL is executed.

0 0 0 0 1 0 0 1 0 0 0 1 1 0 1 0

This operation is equivalent to division by powers of 2 as long as the bitsshifted out of the LSB are zeros.

0 0 0 1 0 0 1 0 0 0 1 1 0 1 0 0 1CF


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Ex. Multiply AX by 10 using shift instructionsSHL AX, 1MOV BX, AXMOV CL,2

SHL AX,CLADD AX, BX

Ex. What is the result of SAR CL, 1if CL initially contains B6H?

DBH

Ex.What is the result of SHL AL, CL

if AL contains 75H and CL contains 3?A8H


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Rotate Instructions

Mnemonic Meaning Format Operation Flags Affected

ROL Rotate left ROL D, Count Rotate the (D) left by thenumber of bit positionsequal to Count. Each bitshifted out from the leftmost bit goes back into therightmost bit position.

CFOF undefined if count 1

ROR Rotate right ROR D, Count Rotate the (D) right by the

number of bit positionsequal to Count. Each bitshifted out from therightmost bit goes back intothe leftmost bit position.

CF

OF undefined if count 1

RCL Rotate leftthroughcarry

RCL D, Count Same as ROL except carryis attached to (D) forrotation.

CFOF undefined if count 1

RCR Rotate right

throughcarry

RCR D, Count Same as ROR except carry

is attached to (D) forrotation.

CF

OF undefined if count 1


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Destination Count

RegisterRegister Memory

Memory

1CL1

CL Allowed Operands for rotate instruction


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CF

RCL

Ex. What is the result of ROL BTRE PTR [SI], 1

if SI is pointing to a memory location that contains 41H? (82H)

CF

ROL

CFRCR

RORCF


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EX: Assume (AX) = 1234HWhat is the result of executing the instruction ROL AX, 1

0 0 0 1 0 0 1 0 0 0 1 1 0 1 0 0

0 0 1 0 0 1 0 0 0 1 1 0 1 0 0 0

0

0

The original value of bit 15 which is 0 is rotated into CF and bit 0 of AX. All other bits have been rotated 1 bit position to the left.

Rotate right ROR instruction operates the same way as ROL except thatData is rotated to the right instead of left.


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In rotate through carry left RCL and rotate through carry right RCR the bitsrotate through the carry flag.

Rotation caused by RCL instruction

Rotation caused by RCR instruction


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Example

Write a program that counts the number of 1s in a byte and writes it into BL

DATA1 DB 97 ; 61hSUB BL, BL ; clear BL to keep the number of 1sMOV DL, 8 ; Counter MOV AL, DATA1

AGAIN: ROL AL,1 ; rotate left onceJNC NEXT ; check for 1

INC BL ; if CF=1 then add one to count NEXT: DEC DL ; go through this 8 times

JNZ AGAIN ; if not finished go back NOP

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Files 3 Handouts Lecture 8