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Machine Instructions

1. Data Transfer Instructions :

Data transfer instructions are used to transfer of data from one address to another without changing the contents.


Data Transfer Instructions

 


Eight Addressing Modes for the Load Instruction


Mode Assembly Convention Register Transfer

Mode

Assembly Convention

Register Transfer

Direct address

LD ADR

AC  M[ADR]

Indirect address

LD @ADR

AC  M[M[ADR]J

Relative address

LD $ADR

AC  M[PC + ADR]

Immediate operand

LD #NBR

AC  NBR

Index addressing

LD ADR(X)

AC  M[ADR + XR]

Register

LD R1

AC  R1

Register indirect

LD (R1)

AC  M[R1]

Autoincrement

LD (R1)

+ AC  M[R1], R1  R1 + 1

 

Abbreviations used has meaning as follows : ADR=address, NBR=number/operand, X=index register, AC=Accumulator, R1=General purpose register, @=indirect addressing mode, $=relative address to the PC,


Data Manipulation Instructions

 

Data manipulation instructions perform various operations on data and provide the computational capabilities for the computer. We can further classify the Data Manipulation Instructions in following 3 types :


I. Arithmetic Instructions:

It includes addition, subtraction, multiplication, and division. Following table shows various arithmetic instructions in a typical computer:


Typical Arithmetic Instructions

 

Name

Mnemonic

Increment

INC

Decrement

DEC

Add

ADD

Subtract

SUB

Multiply

MUL

Divide

DIV

Add with carry

ADDC

Subtract with borrow

SUBB

Negate (2’s complement)

NEG

 

 II. Logical and Bit Manipulation Instructions:

Logical instructions include bitwise AND, bitwise OR etc. Logical and bit manipulation instructions in a typical computer are as follows :


Typical Logical and Bit Manipulation Instructions

 

Name

Mnemonic

Clear

CLR

Complement

COM

AND

AND

OR

OR

Exclussive-OR

XOR

Clear carry

CLRC

Set carry

SETC

Complement carry

COMC

Enable interrupt

EI

Disable interrupt

DI

 

III. Shift Instructions:

These instructions are used to move the bits of a Memory Word or register in a particular directions. Shift instructions have many variants like : logical shifts, arithmetic shifts, or circular shifts. In either case the shift may be to the right or to the left. Various shift instructions in a typical computer are as follows :


Typical Shift Instructions

Name

Mnemonic

Logical shift right

SHR

Logical shift left

SHL

Arithmetic shift right

SHRA

Arithmetic shift left

SHLA

Rotate right

ROL

Rotate left

RORC

Rotate right through carry

ROLC

Rotate left through carry

ROLC

  1. Logical Shifts :
    These instructions insert 0 at the last bit position and shift all the bits of the memory word in specified direction by one bit position.
  2. Arithmetic shifts :
    These instructions are meant for signed-2’s complement numbers. These instructions preserve the sign bit in the leftmost position.
  3. Circular Shifts :
    These instructions treat the bit sequence as a circular list. It means that the rightmost bit position is treated adjacent to the leftmost bit.





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