Instruction & Instruction Sequencing in Computer Architecture

What is Instruction ?

Computer instruction is a machine language instruction set that works to understand and execute a program's language.  

A computer works on the instructions provided. 

A computer instruction includes a set of fields called fields.

A basic computer has three instructions, 

  1.  Memory Reference Instruction
  2.  Register Reference Instruction
  3.  Input-Output Instruction

 So let's know about these three instructions



Memory Reference Instruction

Memory Reference Instruction uses 12 bits of memory to specify the address and 1 bit of memory to specify the addressing mode.


Register Reference Instruction

Register Reference Instruction represents opcode 111 with a0.  The Register Reference Instruction specifies an operation or a test on such a register.


Input-Output Instruction

Input-Output Instruction is like Register Reference Instruction.  

The Input-Output instruction in this instruction requires no memory reference and this operation recognizes a1 with code 11 in the leftmost bit of the instruction.  

The last 12 bits are used to specify the type of input-output operation or test performed.


What is an Operands ?

An operand in a computer is a part of computer instruction that operates and manipulates special data by extension.

Computer instruction describes an operation that is added, subtract, and so forth, and the operand performs the same operation.

As in mathematics, an operand is an object of mathematical operation.


What is Opcode ? (operation code)

Operation code refers to a group of bits in the instruction, and it designs arithmetic and logic operations such as add, subtract, multiplies, shifts, and compliment.


Instruction formats

An instruction is usually a combination of operation and some way of specifying an operand, most commonly by its location or address in memory.

The computer works on some given instructions.

Instruction is of various lengths depending on the number of addresses present in it.

CPUs are generally of three types based on the number of address fields.

  1.  Single accumulator organization
  2.  General register organization
  3.  Stack organization

What is Addressing Mode ?

The term addressing modes refers to how the operation of instruction is specified.  

Addressing mode specifies a rule for the modification of the address field of the instruction before the operand is excited.

The addressing mode in the 8086 instruction is divided into two parts,

  •  Addressing modes for data
  •  Addressing modes for branch

8086 memory address mode provides flexible memory access.


What is Addressing Sequence?

 The microinstructions that are here are all in a group in the control memory, here each group specifies a routine, and how to complete the instruction here.

 Here each instruction has its own routine in the control memory.  Microoperations are generated through routines that execute microoperation instructions.

 Hardware that controls address sequencing must be capable of sequencing microinstructions within a routine and must also be capable of branching from one routine to another.


Process of address sequencing

 Whenever the power on any computer is turned on, the corresponding address is loaded into the control address register.  Which is the address of microinstructions.  And it fetches the instruction and activates the routine key.

 The control is then passed through a memory routine that determines the effective address of the instruction.  Then after this, the microoperation is generated.  It executes the data fetched from memory.

Here each instruction has its own microprogram routine which is stored in the control memory.  In the control memory, the instruction code is converted into the address of the bits, which is called mapping.

 Each microprogram that handles the subroutine requires an external register to store the return address.  Its return address cannot be stored in ROM.


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