Load Add and Store Instructions in Assembly Language | Instruction set Architecture

We already know (from the previous lectures) how an instruction is formed and what the basic structure of an instruction is. Let’s go through few examples of it.

Instruction Set

An instruction set is composed of two parts:

1. Opcode
2. Operand (s)

Opcode

Opcode defines the number of operation supported by this instruction format and used for indexing that operation. For example: 0000 is the opcode for LOADI operation.

Operand (s)

Operands can be a register address, memory address, and immediate value.
In our ISA design, the length of instruction is 16 bits. Such that: 4 bits for opcode, and 12 bits for operand(s).

Opcode (4-bits)

Operands (12-bits)

0

3

15

Operations

For this tutorial we will be focusing on the three basic operations, i.e. how to load, add, and store data in assembly language. Following are the few operations along with their opcodes:

OPCODE	Operation
0000	LOADI
0001	LOADM
0010	STOREI
0011	STORER
0100	ADDI
0101	ADDR
0110	SUBI
…	…
…	…
1111	…

In above operations I is for immediate, M is for memory, and R is for register. Let’s look at the registers and memory first before proceeding to the operations.

Registers

There are 16 registers hence 4 bits are required to address them. Registers are named as R0 to R15.

Register Address	Register
0000	R0
0001	R1
0010	R2
0011	R3
0100	R4
0101	R5
0110	R6
…	…
…	…
1111	R15

Memory

There are 28 memory locations where data may reside. Hence 8 bits are required to address them. Memory locations are named as M0 to M255.

Memory Address	Memory
00000000	M0
00000001	M1
00000010	M2
00000011	M3
00000100	M4
00000101	M5
00000110	M6
…	…
…	…
11111111	M255

LOADM Instruction

Definition

It loads an operand value from a memory location to a register.

Bit allocation

LOADM

REGISTER ADDRESS

MEMORY ADDRESS

0

3

7

15

• 4 bits for Opcode
• 4 bits for Register address
• 8 bits for Memory address

Working

Consider an example:
Assembly Code:                       LOADM   R4    M6
Machine Code:                         0001010000000110

LOADM

R4

M6

In the above instruction operand value i.e. 87 from a memory (M6) copied to a register (R4).

STORER Instruction

Define:

It stores an operand value from a register to a Memory location.

Bit allocation:

STORER

REGISTER ADDRESS

MEMORY ADDRESS

0

3

7

15

• 4 bits for Opcode
• 4 bits for Register address
• 8 bits for Memory address

Working

Consider an example:
Assembly Code:                       STORER   R4    M6
Machine Code:                         0011010000000110

STORER

R4

M6

0

3

7

15

In the above instruction operand value (67) is stored from a register (R4) to a Memory (M6) location.

ADDR Instruction

Define:

It adds register values and keeps the result in the destination register.

Bit allocation:

ADDR

Reg. Address

Reg. Address

0

3

7

11

15

• 4 bits for Opcode
• 4 bits for each Register address
• 4 bits for Source
• 4 bits for Destination

Working

Consider an example:
Assembly Code:                       ADDR   R2 R1
Machine Code:                         0101 0010 0001 0000

ADDR

R2

R1

0

3

7

11

15

Register R1 and R2 are added i.e. 23 and 79 and there result is then stored into R2 register.

Sample Example

LOADM      R1       M25     ; load from memory M25 and store the value in register R1
LOADM      R2      M69      ; load from memory M69 and store the value in register R2
ADDR         R2       R1          ; add values of register R1 and R2
STORER      R2       M2        ; store the value of register R2 to memory M2

Execution of a Assembly Language Instruction

Our CPU will use three stages for executing a single assembly language instruction. These are:

1. Fetch:

   CPU fetches the instruction from Instruction memory (Maintained in a 2D Array).

2. Decode:

   Operation and operands are determined.

3. Execute:

   Operation is performed on the operands.

To see these steps in practical follow the below program written in c language.

Program

A simulator for the discussed ISA design is written in C language. Click on the below link:

Write A Simulator For The Assembly Language ISA Design In C Language.