An assembler is a system program that translates an assembly language program into machine code (object code) that can be executed by the CPU.
Key Functions of an Assembler
- Converts opcode mnemonics (e.g.,
MOV,ADD) into machine instructions. - Assigns memory addresses using a Location Counter (LC).
- Handles symbols (labels), constants, and literals.
- Supports forward referencing (resolves references made before symbol definitions).
- Generates intermediate code and final machine code.
Data Structures Used in Assembler
1. POT – Pseudo Opcode Table
Purpose
The Pseudo Opcode Table (POT) stores assembler directives. These are not real machine instructions but guide the assembler on how to manage memory or control flow.
Contents
- Directive name
- Class (
AD= Assembler Directive) - Code (identifier for processing)
Example
| Directive | Class | Opcode/ID |
|---|---|---|
| START | AD | 01 |
| END | AD | 02 |
| ORIGIN | AD | 03 |
| EQU | AD | 04 |
| LTORG | AD | 05 |
Usage
START 100 sets the Location Counter (LC) to 100. The assembler finds START in the POT to perform this operation.
2. MOT – Mnemonic Opcode Table
Purpose
The Mnemonic Opcode Table (MOT) contains all machine instructions (imperative statements) and their corresponding opcodes, formats, and operand types.
Contents
- Mnemonic
- Instruction Class (
IS= Imperative Statement) - Opcode
- Operand count/type
Example
| Mnemonic | Class | Opcode | Operands |
|---|---|---|---|
| MOVER | IS | 04 | 2 |
| ADD | IS | 01 | 2 |
| SUB | IS | 02 | 2 |
| STOP | IS | 00 | 0 |
Usage
When the assembler sees MOVER AREG, VALUE, it looks up MOVER in MOT to get opcode 04.
3. ST – Symbol Table
Purpose
The Symbol Table (ST) stores all user-defined labels or variables and their corresponding memory addresses.
Contents
- Symbol name
- Address (assigned using LC)
- Length (optional)
Example
| Symbol | Address | Length |
|---|---|---|
| LOOP | 200 | 1 |
| VALUE | 202 | 1 |
| ENDLABEL | 210 | 1 |
Usage
When the assembler encounters JMP LOOP, it looks up LOOP in ST to retrieve its address (200).
4. LT – Literal Table
Purpose
The Literal Table (LT) keeps track of constants/literals used in the program and their resolved memory addresses.
Contents
- Literal (e.g.,
='5') - Address (assigned after
LTORGorEND)
Example
| Literal | Address |
|---|---|
='5' | 211 |
='1' | 212 |
Usage
Instruction like MOVER AREG, =‘5’ adds ='5' to LT during Pass 1, and address 211 is assigned during LTORG.
5. BT – Base Table
Purpose
The Base Table (BT) is used in base register addressing schemes to manage
- Which registers are defined as base
- What addresses are currently loaded in those registers
Contents
- Register name
- Base address
Example
| Register | Address |
|---|---|
| BREG | 100 |
| DREG | 200 |
Usage
In indexed addressing, the assembler uses BT to calculate effective address using base + displacement.
🧪 Example Assembly Code
START 100
LOOP MOVER AREG, =‘5’
ADD AREG, VALUE
VALUE DC ‘1’
END🧾 Tables Populated
-
POT →
START,END -
MOT →
MOVER,ADD,DC -
ST →
LOOP = 100,VALUE = 102 -
LT →
='5' = 103(afterEND) -
BT → If base registers are used (optional)
Summary Table
| Table | Full Name | Purpose |
|---|---|---|
| MOT | Mnemonic Opcode Table | Maps instructions like MOVER or ADD to machine opcodes and formats |
| POT | Pseudo Opcode Table | Holds assembler directives like START, END, EQU, LTORG |
| SYMTAB (ST) | Symbol Table | Stores labels and their memory addresses |
| LITTAB (LT) | Literal Table | Stores literal constants (e.g., ='5') and their resolved addresses |
| BT | Base Table | Tracks registers used for base-addressing and their assigned addresses |