When an instruction needs to access data, it must specify WHERE that data is. Different addressing modes provide different ways to specify the location.
Definition
Addressing modes are the different methods used to specify the location of an operand (the data an instruction operates on).
Immediate Addressing
Definition
In immediate addressing, the operand IS the actual value to be used - no memory lookup needed.
assembly
ADD #5 // Add the literal value 5 to accumulator
LDA #100 // Load the literal value 100 into accumulatorThe # symbol indicates immediate addressing. The value 5 or 100 is used directly.
Direct Addressing
Definition
In direct addressing, the operand specifies the memory address where the data is stored.
assembly
ADD 50 // Add the value STORED AT address 50 to accumulator
LDA 100 // Load the value STORED AT address 100 into accumulatorIf address 50 contains the value 7, then ADD 50 adds 7.
Indirect Addressing
Definition
In indirect addressing, the operand points to a memory location that contains the ADDRESS of the actual data.
Key Point
| Memory address | Value stored |
|---|---|
| 50 | 75 |
| 75 | 42 |
assembly
LDA @50 // indirect addressing, @ symbolKey Point
Step-by-step:
1. Look at address 50 → find value 75
2. Use 75 as the actual address
3. Look at address 75 → find value 42
4. Load 42 into accumulator
Result: Accumulator = 42
1. Look at address 50 → find value 75
2. Use 75 as the actual address
3. Look at address 75 → find value 42
4. Load 42 into accumulator
Result: Accumulator = 42
Indexed Addressing
Definition
In indexed addressing, the final address is calculated by adding a base address to a value in an index register.
Key Point
| Item | Value |
|---|---|
| Base address | 100 |
| Index register (X) | 3 |
| Effective address | 100 + 3 = 103 |
assembly
LDA 100,XAction: Load value from address 103 into accumulator.
Why is this useful? Perfect for arrays - the base address is where the array starts, and the index moves through each element.Key Point
| X value | Instruction | Address used | Value loaded |
|---|---|---|---|
| 0 | LDA 100,X | 100 | 10 |
| 1 | LDA 100,X | 101 | 20 |
| 2 | LDA 100,X | 102 | 30 |
Addressing Modes Summary
Key Point
Immediate
• Operand Contains: The actual value
• Example: ADD #5
• Use Case: Constants, fixed values
Direct
• Operand Contains: Address of data
• Example: ADD 50
• Use Case: Simple variables
Indirect
• Operand Contains: Address of address
• Example: ADD @50
• Use Case: Pointers, dynamic access
Indexed
• Operand Contains: Base + index
• Example: ADD 100,X
• Use Case: Arrays, tables
• Operand Contains: The actual value
• Example: ADD #5
• Use Case: Constants, fixed values
Direct
• Operand Contains: Address of data
• Example: ADD 50
• Use Case: Simple variables
Indirect
• Operand Contains: Address of address
• Example: ADD @50
• Use Case: Pointers, dynamic access
Indexed
• Operand Contains: Base + index
• Example: ADD 100,X
• Use Case: Arrays, tables
Exam Tip
Memory Tip:
• Immediate = the value is RIGHT THERE in the instruction
• Direct = go DIRECTLY to that address for the value
• Indirect = indirect route - address gives you another address
• Indexed = base + index (like array access)
• Immediate = the value is RIGHT THERE in the instruction
• Direct = go DIRECTLY to that address for the value
• Indirect = indirect route - address gives you another address
• Indexed = base + index (like array access)