Addressing Modes in Computer Architecture

Study Notes

The way the CPU accesses information needed for operations is called addressing
Operand is the information being accessed
Instructions need:
- Op-code which defines the operation
- Address field which defines location of operand
Addressing modes differentiate based on how the address of the operand is specified

The value of the operand is directly included within the instruction
Used for constants, initial variable values
Advantage: No extra memory reference needed, improves efficiency
Disadvantage: Limited operand size, restricted by address field size, must be known at assembly time
Example: ADD 5 : The number 5 is directly stored in the instruction for addition

Address field contains the direct address of the operand
Requires only one memory reference, minimal calculation
Limitation: Limited address space
Example: LOAD 1000, Ri : If memory location 1000 contains value 2345, the instruction will load value 2345 into register Ri

Address field refers to a memory location that stores the actual address of the operand
Useful for address range limitations of the address field
Example: Add 100 : Memory location 100 contains another address, which points to the data to be added

Address field refers to a register that holds the operand
Similar to Direct Addressing, but uses register instead of memory
Advantages: Smaller address field, faster access due to no memory reference
Disadvantage: Very limited address space
Example: ADD R1 : The value stored in register R1 is used for the addition operation

Requires two address fields, at least one explicit
One field includes a direct value (A)
The other field refers to a register (R)
Effective Address (EA): Calculated as A + (R)
Common uses:
- Relative Addressing: A is a relative offset, R points to a base address
- Base-Register Addressing: R is a base register, A is an offset from the base
- Indexing: R is an index register, A is a base address, EA points to an element within a data structure