Difference between Direct and Indirect Addressing Modes

Prerequisite: Addressing Modes 

1. Direct Addressing Mode: In direct addressing mode, the address field in the instruction contains the effective address of the operand and no intermediate memory access is required. Nowadays it is rarely used.

Example: Add the content of R1 and 1001 and store back to R1:

Add R1, (1001) 

Here 1001 is the address where the operand is stored. 

2. Indirect Addressing Mode: In Indirect addressing mode, the address field in the instruction contains the memory location or register where the effective address of the operand is present. It requires two memory access. It is further classified into two categories: Register Indirect, and Memory Indirect. 

Example:

LOAD R1, @500

The above instruction is used to load the content of the memory location stored at memory location 500 to register R1. In other words, we can say, effective address is stored at memory location 500. 

Difference between Direct and Indirect Addressing Modes:

Parameters	Direct Addressing Mode	Indirect Addressing Mode
Address Field	Address field contains the effective address of operand.	Address field contains reference of effective address.
Memory References	Requires only one memory reference	Requires two memory references
Processing Speed	This addressing mode has fast addressing compared to indirect addressing mode	It is slower than direct addressing mode.
Classification	No further classification	Further classified into two categories- Memory Indirect and Register Indirect Addressing Mode.
Calculation	No further calculation is required to perform the operation.	Requires further calculation to find the effective address.
Address Space	It occupies a smaller amount of space than the indirect mode.	It occupies a large amount of space than the direct mode.
Overhead	No additional overhead is involved while searching for operand.	Additional overhead is involved while searching for operand.
Advantage	Easy as no intermediary is involved.	Availability of large address space.
Disadvantage	Address space is restricted.	Requires more number of memory references.
Application	It aids in accessing static data and using variables.	It assists in passing arrays as parameters and implementing pointers.

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