Prerequisite – Addressing Modes
1. Direct Addressing Mode :
In direct addressing mode, the address field contains the address of the operand.
Effective Address(EA) = address field of operand
Add the contents of register A to the accumulator.
Look in memory at address A for operand. Only a single memory reference is required to access data. So, no additional calculations are required to work out effective address.
2. Implied Addressing Mode :
Operand is specified implicitly in the definition of the instruction. It is normally used for zero or single address instructions.
Increment the contents of register A.
Here it is implicitly specified that register A is both the source as well as the destination.
Clear the contents of flag register.
The above instruction is an example of zero address instruction.
Difference between Direct and Implied Addressing Modes :
|DIRECT ADDRESSING MODE||IMPLIED ADDRESSING MODE|
|Address fields contains the effective address of operand.||Effective address of operand is specified implicitly.|
|Instruction size is larger since operand has to be explicitly specified.||Instruction size is smaller since operand is specified implicitly.|
|It requires one reference to memory.||No memory references are required.|
|Mostly used in 2 address instructions and more.||Mostly used in zero address and single address instructions.|
|It is slower compared to implied mode.||It is a faster method.|
|It has more range than implied mode.||It has less range than direct mode.|
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