Addressing Modes
The Instruction format consist of 3-bit(12-14) operation code, a 12-address (0-11) and an indirect address mode bit designated by I.
- If
- I=0 -> Direct Addressing Mode
- I=1 -> Indirect Addressing Mode
Direct Addressing Mode
According to above diagram direct address is placed in address 22 in memory.Here I=0 so instruction is recognize as direct address instruction and the address part is the binary equivalent of 457.The control finds the operand in memory at address 457 and add it to content of AC (Accumulator)
Indirect Addressing Mode
According to above diagram Indirect address is placed in address 35 in memory.
Here I=1 , So instruction is recognize as indirect address instruction. The address part is binary equivalent of 300.
The control goes to address of operand.The address of operand in this case is 1350.The operand found in address 1350 is than added to the content of AC (Accumulator)
In Indirect address instruction need two references to memory to fetch an operand. The first reference is needed to read the address of the operand and the second is for the operand itself.
Here I=1 , So instruction is recognize as indirect address instruction. The address part is binary equivalent of 300.
The control goes to address of operand.The address of operand in this case is 1350.The operand found in address 1350 is than added to the content of AC (Accumulator)
In Indirect address instruction need two references to memory to fetch an operand. The first reference is needed to read the address of the operand and the second is for the operand itself.
Effective Address
It is an address of an operand or target address such as 1350 is effective address in indirect address and 457 is effective address in direct address.
- Introduction of Computer Organization
- Addressing Mode
- Common Bus System
- Control Unit
- Type of Instructions
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Content is Reference form Compute System Architecture, MORRIS M. MANO
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