Arithmetic and logic unit Continue...

1. Unit 2 Continue….
Instruction set architecture of a CPU
registers,
 instruction execution cycle
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4.  Instruction Execution and Format:
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11. Immediate Addressing:
The simplest form of addressing is immediate addressing, in which the operand is actually present in the
instruction.
Operand is part of instruction
Operand = address field
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12. Register (Direct) Addressing:
A very simple form of addressing is direct addressing, in which:
Address field contains address of operand
Effective address (EA) = address field (A)
e.g. ADD A ; Add contents of cell A to accumulator
The technique was common in earlier generations of computers but is not common on
contemporary architectures. It requires only one memory reference and no special calculation. The obvious
limitation is that it provides only a limited address space.
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13. Register (Direct) Addressing Conti…..
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14. Indirect Addressing:
With direct addressing, the length of the address field is usually less than the word length, thus limiting the address
range. One solution is to have the address field refer to the address of a word in memory, which in turn contains a
full-length address of the operand.
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15. Displacement Addressing
A very powerful mode of addressing combines the capabilities of direct addressing and register indirect addressing.
It is known by a variety of names depending on the context of its use but the basic mechanism is the same. We will
refer to this as displacement addressing, address field hold two values:
•A = base value
•R = register that holds displacement
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