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Addressing Modes Essay

609 words - 3 pages

Addressing Modes
The way the operands are chosen during program execution is dependent on the addressing mode of the instructions. The addressing mode specifies a rule for interpreting or modifying the address field of the instruction before the operand is actually referenced. Computers use addressing mode techniques for the purpose of accommodating one or both of the following provisions:

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Addressing Modes
Computers use addressing mode techniques for the purpose of accommodating one or both of the following provisions:
1.

2.

to give programming versatility to the user by providing such facilities as pointers to memory, counters for loop control, indexing of data and program relocation. To reduce the number of bits in the addressing field of the instruction.
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Addressing Modes
Immediate Direct Indirect Register Register Indirect Displacement (Indexed) Stack
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Immediate Addressing
Operand is part of instruction Operand = address field e.g. ADD 5
Add ...view middle of the document...

g. ADD (A) Add contents of cell pointed to by contents of A to accumulator

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Indirect Addressing (2)
Large address space 2n where n = word length May be nested, multilevel, cascaded
e.g. EA = (((A)))
Draw the diagram yourself

Multiple memory accesses to find operand Hence slower
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Indirect Addressing Diagram
Instruction Opcode Address A Memory Pointer to operand

Operand

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Register Addressing (1)
Operand is held in register named in address field EA = R Limited number of registers Very small address field needed
Shorter instructions Faster instruction fetch
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Register Addressing (2)
No memory access Very fast execution Very limited address space Multiple registers helps performance
Requires good assembly programming or compiler writing N.B. C programming
register int a;

Like Direct addressing
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Register Addressing Diagram
Instruction Opcode Register Address R Registers

Operand

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Register Indirect Addressing
Like indirect addressing EA = (R) Operand is in memory cell pointed to by contents of register R Large address space (2n) One fewer memory access than indirect addressing

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Register Indirect Addressing Diagram
Instruction Opcode Register Address R Memory

Registers

Pointer to Operand

Operand

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Displacement Addressing
EA = A + (R) Address field hold two values
A = base value R = register that holds displacement or vice versa

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Displacement Addressing Diagram
Instruction Opcode Register R Address A Memory Registers

Pointer to Operand

+

Operand

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Relative Addressing
A version of displacement addressing R = Program counter, PC EA = A + (PC) i.e. get operand from A cells from current location pointed to by PC

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Base-Register Addressing
A holds displacement R holds pointer to base address R may be explicit or implicit e.g. segment registers in 80x86

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Indexed Addressing
A = base R = displacement EA = A + R Good for accessing arrays
EA = A + R R++

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Combinations
Postindex EA = (A) + (R) Preindex EA = (A+(R)) (Draw the diagrams)
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Stack Addressing
Operand is (implicitly) on top of stack e.g.
ADD Pop top two items from stack and add

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