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Types of Addressing Modes

Posted Date: 23-May-2008  Last Updated:   Category: Computer & Technology    
Author: Member Level: Gold    Points: 2

1. One of a small number of high-speed memory locations in a computer's CPU. Registers differ from ordinary random-access memory in several respects:

There are only a small number of registers (the "register set"), typically 32 in a modern processor though some, e.g. SPARC, have as many as 144. A register may be directly addressed with a few bits. In contrast, there are usually millions of words of main memory (RAM), requiring at least twenty bits to specify a memory location. Main memory locations are often specified indirectly, using an indirect addressing mode where the actual memory address is held in a register.

Registers are fast; typically, two registers can be read and a third written -- all in a single cycle. Memory is slower; a single access can require several cycles.

The limited size and high speed of the register set makes it one of the critical resources in most computer architectures. Register allocation, typically one phase of the back-end, controls the use of registers by a compiled program.

2. An addressable location in a memory-mapped peripheral device. E.g. the transmit data register in a UART.

Indirect addressing:
An addressing mode found in many processors' instruction sets where the instruction contains the address of a memory location which contains the address of the operand (the "effective address") or specifies a register which contains the effective address. In the first case (indirection via memory), accessing the operand requires two memory accesses - one to fetch the effective address and another to read or write the actual operand. Register indirect addressing requires only one memory access.

An indirect address may be indicated in assembly language by an operand in parentheses, e.g. in Motorola 68000 assembly

MOV D0,(A0)

writes the contents of register D0 to the location pointed to by the address in register A0.
Indirect addressing is often combined with pre- or post- increment or decrement addressing, allowing the address of the operand to be increased or decreased by one (or some specified number) either before or after using it.

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