Chapter 10

Part II: Addressing Modes

LMC Addressing Mode

uses direct, absolute addressing direct address: the address of the data being

referenced can be reached directly from the address contained in the instruction word

absolute address: the address given in the instruction is the actual memory location where the data is stored

too limited for real computer systems to use only this method

Why alternative modes?

Allow a much larger range of addressable memory locations while using reasonable number of bits in the instruction word

Facilitate implementation of certain frequently occurring program statements - such as loops

Facilitate use of registers and moving data between registers

Register Addressing

Both operands are registers Significantly speeds up the execution Reduces memory accesses RISC machines provide instruction sets

that consist almost entirely of registers

Base Register Addressing

Base Register addressing• use a register to hold the initial

address• include the offset and the R# in the

instruction• address from register and offset are

added to get final address

Base Register Example

IBM S/390 Load Instruction

Data value is loaded into destination register.

Data address is found via indexed base addressing.

Advantages

Provides larger address space - usually up to several GBs

Supports relocatability of program from one memory block to another

Some systems have special base register - others permit use of general purpose registers

Relative Addressing - generate an address relative to the location of the current instruction-similar to base addressing but uses Program Counter instead of base register.

Useful for branching to n locations before or after an instruction => complement arithmetic

Relative Addressing

The desired address consists of the value in the address field added to the value in the program counter

Program Counter (PC)Instruction

OPCODE OPERANDS

1326 42

1326 + 42

1368 Actual (absolute) address in memory

Immediate Addressing

Data operand is stored within the instruction itself

Useful for constants No memory access is required

LMC Immediate Addressing

INDIRECT ADDRESSING

separates the location of the data from the instruction itself

OPCODE OPERANDS

52

47484950515253

72737475

74

DATA

Indirect Addressing

Indirect addressing is useful if one needs to reference a location outside the range of absolute address values

More commonly, register indirect addressing is provided

Register Indirect Addressing

Advantages

Very efficient access to memory Utilized in most processors Usually auto-incrementing or

decrementing of register is supported

Indexed Addressing

Similar to base register addressing Index register is used as a offset

for handling subscripting of arrays

Indexed Addressing

Indexed Addressing w/Base Register