OCR F453 - Addressing Modes

8/12/2019 OCR F453 - Addressing Modes

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Addressing modes


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Addressing

Memory can be visualised as a series of pigeon-holes into which

instructions and data can be stored.

Each block of memory has a unique address.

Data can be written to or read from each address.

Addressing modes are the different ways in which theaddresses can be accessed.

For example, you may want to load the accumulator with a

particular value.

Depending on how and where the value is stored this will

require the use of different addressing modes.


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Immediate addressing (relative

addressing) The value to be loaded is not stored in a memory address at all,but is included as part of the instruction.

For example:

LDAA #12H

LDAmeans load the accumulator

#indicates that it is a value that should be loaded and not an

address

12is the value to be loaded

Hindicates that in this case it is a hexadecimal number.


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Direct addressing (absolute

addressing) Direct addressing tells the CPU the address that contains thedata you want to access.

For example: LDAA 100 means load the accumulator with the

data stored at memory address 100.

With an 8-bit address bus, only 256 different memory locationsare addressable.

Even with 32-bit, you are still limiting the amount of memory that

is addressable using this method.


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Indirect addressing

Indirect addressing tells the CPU an address that contains

another address where the data is stored.

For example, LDAI 132:

points to memory address 132

there will be another address at this location

the accumulator is loaded with the data at this address.

This method makes it easier to alter the actions of a program, as

the data can be altered without having to adjust the original

program.


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Index addressing

The address to be accessed is modified by the addition of a

number held in an index register.

For example, LDX R1 32:

LDXmeans load the accumulator (using indexed

addressing) R1is the register that contains the start number for the

address

32is the number to be added to the number at R1 to locate

the address where the data is stored.

This increases the amount of addressable memory.

This particular method is used for addressing data stored in an

array, as it will be stored in consecutive memory locations.


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Base register addressing

This is the same as indexed addressing, except the start

address is held in a base register.

It uses a larger offset than index addressing.

The contents of this register rarely change during the execution

of the program (unlike index addressing).

OCR F453 - Addressing Modes

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