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Types of Programming Language

Modes of addressing memory

  • 3 things to know
  • Component 01

Overview

Recall questions on immediate, direct, indirect and indexed addressing, what the operand means in each mode, working out what LDA loads under each mode, using indexed addressing for arrays, and memory accesses per mode.

What you need to know

Answer each one before you reveal it.

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Key term

In immediate addressing, what does the operand actually mean?

Key term

In direct addressing, what does the operand actually mean?

Key term

Name the four modes of addressing memory.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What is an addressing mode?

    Model answer

    The rule that says how the operand part of an instruction should be interpreted in order to find the data the instruction is to act on. The same bit pattern in the operand field can mean the value itself, an address, an address of an address, or an offset, depending on which addressing mode the instruction specifies.

  2. 2

    Name the four modes of addressing memory.

    Model answer

    Immediate, direct, indirect and indexed.

  3. 3

    In immediate addressing, what does the operand actually mean?

    Model answer

    The operand is the data itself. No memory access is needed to fetch it, because the value to be used is held in the instruction, so the processor simply uses it. If the instruction is LDA with immediate addressing and the operand is 7, the accumulator is set to 7.

  4. 4

    In direct addressing, what does the operand actually mean?

    Model answer

    The operand is the memory address at which the data is stored. The processor goes to that address and fetches its contents. If the instruction is LDA with direct addressing and the operand is 40, the processor reads whatever is in location 40 and puts that into the accumulator.

  5. 5

    In indirect addressing, what does the operand actually mean?

    Model answer

    The operand is the address of a location that itself holds the address of the data. The processor makes two memory accesses: first it reads the location the operand names to obtain a second address, then it goes to that second address for the actual data.

  6. 6

    In indexed addressing, what does the operand actually mean?

    Model answer

    The operand is a base address, to which the current contents of the index register are added to give the effective address of the data. The data is then fetched from that calculated address. Changing the index register changes which location the same instruction reaches.

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