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Address field

Also called: address part, operand field

The bits of an instruction that give a memory address: where to find an operand, where to store a result, or where to jump.

The address field is the part of an instruction word that names a memory address. In Tiny8 it's the low 4 bits, IR[3:0], the right-hand hex digit.

What it's used for depends on the instruction:

  • LOAD, ADD, SUB: where to read the operand.
  • STORE: where to write the ACC.
  • JMP, JZ: where to jump, copied into the PC.
  • HALT: ignored, written as 0.

Its width decides how much memory an instruction can reach: an n-bit address field names 2ⁿ addresses. Tiny8's 4-bit field reaches 16 words.

In a fixed-width format, the fields share the bits. So the address field gets whatever the opcode doesn't use: a 16-bit instruction with a 6-bit opcode has a 10-bit address field, reaching 2¹⁰ = 1024 words.

Worked example

Example

How much memory can it reach?

A machine has 12-bit instructions: a 5-bit opcode followed by one address field.

  1. 1.

    Address field: 12 − 5 = 7 bits.

  2. 2.

    7 bits name 2⁷ = 128 addresses.

  3. 3.

    So an instruction can reach 128 words directly.

Common mistakes

  • Answering with the bit count instead of the address count: 7 bits reach 128 words, not 7.

  • Treating the address field as the data value.

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Address field is taught in Basic CPU / Computer Architecture.

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