Inside the CPU: ALU, control unit, registers and buses

Overview

Recall questions on the arithmetic and logic unit, the control unit, the program counter, MAR, MDR, CIR and accumulator, the address, data and control buses, how bus width sets addressable memory and transfer size, and how assembly language relates to the registers.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What are the three main parts of a processor, and what does each one do?

   Model answer

   The arithmetic and logic unit (ALU) carries out every calculation and logical operation. The control unit (CU) fetches and decodes instructions and coordinates everything else, sending out the control signals that make the other components act. The registers are a small number of extremely fast storage locations inside the processor itself, holding the addresses, instructions and data the CPU is working on at this instant.

2. 2

   Which operations does the ALU actually perform?

   Model answer

   Arithmetic: addition and subtraction, and multiplication and division built from repeated addition and binary shifts. Logic: AND, OR, NOT and XOR applied bit by bit. Comparisons, which are usually performed as a subtraction and set flags such as zero, negative, carry and overflow. It also performs the binary shifts themselves. The result of an ALU operation is normally placed in the accumulator.

3. 3

   The control unit does not calculate anything, so what is it actually for?

   Model answer

   It decodes the opcode of the current instruction and works out what has to happen. It then generates the control signals that tell each component what to do and when: read from memory, write to memory, use the ALU, move a value between registers. It uses the pulses of the system clock to synchronise the timing of every step, and it directs the flow of data along the buses. In short, it manages the fetch-decode-execute cycle.

4. 4

   Why does a processor need registers at all when it already has main memory?

   Model answer

   Registers sit inside the processor and can be read or written within a single clock cycle, whereas a main memory access takes many cycles and has to cross the buses. Without registers the CPU would spend nearly all its time waiting. Registers are very few and very small because they are expensive in chip area, so they hold only the handful of values in immediate use.

5. 5

   What does the program counter hold?

   Model answer

   The memory address of the next instruction to be fetched. It is copied into the memory address register at the start of each fetch and then incremented, so by default the processor works through memory in order. A jump instruction overwrites it with a different address, which is how the flow of a program is changed.

6. 6

   Explain the purpose of the memory address register.

   Model answer

   The MAR holds the address of the memory location that is about to be read from or written to. Its contents are placed onto the address bus so that the memory hardware can select the right location. It holds an address only, never the value stored there.

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