Components of Processor: Registers, ALU, Bus Control, Instruction Decoder

From our perspective, the processor is the electronic integrated circuit that controls other elements of the computer. Its main ability is to execute instructions. While we will go into details of the instruction set, you will see that some instructions perform calculations or process data, while others do not. This suggests that the processor comprises two main units. One of them is responsible for instruction execution, while the second performs data processing. The first one is called the control unit or instruction processor. The second one is named the execution unit or data processor. We can see them in Fig 1.

The function of the control unit, known also as the instruction processor is to fetch, decode and execute instructions. It also generates signals to the execution unit if the instruction being executed requires so. It is a synchronous and sequential unit. Synchronous means that it changes state synchronously with the clock signal. Sequential means that the next state depends on the states at the inputs and the current internal state. As inputs, we can consider not only physical signals from other units of the computer but also the code of the instruction. To ensure that the computer behaves the same every time it is powered on, the execution unit is set to the known state at the beginning of operation by RESET signal. A typical control unit contains some essential elements:

• Instruction register (IR).
• Instruction decoder.
• Program counter (PC)/instruction pointer (IP).
• Stack pointer (SP).
• Bus interface unit.
• Interrupt controller.

Elements of the control unit are shown in Fig 2.

The control unit executes instructions in a few steps:

• Generates the address of the instruction.
• Fetches instruction code from memory.
• Decodes instructions.
• Generates signals to the execution unit or executes instructions internally.

In detail, the process looks as follows:

1. The control unit takes the address of the instruction to be executed from a special register known as the Instruction Pointer or Program Counter and sends it to the memory via the address. It also generates signals on the control bus to synchronise memory with the processor.
2. Memory takes the code of instruction from the provided address and sends it to the processor using a data bus.
3. The processor stores the instruction code in the instruction register and based on the bit pattern interprets what to do next.
4. If the instruction requires the execution unit operation, the control unit generates signals to control it. In cooperation with the execution unit, it can also read data from or write data to the memory.

The control unit works according to the clock signal generator cycles known as main clock cycles. With every clock cycle, some internal operations are performed. One such operation is reading or writing the memory which sometimes requires more than a single clock cycle. Single memory access is known as a machine cycle. As instruction execution sometimes requires more than one memory access and other actions the execution of the whole instruction is named instruction cycle. Summarising one instruction execution requires one instruction cycle, and several machine cycles, each composed of a few main clock cycles. Modern advanced processors are designed in such a way that they are able to execute a single instruction (sometimes even more than one) every single clock cycle. This requires a more complex design of a control unit, many execution units and other advanced techniques which makes it possible to process more than one instruction at a time.

The control unit also accepts input signals from peripherals enabling the interrupts and direct memory access mechanisms. For proper return from the interrupt subroutine, the control unit uses a special register called stack pointer. Interrupts and direct memory access mechanisms will be explained in detail in further chapters.

An execution unit, known also as the data processor executes instructions. Typically, it is composed of a few essential elements:

• Arithmetic logic unit (ALU).
• Accumulator and set of registers,
• Flags register,
• Temporal register.

The arithmetic logic unit (ALU) is the element that performs logical and arithmetical calculations. It uses data coming from registers, the accumulator or from memory. Data coming from memory data for arithmetic and logic instructions are stored in the temporal register. The result of calculations is stored back in the accumulator, other register or memory. In some legacy CISC processors, the only possible place for storing the result is the accumulator. Besides the result, ALU also returns some additional information about the calculations. It modifies the bits in the flag register which comprises flags that indicate the results from arithmetic and logical operations. For example, if the result of the addition operation is too large to be stored in the resulting argument, the carry flag is set to inform about such a situation.

Typically flags register include:

• Carry flag, set in case a carry or borrow occurs.
• Sign flag, indicating whether the result is negative.
• Zero flag, set in case the result is zero.
• Auxiliary Carry flag used in BCD operations.
• Parity flag, which indicates whether the result has an even number of ones.

The flags are used as conditions for decision-making instructions (like if statements in some high-level languages). Flags register can also implement some control flags to enable/disable processor functionalities. An example of such a flag can be the Interrupt Enable flag from the 8086 microprocessor.

As we mentioned in the chapter about CISC and RISC processors in the first ones there are specialised registers including the accumulator. A typical CISC execution unit is shown in Fig 3.

A typical RISC execution unit does not have a specialised accumulator register. It implements the set of registers as shown in Fig 4.