Microprogrammed Control Unit

A microprogrammed control unit is a control unit that saves binary control values as words in memory. By creating a certain collection of signals at every system clock beat, a controller generates the instructions to be executed. Each one of these output signals causes a single micro-operation, such as register transfer. As a result, defined micro-operations that can be preserved in memory are formed from the sets of control signals.

In this article, we will dive deeper into Microprogrammed Control Unit according to the GATE Syllabus for (Computer Science Engineering) CSE. Keep reading ahead to learn more.

Table of Contents

What is a Microprogrammed Control Unit?
Characteristics of Microprogrammed Control Unit
Difference between Hardwired and Microprogrammed Control Unit
Instruction Words
- Using a Single-level Control Store
- Using a Two-level Control Store
Microprogrammed Control Unit Pros
Microprogrammed Control Unit Cons

What is a Microprogrammed Control Unit?

The programming approach is used to implement a microprogrammed control unit. A program made up of microinstructions is used to carry out a series of micro-operations. The control unit’s control memory stores a microprogram composed of microinstructions. The creation of a set of control signals is dependent on the execution of a microinstruction.

The block diagram of this type of organisation is shown below:

Characteristics of Micro-programmed Control Unit

The microinstruction address is specified in the control memory address register.
All the control information is saved in the control memory, which is considered to be a ROM.
The microinstruction received from memory is stored in the control register.
A control word in the microinstruction specifies one or multiple micro-operations for a data processor.
The next address is calculated in the circuit of the next address generator and then transferred to the control address register for reading the next microinstruction when the micro-operations are being executed.
Because it determines the sequence of addresses received from control memory, the next address generator is also known as a microprogram sequencer.

Difference between Hardwired and Microprogrammed Control Unit

The existence of the control store, which is used to store words containing encoded control signals required for instruction execution, is the main distinction between microprogrammed structures and the hardwired control unit structure.

Each bit in the microinstruction is connected to a single control signal. The control signal is active when the bit is set., and it becomes inactive when it is cleared. A sequence of these microinstructions can be kept in the internal ‘control’ memory. A microprogram-controlled computer’s control unit is a computer within a computer.

Here is the block diagram:

Instruction Words

The following instruction words are normally fetched into the instruction register in micro-programmed control units. The operation code of each instruction, on the other hand, is not directly decoded to enable instant control signal generation; instead, it contains the initial address of a microprogram in the control store.

Using a Single-level Control Store

The control store address register receives the instruction opcode from the instruction register. The first microinstruction of a microprogram that interprets the execution of such an instruction is read to the microinstruction register based on this address. The operation element of this microinstruction contains encoded control signals, usually in the form of a few bit fields. The fields are decoded using a collection of microinstruction field decoders. The address of the next microinstruction in the supplied instruction microprogram is also included in the microinstruction, as well as a control field for controlling the microinstruction address generator’s actions.

The addressing mode or addressing operation that is to be applied to the address encoded in the continuing microinstruction is determined by the last-mentioned field. This address is refined in microinstructions and conditional addressing mode by employing the processor condition flags, which describe the status of calculations in the current program. The microinstruction that fetches the very next instruction to the instruction register from the main memory is the last microinstruction in the instruction of the provided microprogram.

Using a Two-level Control Store

Apart from the control memory for the microinstructions, the nano-instruction memory is also contained in a control unit along with a two-level control store. Microinstructions do not include encoded control signals in such a control unit. The address of a word in the nano-instruction memory that carries encoded control signals is contained in the operation component of microinstructions. The nano-instruction memory stores all combinations of control signals that exist in microprograms that interpret a computer’s whole instruction set, which is written in nano-instructions only once.

It eliminates the need to store the same operation sections of microinstructions several times. Micro-instruction words can be substantially shorter in this situation than in the single level control store. It results in a significantly smaller microinstruction memory in terms of bits and, as a result, an even smaller overall control memory. The control for selecting consecutive microinstructions is stored in the microinstruction memory, whereas those control signals are produced on the basis of nano-instructions. Control signals in nano-instructions are usually encoded using the 1 bit/1 signal technique, which eliminates decoding.