Micro-Programmed Control Unit Detailed Notes

Micro-Programmed Control Unit

Overview of Micro-Programmed Control Units

  • A microprogrammed control unit uses a microprogram, composed of microinstructions, to perform a series of micro-operations.

  • The control memory stores the microprogram containing microinstructions.

  • The creation of control signals is dependent on the execution of these microinstructions.

Characteristics of Micro-Programmed Control Unit

  • The microinstruction address is found in the control memory address register.

  • All control information is stored in a ROM-like control memory.

  • Microinstructions from memory are held in the control register.

  • A control word specifies one or multiple micro-operations for a data processor.

  • The next address generator calculates the address for the next microinstruction and stores it in the control address register for further execution.

  • The next address generator is also known as the microprogram sequencer.

Design Aspects of Micro-Programmed Control Unit

  • The existence of the control store distinguishes it from hardwired control unit structures.

  • In microprogrammed units, each bit in a microinstruction corresponds to a unique control signal, which is only active when set, and becomes inactive when reset, allowing for precise control over the operations of the CPU during instruction execution.

  • A microprogram-controlled computer's control unit functions as a computer within a computer.

Important Terminology

  1. Control Word: Represents various control signals through its individual bits.

  2. Micro-Routine: A sequence of control words for a machine instruction represents its control sequence.

  3. Micro-Instruction: Individual control words in a micro-routine.

  4. Micro-Program: A sequence of micro-instructions stored in ROM/RAM (Control Memory).

  5. Control Store: Holds micro-routines for all instructions in a computer's instruction set.

Operation of Micro-Programmed Control Units

  • Instruction fetching occurs in the instruction register in MCU.

  • The initial address of a microprogram, instead of its direct opcode, is fetched to generate control signals.

  • The control store address register receives the instruction opcode, enabling the reading of the first microinstruction into the microinstruction register.

  • The decoded control signals are generated from the microinstruction field decoders, handling operations as indicated by the microinstruction fields.

Types of Micro-Programmed Control Units

  1. Horizontal Micro-Programmed Control Unit:

  • Control signals are in decoded binary format (1 bit/CS).

  • Supports longer control words (more than 1 signal can be enabled at a time).

  • it is used in parallel processing application.

  • Faster than vertical microprogramming because no additional hardware(decoders) are required.

  • It is more flexible than vertical mircoprogrammed.

  1. Vertical Micro-Programmed Control Unit:

  • Control signals in encoded binary format (Log2(N) bits for N control signals).

  • Supports shorter control words

  • supports easy implementation of new control signals therefore it is more flexible.

Advantages of Micro-Programmed Control Units

  • Facilitates systematic control unit design.

  • Simplifies troubleshooting and modifications.

  • Maintains fundamental control function structure.

  • Less expensive and error-prone; suitable for software-based functions.

Disadvantages of Micro-Programmed Control Units

  • Higher costs due to adaptability.

  • Slower than hardwired control units.

References

  • J.P. Hayes, “Computer Architecture and Organization”, Third Edition.

  • Mano, M., “Computer System Architecture”, Third Edition, Prentice Hall.

  • Stallings, W., “Computer Organization and Architecture”, Eighth Edition, Pearson Education.

  • Carpinelli J.D,” Computer systems organization & Architecture”, Fourth Edition, Addison Wesley.

  • Patterson and Hennessy, “Computer Architecture”, Fifth Edition Morgan Kaufman.