memory organization

1. Control Unit in Computer
Architecture
Hardwired vs Microprogrammed
Control Units

2. Control Unit
• CPU is partitioned into ALU and Control Unit
(CU).
• Generates timing and control signals for
operations.
• Controls data flow between processor,
memory, and peripherals.

3. Control Unit

4. Types of Control Units
• Hardwired Control Unit: Circuit-based
approach using logic gates.
• Microprogrammed Control Unit: Program-
based approach using control memory.

5. Hardwired Control Unit
• Generates control signals using hardware
circuitry.
• Designed for RISC-style instruction sets.
• Components: Instruction Register, Decoder,
Sequence Counter, Logic Gates.

6. Hardwired Control Unit

7. Hardwired Design Methods
• State-table Method
• Delay-element Method (uses D Flip-Flops)
• Sequence-counter Method
• PLA Method (Programmable Logic Array)

8. Hardwired CU: Pros & Cons
• Advantages: Very fast due to combinational
circuits.
• Disadvantages: Complex design, difficult to
modify or add features.

9. Microprogrammed Control Unit
• A micro-programmed control unit is implemented
using programming approach. A sequence of
microoperations are carried out by executing a
program consisting of micro-instructions.
• Micro-program, consisting of micro-instructions is
stored in the control memory of the control unit.
• Execution of a micro-instruction is responsible for
generation of a set of control signals

10. Microprogrammed Control Unit
• Control Word: - The control variables at any time are represented by
1’s and 0’s, known as Control Word. Control words can be
programmed to perform various operations.
• Micro-Operations: The operations performed on the data stored inside
the registers are called micro-operations.
• Micro-Programs: Microprogramming is the concept for generating
control signals using programs. These programs are called micro-
programs.
• Micro-Instructions: The instructions that make micro-program are
called micro-instructions.
• Micro-Code: Micro-program is a group of microinstructions. The
micro-program can also be termed as micro-code.
• Control Memory: Micro-programs are stored in the read only memory
(ROM). That memory is called control memory

11. Micro-Programmed control Organization
• Control Memory
• Control Address Register
• Sequencer (Next Address Generator)
• Control Data Register (Pipeline Register

12. Micro-Programmed control Organization

13. Micro-Programmed control Organization
Instruction code
Mapping
Logic
Multiplexers
Control memory (ROM)
Subroutine
Register
(SBR)
Branch
Logic
Status
Bits
Microoperations
Control Address Register
(CAR)
Incrementer
MUX
Select
Select a status
Bit
Branch address

14. Microprogramming Concepts
• Control Word: Binary representation of control
signals.
• Micro-operations: Operations inside registers.
• Micro-instructions: Form microprograms
stored in control memory.

15. Microprogrammed CU: Address Sequencing
• Sequencer determines the next
microinstruction address.
• Supports Increment, Branch, and Mapping.
• Subroutines supported with return address
stored in SBR.

16. Micro-instruction Types
• Vertical Microprogramming: Narrow, encoded
control signals, less parallelism.
• Horizontal Microprogramming: Wide word,
high parallelism, less encoding.

17. Microprogrammed CU: Pros & Cons
• Advantages: Flexible, easy to modify or extend
instruction set.
• Easier debugging and fault diagnosis.
• Disadvantages: Slower than hardwired CU,
expensive, longer design time.

18. Comparison: Hardwired vs
Microprogrammed
• Hardwired CU: Faster, less flexible, complex to
modify.
• Microprogrammed CU: Slower, more flexible,
easier to modify and debug.