Control Memory | Computer Architecture & Organisation (CAO) - Computer Science Engineering (CSE) PDF Download

Control Memory

• The function of the control unit in a digital computer is to initiate sequences of microoperations.
• When the control signals are generated by hardware using conventional logic design techniques, the control unit is said to be hardwired.
• Microprogramming is a second alternative for designing the control unit of a digital computer.
  • The principle of microprogramming is an elegant and systematic method for controlling the microoperation sequences in a digital computer.
• In a bus-organized systems, the control signals that specify microoperations are groups of bits that select the paths in multiplexers, decoders, and arithmetic logic units.
• A control unit whose binary control variables are stored in memory is called a microprogrammed control unit.
• A memory that is part of a control unit is referred to as a control memory.
  • Each word in control memory contains within it a microinstruction.
  • A sequence of microinstructions constitutes a microprogram.\
  • Can be either read-only memory(ROM) or writable control memory (dynamic microprogramming)
• A computer that employs a microprogrammed control unit will have two separate memories:
  • A main memory
  • A control memory
• The general configuration of a microprogrammed control unit is demonstrated in the block diagram of Fig. 3.1.
  • The control memory is assumed to be a ROM, within which all control information is permanently stored.
  • The control address register specifies the address of the microinstruction.
  • The control data register holds the microinstruction read from memory.
• Thus a microinstruction contains bits for initiating microoperations in the data processor part and bits that determine the address sequence for the control memory.

Extra Stuff:

Microprogram

• Program stored in memory that generates all the control signals required to execute the instruction set correctly
• Consists of microinstructions

Microinstruction

• Contains a control word and a sequencing word
  • Control Word - All the control information required for one clock cycle
  • Sequencing Word - Information needed to decide the next microinstruction address
• Vocabulary to write a microprogram 

Control Memory (Control Storage: CS)

• Storage in the microprogrammed control unit to store the microprogram

Writeable Control Memory(Writeable Control Storage:WCS)

• CS whose contents can be modified
  • Allows the microprogram can be changed
  • Instruction set can be changed or modified

 Dynamic Microprogramming

• Computer system whose control unit is implemented with a microprogram in WCS
• Microprogram can be changed by a systems programmer or a user  

Microrogrammed Sequencer

• The next address generator is sometimes called a microprogram sequencer, as it determines the address sequence that is read from control memory.
• Typical functions of a microprogram sequencer are:
  • Incrementing the control address register by one
  • Loading into the control address register an address from control memory
  • Transferring an external address
  • Loading an initial address to start the control operations

Pipeline Register

• The data register is sometimes called a pipeline register.
  • It allows the execution of the microoperations specified by the control word simultaneously with the generation of the next microinstruction.
    • This configuration requires a two-phase clock
  • The system can operate by applying a single-phase clock to the address register.
    • Without the control data register
    • Thus, the control word and next-address information are taken directly from the control memory.

Advantages

• The main advantage of the microprogrammed control is the fact that once the hardware configuration is established; there should be no need for further hardware or wiring change.
• Most computers based on the reduced instruction set computer (RISC) architecture concept use hardwired control rather than a control memory with a microprogram. (Why?)

A Microprogram Control Unit that determines the Microinstruction Address to be executed in the next clock cycle

• In-line Sequencing
• Branch
• Conditional Branch
• Subroutine
• Loop
• Instruction OP-code mapping