Micro-Programmed Control Unit

  • A control unit whose binary control variables are stored in memory is called a micro programmed control unit.
  • Dynamic microprogramming: It permits a microprogram to be loaded initially from an auxiliary memory such as a magnetic disk. Control units that use dynamic microprogramming employ a writable control memory. This type of memory can be used for writing.
  • Control Memory: Control Memory is the storage in the microprogrammed control unit to store the microprogram.
  • Writeable Control Memory: Control Storage whose contents can be modified, allow the change in microprogram and Instruction set can be changed or modified is referred as Writeable Control Memory.
  • Control Word:The control variables at any given time can be represented by a control word string of 1 ‘s and 0’s called a control word.
  • Microoperations: In computer central processing units, micro-operations (also known as a micro-ops or μops) are detailed low-level instructions used in some designs to implement complex machine instructions (sometimes termed macro-instructions in this context).
  • Micro instruction: A symbolic microprogram can be translated into its binary equivalent by means of an assembler. Each line of the assembly language microprogram defines a symbolic microinstruction.
  • Micro program: A sequence of microinstructions constitutes a microprogram.
  • Microcode: Microinstructions can be saved by employing subroutines that use common sections of microcode.

Organization of micro programmed control unit

  • The address of micro-instruction that is to be executed is stored in the control address register (CAR).
  • Micro-instruction corresponding to the address stored in CAR is fetched from control memory and is stored in the control data register (CDR).
  • This micro-instruction contains control word to execute one or more micro-operations.
  • After the execution of all micro-operations of micro-instruction, the address of next micro-instruction is located.

Advantages

  1. The design of micro-program control unit is less complex because micro-programs are implemented using software routines.
  2. The micro-programmed control unit is more flexible because design modifications, correction and enhancement is easily possible.
  3. The new or modified instruction set of CPU can be easily implemented by simply rewriting or modifying the contents of control memory.
  4. The fault can be easily diagnosed in the micro-program control unit using diagnostics tools by maintaining the contents of flags, registers and counters.

Disadvantages

  1. The micro-program control unit is slower than hardwired control unit.That means to execute an instruction in micro-program control unit requires more time.
  2. The micro-program control unit is expensive than hardwired control unit in case of limited hardware resources.
  3. The design duration of micro-program control unit is more than hardwired control unit for smaller CPU.

Types of Micro-programmed Control Unit – Based on the type of Control Word stored in the Control Memory (CM), it is classified into two types :
1. Horizontal Micro-programmed control Unit :
The control signals are represented in the decoded binary format that is 1 bit/CS. Example: If 53 Control signals are present in the processor than 53 bits are required. More than 1 control signal can be enabled at a time.

  • It supports longer control word.
  • It is used in parallel processing applications.
  • It allows higher degree of parallelism. If degree is n, n CS are enabled at a time.
  • It requires no additional hardware(decoders). It means it is faster than Verical Microprogrammed.

2. Vertical Micro-programmed control Unit :
The control signals re represented in the encoded binary format. For N control signals- Log2(N) bits are required.

  • It supports shorter control words.
  • It supports easy implementation of new control signals therefore it is more flexible.
  • It allows low degree of parallelism i.e., degree of parallelism is either 0 or 1.
  • Requires an additional hardware (decoders) to generate control signals, it implies it is slower than horizontal microprogrammed.

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