Microprogrammed Control Unit, Lecture notes of Computer Vision

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WHY AM I TEACHING WHAT I AM TEACHING COMPUTER ORGANIZATION VNRVJIET: ECE : CO WIT & WIL Contd.... Unit 3

Micro-programmed Control Unit

Unit-

Microprogrammed Control: Control memory, address sequencing, micro program example, and design of control unit, hardwired control, and micro programmed control.

Important Terms

• (^) Hardwired Control Unit : When the control signals are generated by hardware using conventional logic design techniques, the control unit is said to be hardwired.
• (^) Micro programmed control unit : A control unit whose binary control variables are stored in memory is called a micro programmed control unit.
• (^) Dynamic microprogramming : A more advanced development known as dynamic micro-programming permits a micro-program 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 micro-programmed control unit to store the micro- program.
• (^) Writeable Control Memory: Control Storage whose contents can be modified, allow the change in micro- program 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.

• (^) Micro program: A sequence of microinstructions constitutes a micro-program. Since alterations of the microprogram are not needed once the control unit is in operation, the control memory can be a read-only memory (ROM). ROM words are made permanent during the hardware production of the unit. The use of a micro program involves placing all control variables in words of ROM for use by the control unit through successive read operations. The content of the word in ROM at a given address specifies a microinstruction.
• (^) Microcode: Microinstructions can be saved by employing subroutines that use common sections of microcode. For example, the sequence of micro operations needed to generate the effective address of the operand for an instruction is common to all memory reference instructions. This sequence could be a subroutine that is called from within many other routines to execute the effective address computation.

Micro-programmed Control

• (^) Control Memory
• (^) Address Sequencing
• (^) Micro-program Example
• (^) Design of Control Unit

Figure: Computer Hardware Configuration MUX AR 10 0 PC 10 0 Address Memory 2048 x 16 MUX DR 15 0 Arithmetic logic and shift unit AC (^15 ) SBR 6 0 CAR 6 0 Control memory 128 x 20 Control unit (For Reference)

Control Unit for the basic computer I Instruction Register (IR) 15 14 13 12 11 - 0 3 x 8 decoder 7 6 5 4 3 2 1 0 Control logic gates D 0 15 14.... 2 1 0 4 x 16 Sequence decoder 4-bit sequence counter (SC) Increment (INR) Clear (CLR) Clock Other inputs Control outputs D T T 7 15 0 CU consists of:

• Two decoders
• A sequence counter
• No. of control logic gates (For Reference)

• (^) Micro programmed control unit
  • (^) A control unit whose binary control variables are stored in memory.
• (^) Control memory can be a read-only memory (ROM)
• (^) But Dynamic microprogramming (writable control memory) - (^) permits a microprogram to be loaded initially from an auxiliary memory such as a magnetic disk.

• (^) A microprogrammed control unit have two separate memories: - (^) main memory - (^) control memory
• (^) Main memory
  • (^) is available to the user for storing the programs.
  • (^) contents may alter when the data are manipulated
  • (^) user's program in it consists of machine instructions and data

TABLE: Symbolic Microprogram (Partial) (For Reference)

Microprogrammed control Organization

• (^) Control Address Register
  • (^) specifies the address of the microinstruction
• (^) Control Data Register
  • (^) holds the Microinstruction read from Memory
  • (^) Called as Pipeline Register.
• (^) Next Address
  • (^) must be determined, after these operations are executed
  • (^) may also be a function of external input conditions Figure: Microprogrammed control organization

• (^) System can operate without the control data

register by applying a single-phase clock to the

address register.

• (^) Control word and next-address information are

taken directly from the control memory.

• (^) ROM (Control Memory) operates as a

Combinational Circuit, with the address value as the

input and the corresponding word as the output.

• (^) In the example, assume a single-phase clock,
  • (^) Only Address Register receives clock pulses
  • (^) So NO need to use a Control Data Register
  • (^) Sequencer and Control Memory do NOT need a clock as those are combinational circuits

• (^) Main advantage of the micro-programmed control - (^) once the hardware configuration is established, there should be NO NEED for further HARDWARE or WIRING CHANGES. - (^) If we want to establish a different control sequence for the system, all we need to do is specify a DIFFERENT SET OF MICROINSTRUCTIONS (MICROPROGRAM) in control memory.