Design of control unit.pptx
- 1. DISCOVER . LEARN . EMPOWER DESIGN OF CONTROL UNIT University Institute of Engineering DEPARTMENT OF COMPUTER SCIENCE & ENGINEERING Bachelor of Engineering (Computer Science & Engineering) Subject Name: Computer Organization & Architecture Subject Code: CST-252
- 2. Contents • Hardwired control unit • Micro-Programmed control unit • Comparative study 2
- 3. Introduction-Prerequisites • Functional Units of the Computer • CPU, ALU, Memory etc. • Memory Units and its types • Main memory • Secondary memory • Cache • Virtual • Multiprogramming environment 3
- 4. CONTROL UNIT: Implementation • Hardwired • Micro-programmed 4 Instruction code Combinational Logic Circuits Memory Sequence Counter . . Control signals Control signals Next Address Generator (sequencer) CAR Control Memory CDR Decoding Circuit Memory . . CAR: Control Address Register CDR: Control Data Register Instruction code
- 5. • Control signals • Group of bits used to select paths in multiplexers, decoders, arithmetic logic units • Control variables • Binary variables specify microoperations • Certain microoperations initiated while others idle • Control word • String of 1’s and 0’s represent control variables 5 MICRO PROGRAMMED CONTROL UNIT
- 6. Contd.. • Control memory • Memory contains control words • Microinstructions • Control words stored in control memory • Specify control signals for execution of micro operations • Micro program • Sequence of microinstructions 6
- 7. • Read-only memory (ROM) • Content of word in ROM at given address specifies microinstruction • Each computer instruction initiates series of microinstructions (microprogram) in control memory • These microinstructions generate microoperations to • Fetch instruction from main memory • Evaluate effective address • Execute operation specified by instruction • Return control to fetch phase for next instruction 7 CONTROL MEMORY
- 8. 8 Control word Next Address Generator (sequencer) CAR Control Memory (ROM) CDR External input Control memory Contains microprograms (set of microinstructions) Microinstruction contains Bits that initiate microoperations Bits that determine address of next microinstruction Control address register (CAR) Specifies address of next microinstruction
- 9. • Next address generator (microprogram sequencer) • Determines address sequence for control memory • Micro program sequencer functions • Increment CAR by one • Transfer external address into CAR • Load initial address into CAR to start control operations 9 Contd..
- 10. • Control data register (CDR)- or pipeline register • Holds microinstruction read from control memory • Allows execution of micro operations specified by control word simultaneously with generation of next microinstruction • Control unit can operate without CDR 10 Contd.. Control word Next Address Generator (sequencer) CAR Control Memory (ROM) External input
- 11. • Routine • Group of microinstructions stored in control memory • Each computer instruction has its own micro program routine to generate micro operations that execute the instruction 11 MICRO PROGRAM ROUTINES
- 12. DESIGN OF CONTROL UNIT 12 microoperation fields 3 x 8 decoder 7 6 5 4 3 2 1 0 F1 3 x 8 decoder 7 6 5 4 3 2 1 0 F2 3 x 8 decoder 7 6 5 4 3 2 1 0 F3 Arithmetic logic and shift unit AND ADD DRTAC AC Load From PC From DR(0-10) Select 0 1 Multiplexers AR Load Clock AC DR DRTAR PCTAR
- 13. 13 Input Logic for Micro-program Sequencer Input logic I0 I1 T MUX2 Select 1 I S Z Test CD Field of CS From CPU BR field of CS L(load SBR with PC) for subroutine Call S0 S1 for next address selection I1I0T Meaning Source of Address S1S0 L 000 In-Line CAR+1 00 0 001 JMP CS(AD) 01 0 010 In-Line CAR+1 00 0 011 CALL CS(AD) and SBR <- CAR+1 01 1 10x RET SBR 10 0 11x MAP DR(11-14) 11 0 L S1 = I1 S0 = I0I1 + I1’T L = I1’I0T Input Logic
- 14. • Sequencing & micro-operation logic gets complex • Difficult to design, prototype, and test • Resultant design is inflexible, and difficult to build upon (Pipeline, multiple computation units, etc.) • Adding new instructions requires major design and adds complexity quickly 14 Problems with hardwired control unit
- 15. Advantage: • Simplifies design of control unit • Cheaper • Less error-prone • Easier to modify Disadvantage: • Slower 15 Advantages and Disadvantages of Microprogramming
- 16. Micro programmed control • Micro programmed control is a control mechanism to generate control signals by using a memory called control storage (CS), which contains the control signals. Although micro programmed control seems to be advantageous to CISC machines, since CISC requires systematic development of sophisticated control signals, there is no intrinsic difference between these 2 control mechanisms. Hard-wired control • Hardwired control is a control mechanism to generate control signals by using appropriate finite state machine (FSM). The pair of "microinstruction-register" and "control storage address register" can be regarded as a "state register" for the hardwired control. Note that the control storage can be regarded as a kind of combinational logic circuit. We can assign any 0, 1 values to each output corresponding to each address, which can be regarded as the input for a combinational logic circuit. This is a truth table 16 Comparative Study
- 17. APPLICATIONS • Computer organization and architecture course deals with instruction set architecture, micro architecture and efficient implementation of micro architecture. • Understanding the computer architecture concepts is essential for students interested in hardware, processor design, compilers, and operating systems. 17
- 18. REFERENCES Reference Books: 1. J.P. Hayes, “Computer Architecture and Organization”, Third Edition. 2. Mano, M., “Computer System Architecture”, Third Edition, Prentice Hall. 3. Stallings, W., “Computer Organization and Architecture”, Eighth Edition, Pearson Education. Text Books: 1. Carpinelli J.D,” Computer systems organization &Architecture”, Fourth Edition, Addison Wesley. 2. Patterson and Hennessy, “Computer Architecture” , Fifth Edition Morgaon Kauffman. Reference Website 1. https://www.geeksforgeeks.org/computer-organization-and-architecture-tutorials/ 18
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