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Jayson Wright
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1 BASIC PROCESSING UNIT Control Unit has two major functions: To control the sequencing of information-processing tasks performed by machine Guiding and supervising each unit to make sure that each unit carries out every operation assigned at the proper time Control of a computer can be distributed or centralized Early computers used distributed control and a lot of redundant hardware

2 PROCESSING UNIT FEATURES Execution of a Complete Instruction o Add (R3), R1 /* R1 [R1] + [[R3]] o Adds the contents of a memory location pointed to by R3 to register R1. o Sequence of control steps: 1. PCout, MARin, Read, Select4, Add, Zin 2. Zout, PCin, Yin, WMFC 3. MDRout, IRin 4. R3out, MARin, Read 5. R1out, Yin, WMFC 6. MDRout, SelectY, Add, Zin

7. Zout, R1in, End Multiple bus architecture Single-bus structure: Control sequences are long as only one data item can be transferred over the bus in a clock cycle.

3 7. Zout, R1in, End Multiple bus architecture Single-bus structure: Control sequences are long as only one data item can be transferred over the bus in a clock cycle. Figure on next slide shows a three-bus structure. All registers are combined into a single block called register file with three ports: 2 outputs allowing 2 registers to be accessed simultaneously and have their contents put on buses A and B, and 1 input allowing data on bus C to be loaded into a third register.

4 Buses A and B are used to transfer source operands to the A and B inputs of ALU, and result transferred to destination over bus C. For the ALU, R=A (or R=B) means that its A (or B) input is passed unmodified to bus C. Add R4, R5, R6 /* R6 [R4] + [R5] o Adds the contents of R4 and R5 to R6. Sequence of control steps: o PCout, R=B, MARin, Read, IncPC o WMFC o MDRoutB, R=B, IRin o R4outA, R5outB, SelectA, Add, R6in, End

5 Hardwired control The control logic is implemented with gates, F/Fs, decoders, and other digital circuits To execute instructions, a computer's processor must generate the control signals used to perform the processor's actions in the proper sequence. This sequence of actions can either be executed by another processor's software or in hardware. Hardware signals are generated either by hardwired control, in which the instruction bits directly generate the signals hardwired control usually was implemented using discrete components, flip-chips, or even rotating discs or drums. This can be generally done by two methods. The classical method of sequential circuit design. It attempts to minimize theamount of hardwire, in particular, by using only log 2 p flip flops to realize a p state circuit. An approach that uses one flip flop per state. While expensive in terms of flip flops, this method simplifies controller unit design and debuggi Combinational logic Determine outputs at each state. Determine next state. Storage elements Maintain state representation

6 State Machine Inputs Combinational Logic Circuit Outputs Clock Storage Elements Hardwired Implementation The Cycles (Fetch, Indirect, Execute, Interrupt) are constructed as a State Machine The Individual instruction executions can be constructed as State Machines

7 Common sections can be shared. There is a lot of similarity One ALU is implemented. All instructions share it

8 Microprogrammed control A control unit whose binary control variables are stored in memory (control memory). The Control Memory contains sequences of microinstructions that provide the control signals to execute instruction cycles, e.g. Fetch, Indirect, Execute, and Interrupt Microinstruction : Control Word in Control Memory The microinstruction specifies one or more microoperations Microprogram A sequence of microinstruction» Dynamic microprogramming : Control Memory = RAM n RAM can be used for writing (to change a writable control memory) n Microprogram is loaded initially from an auxiliary memory such as a magnetic disk» Static microprogramming : Control Memory = ROM n Control words in ROM are made permanent during the hardware production. Microprogrammed control Organization : 1) Control Memory» A memory is part of a control unit : Microprogram Computer Memory (employs a microprogrammed control unit) --Main Memory : for storing user program (Machine instruction/data) --Control Memory : for storing microprogram (Microinstruction) 2) Control Address Register» Specify the address of the microinstruction 3) Sequencer (= Next Address Generator)» Determine the address sequence that is read from control memory

9 » Next address of the next microinstruction can be specified several way depending on the sequencer input : 4) Control Data Register (= Pipeline Register )» Hold the microinstruction read from control memory» Allows the execution of the microoperations specified by the control word simultaneously with the generation of the next microinstruction

11 Microprogrammed control --Typical Microinstruction Formats --Micro-instruction Types Each micro-instruction specifies single (or few) micro-operations to be performed (vertical micro-programming) Each micro-instruction specifies many different micro-operations to be performed in parallel (horizontal micro-programming) Vertical Micro-programming Width is narrow n control signals encoded into log2 n bits Limited ability to express parallelism Considerable encoding of control information requires external memory word decoder to identify the exact control line being manipulated

12 Function Codes Micro-instruction Address Jump Condition Horizontal Micro-programming Wide memory word High degree of parallel operations possible Little encoding of control information Internal CPU Control Signals Micro-instruction Address System Bus Control Signals Jump Condition

13 Nanoprogramming Use a 2-level control storage organization Top level is a vertical format memory» Output of the top level memory drives the address register of the bottom (nano-level) memory Nanomemory uses the horizontal format» Produces the actual control signal outputs The advantage to this approach is significant saving in control memory size (bits) Disadvantage is more complexity and slower operation (doing 2 memory accesses fro each microinstruction) Example: Supppose that a system is being designed with 200 control points and 2048 microinstructions Assume that only 256 different combinations of control points are ever used A single-level control memory would require 2048x200=409,600 storage bits A nano programmed system would use» Microstore of size 2048x8=16k» Nanostore of size 256x200=51200» Total size = 67,584 storage bits Nano programming has been used in many CISC microprocessors

15 Nano programmed machine

Week 4: Assignment Solutions

Week 4: Assignment Solutions 1. Which of the following statements are true for horizontal microinstruction encoding? a. If there are kcontrol signals, every control word stored in control memory (CM) consists