Processing Unit. Unit II

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1 Processing Unit Unit II

2 Execution of a complete instruction Add (R3), R1 - Adds the contents of a memory location pointed to by R3 to register R1 and store the result in R1. 1. Fetch the instruction 2. Fetch the operand 3. Perform the addition 4. Load the result into R1

3 Control Sequence: Add(R3),R1 Step Action 1 PC out, MAR in, Read, Select4, Add, Z in 2 Z out,pc in, Y in,wmfc 3 MDR out, IR in 4 R3 out, MAR in,read 5 MDR ine, WMFC, MDR out,, Y in 6 R1 out, SelectY, Add, Z in 7 Z out, R1 in,end

4 Sum of N numbers 100 Move N, R1 104 Move #NUM1, R2 108 Clear R0 Loop 112 ADD (R2), R0 116 ADD #4, R2 120 Decrement R1 124 Branch > 0 Loop 128 Move R0, SUM : SUM 200 N NUM1 208 NUM2 212 : Branch Target Address = PC + branch offset

5 Branch Instruction Unconditional branch instruction: jump PC out, MAR in, Read, Select4, Add, Z in 2. Z out,pc in, Y in,wmfc 3. MDR out, IR in 4. offset_field_of_ir out, Add, Z in 5. Z out, PC in, End : 4000 Jump Offset = 46 : 4050

6 Conditional Branch Branch on negative Steps: Step 4 is changed offset_field_of_IR out, Add, Z in, if N=0 then end 5.

7 Multiple Bus Organization

8 ADD R4,R5,R6 1. PC out, R=B, MAR in, Read, IncPC 2. WMFC 3. MDR outb, R=B, IR in 4. R4 outa, R5 outb, SelectA, Add, R6 in, End

9 Generating Control signals Types: 1. Hardwired Control 2. Microprogrammed Control

10 Hardwired Control Clock CLK Control Step Counter : External Inputs IR : Decoder/ Encoder : Condition codes Control Signals

11 Decoding & Encoding Functions Reset Enco der RUN End

12 Generation of Control signal Z in = T1+T6.Add+T4.BR+ T4 Branch Add T6 T1 Z in

13 End Control signal End= T7.Add+T5.BR+(T5.N+T4.N).BRN+..

14 Advantages of Hardwired control High speed unit Disadvantages: Little flexibility Complexity of the instruction set is limited

15 Microprogrammed Control Control signals are generated by a program similar to machine language programs

16 Micro Programmed Control Micro Instruction Horizontal Organization

17 Organization of a Micro programmed Control unit µpc - Micro Program Counter

18 Control Word(CW): A word whose individual bits represent the various control signals Microroutine: A sequence of CWs corresponding to the control sequence of a machine instruction Microinstruction: The individual control word in the microroutine Control Store: The microroutines for all instructions in the instruction set of a computer are stored in a special memory called the control store.

19 Microroutine Instruction: Branch < 0 Address Microinstruction 0 PC out, MAR in, Read, Select4, Add, Z in 1 Z out,pc in, Y in,wmfc 2 MDR out, IR in 3 Branch to starting address of appropriate Microroutine If N=0, then branch to microinstruction 0 26 Offset-field-of-IR out, SelectY, Add, Z in 27 Z out, PC in, End

20 Control unit in micro programmed control µpc - Micro Program Counter

21 Vertical Organization Micro instruction

22 Control Sequence ADD (R3),R1

23 Micro routine

24 Vertical organization Highly encoded schemes that use compact codes to specify only a small number of control functions in each microinstruction Advantages: Highly encoded schemes Disadvantages: Slower operating speed Limited Parallelism

25 Horizontal organization The minimally encoded scheme and many resources can be controlled with a single microinstruction Advantages: Minimally encoded scheme Higher operating speed Parallel use of resources

26 Nanoprogramming Microprogram of Size AB

27 NanoProgramming n -the number of unique micro instructions Where n < A

MC9211Computer Organization. Unit 4 Lesson 1 Processor Design

MC9211Computer Organization. Unit 4 Lesson 1 Processor Design MC92Computer Organization Unit 4 Lesson Processor Design Basic Processing Unit Connection Between the Processor and the Memory Memory MAR PC MDR R Control IR R Processo ALU R n- n general purpose registers