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Controller Implementation

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1 Controller Implementation
Alternative controller FSM implementation approaches based on: Classical Moore and Mealy machines Time-State: Divide and Conquer Jump counters Microprogramming (ROM) based approaches Branch sequencers Horizontal microcode Vertical microcode 2015 – Lec #24: Microprogramming

2 2015 – Lec #24: Microprogramming
Branch Sequencers Concept Implement Next State Logic via ROM Address ROM with current state and inputs Problem: ROM doubles in size for each additional input Note: Jump counter trades off ROM size vs. external logic Only jump states kept in ROM Even in hybrid approach, state + input subset form ROM address Branch Sequencer: between the extremes Next State stored in ROM Each state limited to small number of next states Always a power of 2 Observe: only a small set of inputs are examined in any state 2015 – Lec #24: Microprogramming

3 2015 – Lec #24: Microprogramming
Branch Sequencers 4 Way Branch Sequencer 64 Word ROM I Mux n N Mux b C S p a0 a o i u a1 x11 Z a b a b x10 Y n g a b t a b a2 s x01 X t n a3 Z x00 W r a W X Y a4 o l a5 l s state Current State selects two inputs to form part of ROM address These select one of four possible next states (and output sets) Every state has exactly four possible next states 2015 – Lec #24: Microprogramming

4 2015 – Lec #24: Microprogramming
Branch Sequencer Processor CPU Design Example Alpha, Beta multiplexer input setup 2015 – Lec #24: Microprogramming

5 2015 – Lec #24: Microprogramming
Example Processor FSM ROM ADDRESS ROM CONTENTS (Reset, Current State, a, b) Next State Register Transfer Operations RES X X 0001 (IF0) PC  MAR, PC + 1  PC IF (IF0) (IF1) MAR  Mem, Read, Request IF (IF2) MAR  Mem, Read, Request (IF1) Mem  MBR IF (IF2) (OD) MBR  IR OD (LD0) IR  MAR (ST0) IR  MAR, AC  MBR (AD0) IR  MAR (BR0) IR  MAR 2015 – Lec #24: Microprogramming

6 2015 – Lec #24: Microprogramming
Example Processor FSM ROM ADDRESS ROM CONTENTS (Reset, Current State, a, b) Next State Register Transfer Operations LD X X 0110 (LD1) MAR  Mem, Read, Request LD (LD2) Mem  MBR (LD1) MAR  Mem, Read, Request LD X X 0000 (RES) MBR  AC ST X X 1001 (ST1) MAR  Mem, Write, Request, MBR  Mem ST (RES) (ST1) MAR  Mem, Write, Request, MBR  Mem AD X X 1011 (AD1) MAR  Mem, Read, Request AD (AD2) (AD1) MAR  Mem, Read, Request AD X X 0000 (RES) MBR + AC  AC BR (RES) (RES) IR  PC 2015 – Lec #24: Microprogramming

7 2015 – Lec #24: Microprogramming
Branch Sequencers Alternative Horizontal Implementation Input MUX controlled by encoded signals, not state Much fewer inputs than unique states! In example FSM, input MUX can be 2:1! Adding length to ROM word saves on bits vs. doubling words Vertical format: (14 + 4) x 64 = 1152 ROM bits Horizontal format: ( x 4 + 2) x 16 = 512 ROM bits 2015 – Lec #24: Microprogramming

8 2015 – Lec #24: Microprogramming
How to organize the control signals Implement control signals by storing 1's and 0's in a ROM Horizontal vs. vertical microprogramming Horizontal: 1 ROM output for each control signal Vertical: encoded control signals in ROM, decoded externally some mutually exclusive signals can be combined helps reduce ROM length 2015 – Lec #24: Microprogramming

9 2015 – Lec #24: Microprogramming
Register Transfer/Microoperations 14 Register Transfer operations become 22 Microoperations: PC  ABUS IR  ABUS MBR  ABUS RBUS  AC AC  ALU A MBUS  ALU B ALU ADD ALU PASS B MAR  Address Bus MBR  Data Bus ABUS  IR ABUS  MAR Data Bus  MBR RBUS  MBR MBR  MBUS 0 PC PC + 1  PC ABUS  PC Read/Write Request AC  RBUS ALU Result  RBUS 2015 – Lec #24: Microprogramming

10 Horizontal Microprogramming
Horizontal Branch Sequencer ,  Mux bits 4 x 4 Next State bits 22 Control operation bits 40 bits total RBUS Address Bus MBR ALU B Data Bus ABUS AC MAR MBR MBUS PC IR PC ABUS ALU A RBUS ABUS PC ALU PASS B Read/Write ALU Result ALU ADD Data Bus Request mux mux MBR RBUS MBUS PC + 1 ABUS MAR MBR ABUS ABUS RBUS MBR PC AC AC a IR b Next States A0 A1 A2 A3 2015 – Lec #24: Microprogramming

11 Horizontal Microprogramming
Moore Processor ROM RBUS Address Bus MBR ALU B Data Bus ABUS AC MAR MBR MBUS PC PC ABUS ALU A IR ABUS RBUS ALU PASS B PC Read/Write ALU Result ALU ADD Request mux Data Bus mux Next States MBR RBUS MBUS PC + 1 Current State PC AC MAR MBR ABUS ABUS RBUS MBR ABUS a b IR AC (Address) A0 A1 A2 A3 RES (0000) 0 0 0001 0001 0001 0001 1 IF0 (0001) 0 0 0010 0010 0010 0010 1 1 IF1 (0010) 0 0 0010 0010 0011 0011 IF2 (0011) 0 0 0100 0100 0011 0011 1 IF3 (0100) 0 0 0100 0100 0101 0101 1 OD (0101) 1 1 0110 1001 1011 1110 LD0 (0110) 0 0 0111 0111 0111 0111 1 LD1 (0111) 0 0 1000 1000 0111 0111 1 LD2 (1000) 0 0 0001 0001 0001 0001 1 1 ST0 (1001) 0 0 1010 1010 1010 1010 1 1 ST1 (1010) 0 0 0001 0001 1010 1010 1 AD0 (1011) 0 0 1100 1100 1100 1100 1 AD1 (1100) 0 0 1101 1101 1100 1100 1 1 AD2 (1101) 0 0 0001 0001 0001 0001 1 1 BR0 (1110) 0 1 0001 1111 0001 1111 BR1 (1111) 0 0 0001 0001 0001 0001 1 1 Alpha inputs: 0 = Wait, = IR<15> Beta inputs: 0 = AC<15>, 1 = IR<14> 2015 – Lec #24: Microprogramming

12 Horizontal Microprogramming
Advantages: most flexibility -- complete parallel access to datapath control points Disadvantages: very long control words bits for real processors NOTE: Not all microoperation combinations make sense! Output Encodings: Group mutually exclusive signals Use external logic to decode Example: 0  PC, PC + 1  PC, ABUS  PC mutually exclusive Save ROM bit with external 2:4 Decoder 2015 – Lec #24: Microprogramming

13 Horizontal Microprogramming
Partially Encoded Control Outputs 2015 – Lec #24: Microprogramming

14 Vertical Microprogramming
More extensive encoding to reduce ROM word length Typically use multiple microword formats: Horizontal microcode -- next state + control bits in same word Separate formats for control outputs and "branch jumps" may require several microwords in a sequence to implement same function as single horizontal word In the extreme, very much like assembly language programming 2015 – Lec #24: Microprogramming

15 Vertical Microprogramming
Branch Jump Compare indicated signal to 0 or 1 Register Transfer Source, Destination, Operation 10 ROM Bits 2015 – Lec #24: Microprogramming

16 Vertical Microprogramming
ROM ADDRESS SYMBOLIC CONTENTS BINARY CONTENTS RES RT PC  MAR, PC +1  PC IF0 RT MAR  M, Read BJ Wait=0, IF IF1 RT MAR  M, M  MBR, Read BJ Wait=1, IF IF2 RT MBR  IR BJ Wait=0, IF RT IR  MAR OD BJ IR<15>=1, OD BJ IR<14>=1, ST LD0 RT MAR  M, Read LD1 RT MAR  M, M  MBR, Read BJ Wait=1, LD LD2 RT MBR  AC BJ Wait=0, RES BJ Wait=1, RES 2015 – Lec #24: Microprogramming

17 Vertical Microprogramming
ROM ADDRESS SYMBOLIC CONTENTS BINARY CONTENTS ST0 RT AC  MBR RT MAR  M, MBR  M, Write ST1 RT MAR  M, MBR  M, Write BJ Wait=0, RES BJ Wait=1, ST OD1 BJ IR<14>=1, BR AD0 RT MAR  M, Read AD1 RT MAR  M, M  MBR, Read BJ Wait=1, AD AD2 RT AC + MBR  AC BJ Wait=0, RES BJ Wait=1, RES BR0 BJ AC<15>=0, RES RT IR  PC BJ AC<15>=1, RES 31 words x 10 ROM bits = 310 bits total versus 16 x 38 = 608 bits horizontal 2015 – Lec #24: Microprogramming

18 2015 – Lec #24: Microprogramming
Vertical Programming Controller Block Diagram 2015 – Lec #24: Microprogramming

19 Vertical Microprogramming
Condition Logic 2015 – Lec #24: Microprogramming

20 Vertical Microprogramming
Writeable Control Store Part of control store addresses map into RAM Allows assembly language programmer to implement own instructions Extend "native" instruction set with application specific instructions Requires considerable sophistication to write microcode Not a popular approach with today's processors Make the native instruction set simple and fast Write "higher level" functions as assembly language sequences 2015 – Lec #24: Microprogramming

21 Controller Implementation Summary
Control Unit Organization Register transfer operation Classical Moore and Mealy machines Time State Approach Jump Counter Branch Sequencers Horizontal and Vertical Microprogramming 2015 – Lec #24: Microprogramming

22 Winter 2005 – Lecture #14: Review - 22
Design hierarchy digital system data-path control code registers state registers combinational logic multiplexer comparator register logic switching networks Winter 2005 – Lecture #14: Review - 22

23 Combinational Logic Implementation
K-map method to map truth tables into minimized gate level descriptions Alternative implementation approaches: Two-level logic, multi-level logic, logic implementation with multiplexers Programmable logic in the form of PLAs, ROMs, Muxes, … Field programmable logic in the form of devices like Xilinx Combinational logic building blocks Arithmetic and logic units, including adders/subtractors and other arithmetic functions (e.g., combinational multipliers) Winter 2005 – Lecture #14: Review - 23

24 Sequential Logic Implementation
Models for representing sequential circuits Abstraction of sequential elements Finite state machines and their state diagrams Inputs/outputs Mealy, Moore, and synchronous Mealy machines Finite state machine design procedure Deriving state diagram Deriving state transition table Determining next state and output functions Implementing combinational logic Sequential logic building blocks Registers, Register files (with multiple read and write ports), Shifters, Counters, RAMs Arbitrators Winter 2005 – Lecture #14: Review - 24

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