CPSC 321 Computer Architecture and Engineering Lecture 7 Designing a Multi-cycle Processor

1
CPSC 321Computer Architecture and
EngineeringLecture 7 Designing a Multi-cycle
Processor

• Instructor Praveen Bhojwani
• Adapted from the lecture notes of John
  Kubiatowicz (UCB)

2
Recap A Single Cycle Datapath
Instructionlt310gt
nPC_sel
Instruction Fetch Unit
Rt
Rd
lt2125gt
lt1620gt
lt1115gt
lt015gt
Clk
RegDst
0
1
Mux
Imm16
Rd
Rt
Rs
Rs
Rt
RegWr
ALUctr
5
5
5
MemtoReg
busA
Equal
MemWr
Rw
Ra
Rb
busW
32
32 32-bit Registers
0
ALU
32
busB
32
0
Clk
Mux
32
Mux
32
1
WrEn
Adr
1
Data In
32
Data Memory
Extender
imm16
32
16
Clk
ALUSrc
ExtOp
3
Recap The Truth Table for the Main Control
4
The Big Picture Where are We Now?

• The Five Classic Components of a Computer
• Todays Topic Designing the Datapath for the
  Multiple Clock Cycle Datapath

Processor
Input
Control
Memory
Datapath
Output
5
Abstract View of our single cycle processor
Main Control
op
ALU control
fun
ALUSrc
Equal
ExtOp
MemWr
MemWr
MemRd
RegDst
RegWr
nPC_sel
ALUctr
Reg. Wrt
ALU
Register Fetch
Ext
Mem Access
PC
Instruction Fetch
Next PC
Result Store
Data Mem

• looks like a FSM with PC as state

6
Whats wrong with our CPI1 processor?
Arithmetic Logical
PC
Reg File
Inst Memory
ALU
setup
mux
mux
Load
PC
Inst Memory
ALU
Data Mem
Reg File
setup
mux
mux
Critical Path
Store
PC
Inst Memory
ALU
Data Mem
Reg File
mux
Branch
PC
Inst Memory
cmp
Reg File
mux

• Long Cycle Time
• All instructions take as much time as the slowest
• Real memory is not as nice as our idealized
  memory
• cannot always get the job done in one (short)
  cycle

7
Reducing Cycle Time

• Cut combinational dependency graph and insert
  register / latch
• Do same work in two fast cycles, rather than one
  slow one
• May be able to short-circuit path and remove some
  components for some instructions!

storage element
Acyclic Combinational Logic (A)
?
storage element
Acyclic Combinational Logic (B)
storage element
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Basic Limits on Cycle Time

• Next address logic
• PC lt branch ? PC offset PC 4
• Instruction Fetch
• InstructionReg lt MemPC
• Register Access
• A lt Rrs
• ALU operation
• R lt A B

Control
MemWr
MemWr
MemRd
RegDst
RegWr
nPC_sel
ALUctr
ALUSrc
ExtOp
Reg. File
Exec
Operand Fetch
Mem Access
Instruction Fetch
PC
Next PC
Result Store
Data Mem
9
Partitioning the CPI1 Datapath

• Add registers between smallest steps
• Place enables on all registers

MemWr
MemWr
MemRd
RegDst
RegWr
nPC_sel
ALUSrc
ExtOp
ALUctr
Reg. File
Exec
Operand Fetch
Mem Access
Instruction Fetch
PC
Next PC
Result Store
Data Mem
10
Example Multicycle Datapath
Equal
nPC_sel
E
Reg File
A
PC
IR
Next PC
B
Instruction Fetch
Operand Fetch

• Critical Path ?

11
Recall Step-by-step Processor Design

• Step 1 ISA gt Logical Register Transfers
• Step 2 Components of the Datapath
• Step 3 RTL Components gt Datapath
• Step 4 Datapath Logical RTs gt Physical RTs
• Step 5 Physical RTs gt Control

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Step 4 R-type (add, sub, . . .)
inst Logical Register Transfers ADDU Rrd lt
Rrs Rrt PC lt PC 4

• Logical Register Transfer
• Physical Register Transfers

inst Physical Register Transfers IR lt
MEMpc ADDU Alt Rrs B lt Rrt S lt A
B Rrd lt S PC lt PC 4
E
Reg. File
Reg File
Exec
PC
IR
Next PC
Inst. Mem
Mem Access
Data Mem
13
Step 4 Logical immed
inst Logical Register Transfers ORI Rrt lt
Rrs OR ZExt(Im16) PC lt PC 4

• Logical Register Transfer
• Physical Register Transfers

inst Physical Register Transfers IR lt
MEMpc ORI Alt Rrs B lt Rrt S lt A or
ZExt(Im16) Rrt lt S PC lt PC 4
E
Reg. File
Reg File
Exec
PC
IR
Next PC
Inst. Mem
B
Mem Access
Data Mem
14
Step 4 Load
inst Logical Register Transfers LW Rrt lt
MEMRrs SExt(Im16) PC lt PC 4

• Logical Register Transfer
• Physical Register Transfers

E
Reg. File
Reg File
Exec
PC
IR
Next PC
Inst. Mem
B
Mem Access
Data Mem
15
Step 4 Store
inst Logical Register Transfers SW MEMRrs
SExt(Im16) lt Rrt PC lt PC 4

• Logical Register Transfer
• Physical Register Transfers

inst Physical Register Transfers IR lt
MEMpc SW Alt Rrs B lt Rrt S lt A
SExt(Im16) MEMS lt B PC lt PC 4
E
Reg. File
Reg File
Exec
PC
IR
Next PC
Inst. Mem
B
Mem Access
Data Mem
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Step 4 Branch

• Logical Register Transfer
• Physical Register Transfers

inst Logical Register Transfers BEQ if Rrs
Rrt then PC lt PC 4SExt(Im16) 00 else
PC lt PC 4
inst Physical Register Transfers IR lt
MEMpc BEQ Elt (Rrs Rrt) if !E then PC lt
PC 4 else PC ltPC4SExt(Im16)00
E
Reg. File
Reg File
A
Exec
PC
IR
Next PC
Inst. Mem
B
Mem Access
Data Mem
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Alternative data-path (book) Multiple Cycle
Datapath

• Minimizes Hardware 1 memory, 1 adder

PCWr
PCWrCond
PCSrc
BrWr
Zero
ALUSelA
MemWr
IRWr
RegWr
RegDst
IorD
1
Mux
32
PC
0
Zero
32
Rs
Ra
RAdr
5
32
32
Rt
Rb
busA
32
ALU
Ideal Memory
32
Instruction Reg
Reg File
5
32
ALU Out
4
Rt
0
32
Rw
WrAdr
32
1
32
Rd
Din
Dout
busW
32
busB
2
32
3
Imm
32
ALUOp
MemtoReg
ExtOp
ALUSelB
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Our Control Model

• State specifies control points for Register
  Transfer
• Transfer occurs upon exiting state (same falling
  edge)

inputs (conditions)
Next State Logic
State X
Register Transfer Control Points
Control State
Depends on Input
Output Logic
outputs (control points)
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Step 4 ? Control Specification for multicycle
proc
instruction fetch
IR lt MEMPC
decode / operand fetch
A lt Rrs B lt Rrt
LW
R-type
ORi
SW
BEQ
PC lt Next(PC,Equal)
S lt A fun B
S lt A or ZX
S lt A SX
S lt A SX
M lt MEMS
MEMS lt B PC lt PC 4
Rrd lt S PC lt PC 4
Rrt lt S PC lt PC 4
Rrt lt M PC lt PC 4
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Traditional FSM Controller
next state
state
op
cond
control points
Truth Table
next State
control points
11
Equal
6
State
4
op
datapath State
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Step 5 ? (datapath state diagram?? control)

• Translate RTs into control points
• Assign states
• Then go build the controller

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Mapping RTs to Control Points
IR lt MEMPC
instruction fetch
imem_rd, IRen
A lt Rrs B lt Rrt
decode
Aen, Ben, Een
LW
R-type
ORi
SW
BEQ
S lt A fun B
PC lt Next(PC,Equal)
S lt A or ZX
S lt A SX
S lt A SX
ALUfun, Sen
M lt MEMS
MEMS lt B PC lt PC 4
Rrd lt S PC lt PC 4
RegDst, RegWr, PCen
Rrt lt S PC lt PC 4
Rrt lt M PC lt PC 4
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Assigning States
instruction fetch
IR lt MEMPC
0000
decode
A lt Rrs B lt Rrt
0001
LW
R-type
ORi
SW
BEQ
PC lt Next(PC)
S lt A fun B
S lt A or ZX
S lt A SX
S lt A SX
0100
0110
1000
0011
1011
M lt MEMS
MEMS lt B PC lt PC 4
1001
1100
Rrd lt S PC lt PC 4
Rrt lt S PC lt PC 4
Rrt lt M PC lt PC 4
0101
0111
1010
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(Mostly) Detailed Control Specification
(missing?0)
State Op field Eq Next IR PC Ops Exec Mem Write-B
ack en sel A B E Ex Sr ALU S R W M M-R Wr
Dst

• 0000 ?????? ? 0001 1
• 0001 BEQ x 0011 1 1 1
• 0001 R-type x 0100 1 1 1
• 0001 ORI x 0110 1 1 1
• 0001 LW x 1000 1 1 1
• 0001 SW x 1011 1 1 1
• 0011 xxxxxx 0 0000 1 0 x 0 x
• 0011 xxxxxx 1 0000 1 1 x 0 x
• 0100 xxxxxx x 0101 0 1 fun 1
• 0101 xxxxxx x 0000 1 0 0 1 1
• 0110 xxxxxx x 0111 0 0 or 1
• 0111 xxxxxx x 0000 1 0 0 1 0
• 1000 xxxxxx x 1001 1 0 add 1
• 1001 xxxxxx x 1010 1 0 1
• 1010 xxxxxx x 0000 1 0 1 1 0
• 1011 xxxxxx x 1100 1 0 add 1
• 1100 xxxxxx x 0000 1 0 0 1 0

-all same in Moore machine
BEQ
R
ORi
LW
SW
25
Performance Evaluation

• What is the average CPI?
• state diagram gives CPI for each instruction type
• workload gives frequency of each type

Type CPIi for type Frequency CPIi x freqIi
Arith/Logic 4 40 1.6 Load 5 30 1.5 Store 4 10
0.4 branch 3 20 0.6 Average CPI 4.1
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Controller Design

• The state digrams that arise define the
  controller for an instruction set processor are
  highly structured
• Use this structure to construct a simple
  microsequencer
• Control reduces to programming this very simple
  device
• ? microprogramming

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Our Microsequencer
taken
datapath control
Z I L
Micro-PC
op-code
Map ROM
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Microprogram Control Specification
µPC Taken Next IR PC Ops Exec Mem Write-Bac
k en sel A B Ex Sr ALU S R W M M-R Wr
Dst

• 0000 ? inc 1
• 0001 0 load 1 1
• 0011 0 zero 1 0
• 0011 1 zero 1 1
• 0100 x inc 0 1 fun 1
• 0101 x zero 1 0 0 1 1
• 0110 x inc 0 0 or 1
• 0111 x zero 1 0 0 1 0
• 1000 x inc 1 0 add 1
• 1001 x inc 1 0 1
• 1010 x zero 1 0 1 1 0
• 1011 x inc 1 0 add 1
• 1100 x zero 1 0 0 1 0

BEQ
R
ORi
LW
SW
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Overview of Control

• Control may be designed using one of several
  initial representations. The choice of sequence
  control, and how logic is represented, can then
  be determined independently the control can then
  be implemented with one of several methods using
  a structured logic technique.
• Initial Representation Finite State Diagram
  Microprogram
• Sequencing Control Explicit Next State
  Microprogram counter Function Dispatch ROMs
  
• Logic Representation Logic Equations Truth Tables
• Implementation PLA ROM Technique

hardwired control
microprogrammed control
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Microprogramming (Maurice Wilkes)

• Control is the hard part of processor design
• Datapath is fairly regular and well-organized
• Memory is highly regular
• Control is irregular and global

Microprogramming -- A Particular Strategy for
Implementing the Control Unit of a processor
by "programming" at the level of register
transfer operations Microarchitecture --
Logical structure and functional capabilities of
the hardware as seen by the
microprogrammer Historical Note IBM 360
Series first to distinguish between architecture
organization Same instruction set across wide
range of implementations, each with
different cost/performance
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Macroinstruction Interpretation
User program plus Data this can change!
Main Memory
ADD SUB AND
. . .
one of these is mapped into one of these
DATA
execution unit
AND microsequence e.g., Fetch Calc
Operand Addr Fetch Operand(s)
Calculate Save Answer(s)
control memory
CPU
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Microprogramming
?-Code ROM

• Microprogramming is a fundamental concept
• implement an instruction set by building a very
  simple processor and interpreting the
  instructions
• essential for very complex instructions and when
  few register transfers are possible
• overkill when ISA matches datapath 1-1

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Designing a Microinstruction Set

• 1) Start with list of control signals
• 2) Group signals together that make sense (vs.
  random) called fields
• 3) Place fields in some logical order (e.g., ALU
  operation ALU operands first and
  microinstruction sequencing last)
• 4) To minimize the width, encode operations that
  will never be used at the same time
• 5) Create a symbolic legend for the
  microinstruction format, showing name of field
  values and how they set the control signals
• Use computers to design computers

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12) Start with list of control signals, grouped
into fields

• Signal name Effect when deasserted Effect when
  assertedALUSelA 1st ALU operand PC 1st ALU
  operand RegrsRegWrite None Reg. is written
  MemtoReg Reg. write data input ALU Reg. write
  data input memory RegDst Reg. dest. no.
  rt Reg. dest. no. rdMemRead None Memory at
  address is read, MDR lt MemaddrMemWrite Non
  e Memory at address is written IorD Memory
  address PC Memory address SIRWrite None IR
  lt MemoryPCWrite None PC lt PCSourcePCWriteCond
  None IF ALUzero then PC lt PCSourcePCSource
  PCSource ALU PCSource ALUoutExtOp Zero
  Extended Sign Extended

Single Bit Control
Signal name Value Effect ALUOp 00 ALU adds
01 ALU subtracts 10 ALU does function
code 11 ALU does logical OR ALUSelB 00 2nd ALU
input 4 01 2nd ALU input Regrt 10 2nd
ALU input extended,shift left 2 11 2nd ALU
input extended
Multiple Bit Control
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34) Microinstruction Format unencoded vs.
encoded fields

• Field Name Width Control Signals Set
• wide narrow
• ALU Control 4 2 ALUOp
• SRC1 2 1 ALUSelA
• SRC2 5 3 ALUSelB, ExtOp
• ALU Destination 3 2 RegWrite, MemtoReg, RegDst
• Memory 3 2 MemRead, MemWrite, IorD
• Memory Register 1 1 IRWrite
• PCWrite Control 3 2 PCWrite, PCWriteCond,
  PCSource
• Sequencing 3 2 AddrCtl
• Total width 24 15 bits

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5) Legend of Fields and Symbolic Names

• Field Name Values for Field Function of Field
  with Specific ValueALU Add ALU adds Subt. ALU
  subtracts Func code ALU does function
  code Or ALU does logical ORSRC1 PC 1st ALU
  input PC rs 1st ALU input RegrsSRC2 4 2nd
  ALU input 4 Extend 2nd ALU input sign ext.
  IR15-0 Extend0 2nd ALU input zero ext.
  IR15-0 Extshft 2nd ALU input sign ex., sl
  IR15-0 rt 2nd ALU input Regrtdestination r
  d ALU Regrd ALUout rt ALU Regrt ALUout
  rt Mem Regrt Mem Memory Read PC Read
  memory using PC Read ALU Read memory using
  ALUout for addr Write ALU Write memory using
  ALUout for addrMemory register IR IR MemPC
  write ALU PC ALU ALUoutCond IF ALU Zero then
  PC ALUoutSequencing Seq Go to sequential
  µinstruction Fetch Go to the first
  microinstruction Dispatch Dispatch using ROM.

37
Quick check what do these fieldnames mean?
Destination

• Code Name RegWrite MemToReg RegDest
• 00 --- 0 X X
• 01 rd ALU 1 0 1
• 10 rt ALU 1 0 0
• 11 rt MEM 1 1 0

SRC2
Code Name ALUSelB ExtOp 000 --- X X 001 4 00
X 010 rt 01 X 011 ExtShft 10 1 100 Extend 11
1 111 Extend0 11 0
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Specific Sequencer

• Sequencer-based control unit
• Called microPC or µPC vs. state register
• Code Name Effect 00 fetch Next µaddress
  0 01 dispatch Next µaddress dispatch ROM
  10 seq Next µaddress µaddress 1
• ROM

R-type 000000 0100 BEQ 000100 0011 ori 001101 0110
LW 100011 1000 SW 101011 1011
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Microprogram it yourself!

• Label ALU SRC1 SRC2 Dest. Memory Mem. Reg. PC
  Write Sequencing
• Fetch Add PC 4 Read PC IR ALU Seq

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Microprogram it yourself!

• Label ALU SRC1 SRC2 Dest. Memory Mem. Reg. PC
  Write Sequencing
• Fetch Add PC 4 Read PC IR ALU Seq
• Add PC Extshft Dispatch
• Rtype Func rs rt Seq
• rd ALU Fetch
• Lw Add rs Extend Seq
• Read ALU Seq
• rt MEM Fetch
• Sw Add rs Extend Seq
• Write ALU Fetch
• Ori Or rs Extend0 Seq
• rt ALU Fetch
• Beq Subt. rs rt ALUoutCond. Fetch

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Microprogramming Pros and Cons

• Ease of design
• Flexibility
• Easy to adapt to changes in organization, timing,
  technology
• Can make changes late in design cycle, or even in
  the field
• Can implement very powerful instruction sets
  (just more control memory)
• Generality
• Can implement multiple instruction sets on same
  machine.
• Can tailor instruction set to application.
• Compatibility
• Many organizations, same instruction set
• Costly to implement
• Slow

42
Summary

• Microprogramming is a fundamental concept
• implement an instruction set by building a very
  simple processor and interpreting the
  instructions
• essential for very complex instructions and when
  few register transfers are possible
• Control design reduces to Microprogramming
• Design of a Microprogramming language
• Start with list of control signals
• Group signals together that make sense (vs.
  random) called fields
• Place fields in some logical order (e.g., ALU
  operation ALU operands first and
  microinstruction sequencing last)
• To minimize the width, encode operations that
  will never be used at the same time
• Create a symbolic legend for the microinstruction
  format, showing name of field values and how they
  set the control signals

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Administrivia

• Midterm duration 1 hr 55 mins
• Bring a calculator.
• Handwritten notes, single sheet 8.5 x 11.
• Be in class 5 minutes before start time.