the big picture: where are we now?
DESCRIPTION
CS 152 Computer Architecture and Engineering Lecture 11 Multicycle Controller Design (Continued) . Processor. Input. Control. Memory. Datapath. Output. The Big Picture: Where are We Now? . The Five Classic Components of a Computer Today’s Topics: Microprogramed control Administrivia - PowerPoint PPT PresentationTRANSCRIPT
CS152Lec11.1
CS 152 Computer Architecture and Engineering
Lecture 11
Multicycle Controller Design (Continued)
CS152Lec11.2
The Big Picture: Where are We Now?
° The Five Classic Components of a Computer
° Today’s Topics: • Microprogramed control• Administrivia• Microprogram it yourself• Exceptions
Control
Datapath
Memory
ProcessorInput
Output
CS152Lec11.3
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 sequencercontrol
datapath control
micro-PCsequencer
microinstruction
CS152Lec11.4
Multicycle Datapath
PC
Nex
t PC
Ope
rand
Fetc
h
Inst
ruct
ion
Fetc
h
nPC
_sel
IRRegFile E
xtA
LU Reg
. Fi
le
Mem
Acc
ess
Dat
aM
em
Res
ult S
tore
Reg
Dst
Reg
Wr
Mem
Wr
Mem
Rd
S
M
Mem
ToR
eg
Equ
al
ALU
ctr
ALU
Src
ExtO
p
A
B
E
CS152Lec11.5
State Diagram of Controller
IR <= MEM[PC]
R-type
A <= R[rs]B <= R[rt]
S <= A fun B
R[rd] <= SPC <= PC + 4
S <= A or ZX
R[rt] <= SPC <= PC + 4
ORi
S <= A + SX
R[rt] <= MPC <= PC + 4
M <= MEM[S]
LW
S <= A + SX
MEM[S] <= BPC <= PC + 4
BEQPC <= Next(PC)
SW
“instruction fetch”
“decode”
0000
0001
0100
0101
0110
0111
1000
1001
1010
00111011
1100
Exe
cute
Mem
ory
Writ
e-ba
ck
CS152Lec11.6
Using a Jump Counter
IR <= MEM[PC]
R-type
A <= R[rs]B <= R[rt]
S <= A fun B
R[rd] <= SPC <= PC + 4
S <= A or ZX
R[rt] <= SPC <= PC + 4
ORi
S <= A + SX
R[rt] <= MPC <= PC + 4
M <= MEM[S]
LW
S <= A + SX
MEM[S] <= BPC <= PC + 4
BEQPC <= Next(PC)
SW
“instruction fetch”
“decode”
0000
0001
0100
0101
0110
0111
1000
1001
1010
00111011
1100
inc
load
zero zerozero
zero
zeroinc inc inc inc
inc
Exe
cute
Mem
ory
Writ
e-ba
ck
CS152Lec11.7
Example: Jump-Counter
op-codeMap ROM
Counterzeroincload
0000i
i+1
i
None of above: Do nothing (for wait states)
CS152Lec11.8
Sequencer°Sequencer-based control unit
• Called “microPC” or “µPC” vs. state register
Control Value Effect 00 Next µaddress = 0 01 Next µaddress = dispatch ROM 10 Next µaddress = µaddress + 1
DispatchROM:
Opcode
microPC
1
µAddressSelectLogic
Adder
DispatchROM
Mux
0012
Opcode DispatchState
R-type 000000 0100BEQ 000100 0011ori 001101 0110LW 100011 1000SW 101011 1011
Microprogram
CS152Lec11.9
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 & organizationSame instruction set across wide range of implementations, each with different cost/performance
CS152Lec11.10
“Macro and micro - instruction” Interpretation
MainMemory
executionunit
controlmemory
CPU
ADDSUBAND
DATA
.
.
.
User program plus Data
this can change!
AND microsequence
e.g., Fetch Calc Operand Addr Fetch Operand(s) Calculate Save Answer(s)
one of these ismapped into oneof these
CS152Lec11.11
Variations on Microprogramming
° “Horizontal” Microcode– control field for each control point in the machine
° “Vertical” Microcode– compact microinstruction format for each class of microoperation
– local decode to generate all control points (remember ALU?)branch: µseq-op µaddexecute: ALU-op A,B,Rmemory: mem-op S, D
µseq µaddr A-mux B-mux bus enables register enables
HorizontalVertical
CS152Lec11.12
Extreme Horizontal
inputselectN3 N2 N1 N0. . .
13
Incr PCALU control
1 bit for each loadable register enbMAR enbAC . . .
Depending on bus organization, many potential control combinations simply wrong, i.e., implies transfers that can never happen at the same time.
Makes sense to encode fields to save ROM space
Example: mem_to_reg and ALU_to_reg should never happen simultaneously; => encode in single bit which is decoded rather than two separate bits
NOTE: the encoding should be only wide enough so that parallel actions that the datapath supports should still be specifiable in a single microinstruction
CS152Lec11.13
More Vertical Formatsrc dst
DEC
DEC
other control fields next states inputs
MUX
Multiformat Microcode:1 3 6
1 3 3 3
0 cond next address
1 dst src alu
DEC
DEC
Branch Jump
Register Xfer Operation
CS152Lec11.14
Hybrid Control
Not all critical control information is derived from control logic
E.g., Instruction Register (IR) contains useful control information, such as register sources, destinations, opcodes, etc.
RegisterFile
RS1
DEC
RS2
DEC
RD
DEC
op rs1 rs2 rdIRtocontrol
enablesignalsfromcontrol
CS152Lec11.15
Summary: Horizontal vs. Vertical Microprogramming
NOTE: previous organization is not TRUE horizontal microprogramming; register decoders give flavor of encoded microoperations
Most microprogramming-based controllers vary between:
horizontal organization (1 control bit per control point)
vertical organization (fields encoded in the control memory and must be decoded to control something)
Horizontal
+ more control over the potential parallelism of operations in the datapath
- uses up lots of control store
Vertical
+ easier to program, not very different from programming a RISC machine in assembly language
- extra level of decoding may slow the machine down
CS152Lec11.16
Microprogramming a multicycle processor
° 1) Choose datapath and sequencer architecture° 2) Assign states and sequence of each (multicycle)
instruction (i.e. define the controller FSM)° 2) Choose microinstruction format (minimum bits to
describe all allowable functions of sequencer and datapath)
° 3) Map instructions into microinstruction sequences
CS152Lec11.17
Sequencer°Sequencer-based control unit
• Called “microPC” or “µPC” vs. state register
Control Value Effect 00 Next µaddress = 0 01 Next µaddress = dispatch ROM 10 Next µaddress = µaddress + 1
DispatchROM:
Opcode
microPC
1
µAddressSelectLogic
Adder
DispatchROM
Mux
0012
Opcode DispatchState
R-type 000000 0100BEQ 000100 0011ori 001101 0110LW 100011 1000SW 101011 1011
Microprogram
CS152Lec11.18
Datapath – single memory, single regfile
° Miminizes Hardware: 1 memory, 1 adder
IdealMemoryWrAdrDin
RAdr
32
32
32Dout
MemWr32
AL
U
3232
ALUOp
ALUControl
32
IRWr
Instruction Reg
32
Reg File
Ra
Rw
busW
Rb5
5
32busA
32busB
RegWr
Rs
Rt
Mux
0
1
Rt
Rd
PCWr
ALUSelA
Mux 01
RegDst
Mux
0
1
32
PC
MemtoReg
Extend
ExtOp
Mux
0
132
0
1
23
4
16Imm 32
<< 2
ALUSelB
Mux
1
0
32
Zero
ZeroPCWrCond PCSrc
32
IorD
Mem
Data R
eg
AL
U O
ut
B
A
CS152Lec11.19
Finite State Machine (FSM) Spec
IR <= MEM[PC]PC <= PC + 4
R-type
ALUout <= A fun B
R[rd] <= ALUout
ALUout <= A or ZX
R[rt] <= ALUout
ORi
ALUout <= A + SX
R[rt] <= M
M <= MEM[ALUout]
LW
ALUout <= A + SX
MEM[ALUout] <= B
SW
“instruction fetch”
“decode”
Exe
cute
Mem
ory
Writ
e-ba
ck
0000
0001
0100
0101
0110
0111
1000
1001
1010
1011
1100
BEQ
0010
0011
If A = B then PC <= ALUout
ALUout <= PC +SX
Q: What can wedo instate 0001?
CS152Lec11.20
Finite State Machine (FSM) Spec (improved)
IR <= MEM[PC]PC <= PC + 4
R-type
ALUout <= A fun B
R[rd] <= ALUout
ALUout <= A or ZX
R[rt] <= ALUout
ORi
ALUout <= A + SX
R[rt] <= M
M <= MEM[ALUout]
LW
ALUout <= A + SX
MEM[ALUout] <= B
SW
“instruction fetch”
“decode”
0000
0001
0100
0101
0110
0111
1000
1001
1010
1011
1100
BEQ
0010
If A = B then PC <= ALUout
ALUout <= PC +SX
Exe
cute
Mem
ory
Writ
e-ba
ck
CS152Lec11.21
Designing a Microinstruction Set
1) Start with list of control signals2) 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) Create a symbolic legend for the microinstruction format, showing name of field values and how they set the control signals
5) To minimize the width, encode operations that will never be used at the same time
CS152Lec11.22
1) Start with list of control signalsSignal name Effect when deasserted Effect when asserted
ALUSelA 1st ALU operand = PC 1st ALU operand = Reg[rs]RegWr None Reg. is written MemtoReg Reg. write data input = ALU Reg. write data input = memory RegDst Reg. dest. no. = rt Reg. dest. no. = rdMemRd None Memory at address is read, MemWr None Memory at address is written IorD Memory address = PC Memory address = SIRWr None IR <= MemoryPCWr None PC <= PCSourcePCWrCond None IF ALUzero then PC <= PCSourcePCSrc PCSource = ALU PCSource = ALUoutExtOp Zero Extended Sign Extended
Sing
le B
it C
ontr
ol
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 = Reg[rt] 10 2nd ALU input = extended,shift left 2 11 2nd ALU input = extended
Mul
tiple
Bit
Con
trol
CS152Lec11.23
2) Group into fields of unrelated signals
° Miminizes Hardware: 1 memory, 1 adder
IdealMemoryWrAdrDin
RAdr
32
32
32Dout
MemWr32
AL
U
3232
ALUOp
ALUControl
32
IRWr
Instruction Reg
32
Reg File
Ra
Rw
busW
Rb5
5
32busA
32busB
RegWr
Rs
Rt
Mux
0
1
Rt
Rd
PCWr
ALUSelA
Mux 01
RegDst
Mux
0
1
32
PC
MemtoReg
Extend
ExtOp
Mux
0
132
0
1
23
4
16Imm 32
<< 2
ALUSelB
Mux
1
0
32
Zero
ZeroPCWrCond PCSrc
32
IorD
Mem
Data R
eg
AL
U O
ut
B
A
ALU
SRC1
SRC2
MemRd
PCWrite
CS152Lec11.24
2,3 & 4 ) Group into fields, order and assign names
Field Name Values for Field Function of Field with Specific ValueALU Add ALU adds
Subt. ALU subtractsFunc ALU does function codeOr ALU does logical OR
SRC1 PC 1st ALU input <= PCrs 1st ALU input <= Reg[rs]
SRC2 4 2nd ALU input <= 4Extend 2nd ALU input <= sign ext. IR[15-0]Extend0 2nd ALU input <= zero ext. IR[15-0] Extshft 2nd ALU input <= sign ex., sl IR[15-0]rt 2nd ALU input <= Reg[rt]
dest(ination) rd ALU Reg[rd] <= ALUout rt ALU Reg[rt] <= ALUout
rt Mem Reg[rt] <= Mem Mem(ory) Read PC Read memory using PC
Read ALU Read memory using ALUout for addrWrite ALU Write memory using ALUout for addr
Memreg IR IR <= MemPCwrite PCwr PC <= PCSource
PCSrc IF Zero then PCSource <= ALUout else ALUPCWrCond IF Zero then PC <= PCSource
Seq(uencing) Seq Go to sequential µinstructionFetch Go to the first microinstructionDispatch Dispatch using ROM.
ALU SRC1
SRC2
Dest Mem Memreg PCwrite Seq
CS152Lec11.25
5) Encode each field
Field Name Width Control Signals Setwide narrow
ALU 4 2ALUOpSRC1 2 1ALUSelASRC2 5 3ALUSelB, ExtOpDest 3 2RegWrite, MemtoReg, RegDstMem 3 2MemRd, MemWre, IorDMemreg 1 1IRWritePCWrite 3 1PCWr, PCSrc, PCWrCondSeq 3 2AddrCtlTotal width 24 14bits
CS152Lec11.26
5) Encode each field (cont.)
Code Name RegWrite MemToReg RegDest00 --- 0 X X01rd ALU 1 0 110rt ALU 1 0 011rt MEM 1 1 0
Code Name ALUSelB ExtOp000 --- X X001 4 00 X010 rt 01 X011 ExtShft 10 1100 Extend 11 1111 Extend0 11 0
Dest:
SRC2:
CS152Lec11.27
Finally – Do the microprogram….
Label ALU SRC1 SRC2 Dest. Memory MemReg. PCWrite SeqFetch: Add PC 4 Read PC IR PCwr Seq
Add PC Extshft Dispatch
Rtype: Func rs rt Seqrd ALU Fetch
Ori: Or rs Extend0 Seqrt ALU Fetch
Lw: Add rs Extend SeqRead ALU Seq
rt MEM Fetch
Sw: Add rs Extend SeqWrite ALU Fetch
Beq: Subt. rs rt PCWrCond. Fetch
0000 0001
01000101
01100111
100010011010
10111100
0010
CS152Lec11.28
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
CS152Lec11.29
Adding a more complex memory model
PC
InstructionMemory
Inst. Reg
addr
data
IR_en
InstMem_rd
IM_wait
Add a wait flag of indeterminate length – IM_wait(due to caching)
CS152Lec11.30
Controller handles non-ideal memory
IR <= MEM[PC]
R-type
A <= R[rs]B <= R[rt]
S <= A fun B
R[rd] <= SPC <= PC + 4
S <= A or ZX
R[rt] <= SPC <= PC + 4
ORi
S <= A + SX
R[rt] <= MPC <= PC + 4
M <= MEM[S]
LW
S <= A + SX
MEM[S] <= B
BEQPC <=
Next(PC)
SW
“instruction fetch”
“decode / operand fetch”
Exe
cute
Mem
ory
Writ
e-ba
ck
~IMwait IMwait
~wait wait
PC <= PC + 4
~wait wait
CS152Lec11.31
Really Simple Time-State Control
inst
ruct
ion
fetc
hde
code
Exe
cute
Mem
ory
IR <= MEM[PC]
R-type
A <= R[rs]B <= R[rt]
S <= A fun B
R[rd] <= SPC <= PC + 4
S <= A or ZX
R[rt] <= SPC <= PC + 4
ORi
S <= A + SX
R[rt] <= MPC <= PC + 4
M <= MEM[S]
LW
S <= A + SX
MEM[S] <= B
BEQ
PC <= Next(PC)
SW
~IMwait IMwait
wait
PC <= PC + 4
wait
writ
e-ba
ck
CS152Lec11.32
Time-state Control Path
° Local decode and control at each stage
Exe
c
Reg
. Fi
le
Mem
Acc
ess
Dat
aM
em
A
B
S
M
Reg
File
Equ
al
PC
Nex
t PC
IR
Inst
. Mem
Valid
IRex
Dcd
Ctrl
IRm
em
Ex
Ctrl
IRw
b
Mem
Ctrl
WB
Ctrl
CS152Lec11.33
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”
CS152Lec11.34
Summary° Specialize state-diagrams easily captured by
microsequencer• simple increment & “branch” fields
• datapath control fields
° Most microprogramming-based controllers vary between:• horizontal organization (1 control bit per control point)
• vertical organization (fields encoded in the control memory and must be decoded to control something)
° Steps: • identify control signals, group them, develop “mini language”, then
microprogram
° Control design reduces to Microprogramming • Arbitrarily complicated instructions possible
CS152Lec11.35
Summary: Microprogramming one inspiration for RISC
° If simple instruction could execute at very high clock rate…
° If you could even write compilers to produce microinstructions…
° If most programs use simple instructions and addressing modes…
° If microcode is kept in RAM instead of ROM so as to fix bugs …
° If same memory used for control memory could be used instead as cache for “macroinstructions”…
° Then why not skip instruction interpretation by a microprogram and simply compile directly into lowest language of machine?