1
Ch 5: Designing a Single Cycle Datapath
Computer Systems Architecture
CS 365
The Big Picture: Where are We Now?
The Five Classic Components of a Computer
Todays Topic: Design a Single Cycle Processor
Control
Datapath
Memory
Processor
Input
Output
inst. set design (Ch 3)
technology
machine
design
Arithmetic (Ch 4)
2
The Big Picture: The Performance Perspective
Performance of a machine is determined by:
Instruction count
Clock cycle time
Clock cycles per instruction
Processor design (datapath and control) will determine:
Clock cycle time
Clock cycles per instruction
Today:
Single cycle processor:
Advantage: One clock cycle per instruction
Disadvantage: long cycle time
CPI
Inst. Count Cycle Time
How to Design a Processor: step-by-step
1. Analyze instruction set => datapath requirements
the meaning of each instruction is given by the register transfers
datapath must include storage element for ISA registers
possibly more
datapath must support each register transfer
2. Select set of datapath components and establish clocking methodology
3. Assemble datapath meeting the requirements
4. Analyze implementation of each instruction to determine setting of control
points that effects the register transfer.
5. Assemble the control logic
3
The MIPS Instruction Formats
All MIPS instructions are 32 bits long. The three instruction formats:
R-type
I-type
J-type
The different fields are:
op: operation of the instruction
rs, rt, rd: the source and destination register specifiers
shamt: shift amount
funct: selects the variant of the operation in the op field
address / immediate: address offset or immediate value
target address: target address of the jump instruction
op target address
0 26 31
6 bits 26 bits
op rs rt rd shamt funct
0 6 11 16 21 26 31
6 bits 6 bits 5 bits 5 bits 5 bits 5 bits
op rs rt immediate
0 16 21 26 31
6 bits 16 bits 5 bits 5 bits
Step 1a: The MIPS-lite Subset
ADD, SUB, AND, OR
add rd, rs, rt
sub rd, rs, rt
and rd, rs,rt
or rd,rs,rt
LOAD and STORE Word
lw rt, rs, imm16
sw rt, rs, imm16
BRANCH:
beq rs, rt, imm16
op rs rt rd shamt funct
0 6 11 16 21 26 31
6 bits 6 bits 5 bits 5 bits 5 bits 5 bits
op rs rt immediate
0 16 21 26 31
6 bits 16 bits 5 bits 5 bits
op rs rt immediate
0 16 21 26 31
6 bits 16 bits 5 bits 5 bits
4
Logical Register Transfers
RTL gives the meaning of the instructions
First step is to fetch the instruction from memory
op | rs | rt | rd | shamt | funct = MEM[ PC ]
op | rs | rt | Imm16 = MEM[ PC ]
inst Register Transfers
ADD R[rd] < R[rs] + R[rt]; PC < PC + 4
SUB R[rd] < R[rs] R[rt]; PC < PC + 4
OR R[rt] < R[rs] | R[rt]; PC < PC + 4
LOAD R[rt] < MEM[ R[rs] + sign_ext(Imm16)]; PC < PC + 4
STORE MEM[ R[rs] + sign_ext(Imm16) ] < R[rt]; PC < PC + 4
BEQ if ( R[rs] == R[rt] ) then PC < PC +
sign_ext(Imm16)] || 00
else PC < PC + 4
Step 1: Requirements of the Instruction Set
Memory
instruction & data
Registers (32 x 32)
read RS
read RT
Write RT or RD
PC
Extender
Add and Sub register or extended immediate
Add 4 or extended immediate to PC
5
Step 2: Components of the Datapath
Combinational Elements
Storage Elements
Clocking methodology
Registers
Register #
Data
Register #
Data
memory
Address
Data
Register #
PC Instruction ALU
Instruction
memory
Address
Abstract/Simplified View of Datapath
Two types of functional units:
elements that operate on data values (combinational)
elements that contain state (sequential)
6
Combinational Logic Elements (Basic Building Blocks)
Adder
MUX
ALU
32
32
A
B
32
Sum
Carry
32
32
A
B
32
Result
O
P
32
A
B
32
Y
32
Selec
t
A
d
d
e
r
M
U
X
A
L
U
CarryIn
Unclocked vs. Clocked
Clocks used in synchronous logic
when should an element that contains state be updated?
cycle time
rising edge
falling edge
State Elements: Review
7
The set-reset latch
output depends on present inputs and also on past inputs
An unclocked state element
Q
_
Q
R
S
Output is equal to the stored value inside the element
(don't need to ask for permission to look at the value)
Change of state (value) is based on the clock
Latches: whenever the inputs change, and the clock is asserted
Flip-flop: state changes only on a clock edge
(edge-triggered methodology)
"logically true",
could mean electrically low
A clocking methodology defines when signals can be read and written
wouldn't want to read a signal at the same time it was being written
Latches and Flip-flops
8
Two inputs:
the data value to be stored (D)
the clock signal (C) indicating when to read & store D
Two outputs:
the value of the internal state (Q) and its complement
D-latch
Q
C
D
_
Q
D
C
Q
D flip-flop
Output changes only on the clock edge
Q Q
_
Q
Q
_
Q
D
latch
D
C
D
latch
D D
C
C
D
C
Q
9
Our Implementation
An edge triggered methodology
Typical execution:
read contents of some state elements,
send values through some combinational logic
write results to one or more state elements
Clock cycle
State
element
1
Combinational logic
State
element
2
State
element
Combinational logic
Storage Element: Register (Basic Building Block)
Register
Similar to the D Flip Flop except
N-bit input and output
Write Enable input
Write Enable:
negated (0): Data Out will not
change
asserted (1): Data Out will become
Data In
Clk
Data In
Write Enable
N N
Data Out
10
Built using D flip-flops
Register File
M
u
x
Register 0
Register 1
Register n 1
Register n
M
u
x
Read data 1
Read data 2
Read register
number 1
Read register
number 2
Read register
number 1 Read
data 1
Read
data 2
Read register
number 2
Register file
Write
register
Write
data Write
Register File
Note: we still use the clock to determine when to write
n-to-1
decoder
Regi ster 0
Regi ster 1
Regi ster n 1
C
C
D
D
Regi ster n
C
C
D
D
Regi ster number
Write
Regi st er dat a
0
1
n 1
n
11
Storage Element: Register File
Register File consists of 32 registers:
Two 32-bit output busses:
busA and busB
One 32-bit input bus: busW
Register is selected by:
RA (number) selects the register to put on busA (data)
RB (number) selects the register to put on busB (data)
RW (number) selects the register to be written
via busW (data) when Write Enable is 1
Clock input (CLK)
The CLK input is a factor ONLY during write operation
During read operation, behaves as a combinational logic
block:
RA or RB valid => busA or busB valid after access time.
Clk
busW
Write Enable
32
32
busA
32
busB
5 5 5
RWRARB
32 32-bit
Registers
Storage Element: Idealized Memory
Memory (idealized)
One input bus: Data In
One output bus: Data Out
Memory word is selected by:
Address selects the word to put on Data Out
Write Enable = 1: address selects the memory
word to be written via the Data In bus
Clock input (CLK)
The CLK input is a factor ONLY during write operation
During read operation, behaves as a combinational logic
block:
Address valid => Data Out valid after access time.
Clk
Data In
Write Enable
32 32
DataOut
Address
12
Clocking Methodology
All storage elements are clocked by the same clock edge
Cycle Time = CLK-to-Q + Longest Delay Path + Setup + Clock Skew
(CLK-to-Q + Shortest Delay Path - Clock Skew) > Hold Time
Clk
Dont Care
Setup Hold
.
.
.
.
.
.
.
.
.
.
.
.
Setup Hold
Step 3
Register Transfer Requirements
> Datapath Assembly
Instruction Fetch
Read Operands and Execute Operation
13
3a: Overview of the Instruction Fetch Unit
The common RTL operations
Fetch the Instruction: mem[PC]
Update the program counter:
Sequential Code: PC <- PC + 4
Branch and Jump: PC <- something else
We dont know if instruction is a Branch/Jump or one of the
other instructions until we have fetched and interpreted the
instruction from memory. So all instructions initially increment
the PC
PC
Instruction
memory
Instruction
address
Instruction
a. Instruction memory b. Program counter
Add Sum
c. Adder
14
PC
Instruction
memory
Read
address
Instruction
4
Add
Datapath for Instruction Fetch
3b: R-format instructions: add, sub, and, or, slt
R[rd] <- R[rs] op R[rt] Example: add rd, rs, rt
Read register 1, Read register 2, and Write register come from
instructions rs, rt, and rd fields
ALU control and RegWrite: control logic after decoding the
instruction
op rs rt rd shamt funct
0 6 11 16 21 26 31
6 bits 6 bits 5 bits 5 bits 5 bits 5 bits
ALU control
RegWrite
Registers
Write
register
Read
data 1
Read
data 2
Read
register 1
Read
register 2
Write
data
ALU
result
ALU
Data
Data
Register
numbers
a. Registers b. ALU
Zero
5
5
5 3
15
Instruction
Registers
Write
register
Read
data 1
Read
data 2
Read
register 1
Read
register 2
Write
data
ALU
result
ALU
Zero
RegWrite
ALU operation
3
Datapath for R-format instructions
Register-Register Timing
32
Result
ALUctr
Clk
busW
RegWr
32
32
busA
32
busB
5 5 5
Rw Ra Rb
32 32-bit
Registers
Rs Rt Rd
A
L
U
Clk
PC
Rs, Rt, Rd,
Op, Func
Clk-to-Q
ALUctr
Instruction Memory Access Time
Old Value New Value
RegWr Old Value New Value
Delay through Control Logic
busA, B
Register File Access Time
Old Value New Value
busW
ALU Delay
Old Value New Value
Old Value New Value
New Value Old Value
Register Write
Occurs Here
16
3d: Load & Store Operations
R[rt] <- Mem[R[rs] + SignExt[imm16]] Example: lw rt, rs, imm16
Mem[ R[rs] + SignExt[imm16] <- R[rt] ] Example: sw rt, rs, imm16
op rs rt immediate
0 16 21 26 31
6 bits 16 bits 5 bits 5 bits
16 32
Sign
extend
b. Sign-extension unit
MemRead
MemWrite
Data
memory
Write
data
Read
data
a. Data memory unit
Address
Instruction
16 32
Registers
Write
register
Read
data 1
Read
data 2
Read
register 1
Read
register 2
Data
memory
Write
data
Read
data
Write
data
Sign
extend
ALU
result
Zero
ALU
Address
MemRead
MemWrite
RegWrite
ALU operation
3
Datapath for lw & sw
17
3f: The Branch Instruction
beq rs, rt, imm16
mem[PC] Fetch the instruction from memory
Equal <- R[rs] == R[rt] Calculate the branch condition
if (COND eq 0) Calculate the next instructions address
PC <- PC + 4 + ( SignExt(imm16) x 4 )
else
PC <- PC + 4
op rs rt immediate
0 16 21 26 31
6 bits 16 bits 5 bits 5 bits
16 32
Sign
extend
Zero ALU
Sum
Shift
left 2
To branch
control logic
Branch target
PC + 4 from instruction datapath
Instruction
Add
Registers
Write
register
Read
data 1
Read
data 2
Read
register 1
Read
register 2
Write
data
RegWrite
ALU operation
3
Datapath for branch instruction
18
PC
Instruction
memory
Read
address
Instruction
16 32
Registers
Write
register
Write
data
Read
data 1
Read
data 2
Read
register 1
Read
register 2
Sign
extend
ALU
result
Zero
Data
memory
Address
Write
data
Read
data
M
u
x
4
Add
M
u
x
ALU
RegWrite
ALU operation
3
MemRead
MemWrite
ALUSrc
MemtoReg
Using multiplexors to stitch together the datapath for
memory access and R-format instructions
PC
Instruction
memory
Read
address
Instruction
16 32
Add ALU
result
M
u
x
Registers
Write
register
Write
data
Read
data 1
Read
data 2
Read
register 1
Read
register 2
Shift
left 2
4
M
u
x
ALU operation
3
RegWrite
MemRead
MemWrite
PCSrc
ALUSrc
MemtoReg
ALU
result
Zero
ALU
Data
memory
Address
Write
data
Read
data
M
u
x
Sign
extend
Add
Putting it all together
19
MemtoReg
MemRead
MemWrite
ALUOp
ALUSrc
RegDst
PC
Instruction
memory
Read
address
Instruction
[310]
Instruction [2016]
Instruction [2521]
Add
Instruction [50]
RegWrite
4
16 32
Instruction [150]
0
Registers
Write
register
Write
data
Write
data
Read
data 1
Read
data 2
Read
register 1
Read
register 2
Sign
extend
ALU
result
Zero
Data
memory
Address
Read
data
M
u
x
1
0
M
u
x
1
0
M
u
x
1
0
M
u
x
1
Instruction [1511]
ALU
control
Shift
left 2
PCSrc
ALU
Add
ALU
result
Putting it all together contd
PC
Instruction
memory
Read
address
Instruction
[31 0]
Instruction [20 16]
Instruction [25 21]
Add
Instruction [5 0]
MemtoReg
ALUOp
MemWrite
RegWrite
MemRead
Branch
RegDst
ALUSrc
Instruction [31 26]
4
16 32
Instruction [15 0]
0
0
M
u
x
0
1
Control
Add
ALU
result
M
u
x
0
1
Registers
Write
register
Write
data
Read
data 1
Read
data 2
Read
register 1
Read
register 2
Sign
extend
M
u
x
1
ALU
result
Zero
PCSrc
Data
memory
Write
data
Read
data
M
u
x
1
Instruction [15 11]
ALU
control
Shift
left 2
ALU
Address
Adding the control unit
20
An Abstract View of the Critical Path
Register file and ideal memory:
The CLK input is a factor ONLY during write operation
During read operation, behave as combinational logic:
Address valid => Output valid after access time.
Critical Path (Load Operation) =
PCs Clk-to-Q +
Instruction Memorys Access Time +
Register Files Access Time +
ALU to Perform a 32-bit Add +
Data Memory Access Time +
Setup Time for Register File Write +
Clock Skew
Clk
5
Rw Ra Rb
32 32-bit
Registers
Rd
A
L
U
Clk
Data
In
Data
Address
Ideal
Data
Memory
Instruction
Instruction
Address
Ideal
Instruction
Memory
C
l
k
P
C
5
Rs
5
Rt
16
Imm
32
32
32
32
A
B
N
e
x
t
A
d
d
r
e
s
s
Step 4: Given Datapath: RTL -> Control
ALUop RegDst ALUSrc
MemRd MemtoReg MemWr
Zero
Instruction<31:0>
<
2
1
:
2
5
>
<
1
6
:
2
0
>
<
1
1
:
1
5
>
<
0
:
1
5
>
Imm16 Rd Rs Rt
Branch
Adr
Inst
Memory
DATA PATH
Control
Op
<
2
1
:
2
5
>
Fun
RegWr
21
Control
Selecting the operations to perform (ALU, read/write, etc.)
Design the ALU Control Unit
Controlling the flow of data (multiplexor inputs)
Design the Main Control Unit
Information comes from the 32 bits of the instruction
Example:
add $8, $17, $18 Instruction Format:
000000 10001 10010 01000 00000 100000
op rs rt rd shamt funct
ALU's operation based on instruction type and function code
e.g., what should the ALU do with this instruction
Example: lw $1, 100($2)
35 2 1 100
op rs rt 16 bit offset
ALU control input
000 AND
001 OR
010 add
110 subtract
111 set-on-less-than
Why is the code for subtract 110 and not 011?)
ALU Control
(Recall design of ALU from Chapter 4. Bnegate input for adder set to
1 for subtraction
22
ALU Control Design
111 Set on less
than
101010 Set on less
than
10 R-type
001 Or 1000101 OR 10 R-type
000 And 100100 AND 10 R-type
110 Subtract 100010 Subtract 10 R-type
010 Add 100000 Add 10 R-type
110 Subtract xxxxxx Branch eq 01 BEQ
010 Add xxxxxx Store word 00 SW
010 Add xxxxxx Load word 00 LW
ALU control
input
Desired
ALU action
Funct field
Instruction
operation
ALUOp
Instruction
opcode
Must describe hardware to compute 3-bit ALU control input
given instruction type
00 = lw, sw
01 = beq
10 = arithmetic
function code for arithmetic
Describe it using a truth table (can turn into gates):
ALUOp
computed from instruction type
Control
ALUOp Funct field Operation
ALUOp1 ALUOp0 F5 F4 F3 F2 F1 F0
0 0 X X X X X X 010
X 1 X X X X X X 110
1 X X X 0 0 0 0 010
1 X X X 0 0 1 0 110
1 X X X 0 1 0 0 000
1 X X X 0 1 0 1 001
1 X X X 1 0 1 0 111
23
Design the main control unit
Seven control signals
RegDst
RegWrite
ALUSrc
PCSrc
MemRead
MemWrite
MemtoReg
Control Signals
1. RegDst = 0 => Register destination number for the Write register
comes from the rt field (bits 20-16)
RegDst = 1 => Register destination number for the Write register
comes from the rd field (bits 15-11)
2. RegWrite = 1 => The register on the Write register input is written with
the data on the Write data input (at the next clock edge)
3. ALUSrc = 0 => The second ALU operand comes from Read data 2
ALUSrc = 1 => The second ALU operand comes from the sign-
extension unit
4. PCSrc = 0 => The PC is replaced with PC+4
PCSrc = 1 => The PC is replaced with the branch target address
5. MemtoReg = 0 => The value fed to the register write data input comes
from the ALU
MemtoReg = 1 => The value fed to the register write data input comes
from the data memory
6. MemRead = 1 => Read data memory
7. MemWrite = 1 => Write data memory
24
R-format instructions
RegDst = 1
RegWrite = 1
ALUSrc = 0
Branch = 0
MemtoReg = 0
MemRead = 0
MemWrite = 0
ALUOp = 10
RegDst = 0
RegWrite = 1
ALUSrc = 1
Branch = 0
MemtoReg = 1
MemRead = 1
MemWrite = 0
ALUOp = 00
Memory access instructions
RegDst = X
RegWrite = 0
ALUSrc = 1
Branch = 0
MemtoReg = X
MemRead = 0
MemWrite = 1
ALUOp = 00
Load word
Store Word
0
25
Branch Equal
RegDst = X
RegWrite = 0
ALUSrc = 0
Branch = 1
MemtoReg = X
MemRead = 0
MemWrite = 0
ALUOp = 01
Control
Instruction RegDst ALUSrc
Memto-
Reg
Reg
Write
Mem
Read
Mem
Write Branch ALUOp1 ALUp0
R-format 1 0 0 1 0 0 0 1 0
lw
0 1 1 1 1 0 0 0 0
sw
X 1 X 0 0 1 0 0 0
beq
X 0 X 0 0 0 1 0 1
PC
Instruction
memory
Read
address
Instruction
[31 0]
Instruction [20 16]
Instruction [25 21]
Add
Instruction [5 0]
MemtoReg
ALUOp
MemWrite
RegWrite
MemRead
Branch
RegDst
ALUSrc
Instruction [31 26]
4
16 32
Instruction [15 0]
0
0
M
u
x
0
1
Control
Add
ALU
result
M
u
x
0
1
Registers
Write
register
Write
data
Read
data 1
Read
data 2
Read
register 1
Read
register 2
Sign
extend
Shift
left 2
M
u
x
1
ALU
result
Zero
Data
memory
Write
data
Read
data
M
u
x
1
Instruction [15 11]
ALU
control
ALU
Address
26
Step 5: Implementing Control
Simple combinational logic
(truth tables)
Operation2
Operation1
Operation0
Operation
ALUOp1
F3
F2
F1
F0
F (5 0)
ALUOp0
ALUOp
ALU control block
R-format Iw sw beq
Op0
Op1
Op2
Op3
Op4
Op5
Inputs
Outputs
RegDst
ALUSrc
MemtoReg
RegWrite
MemRead
MemWrite
Branch
ALUOp1
ALUOpO
ALU Control Unit
Main Control Unit
All of the logic is combinational
We wait for everything to settle down, and the right thing to be done
ALU might not produce right answer right away
we use write signals along with clock to determine when to write
Cycle time determined by length of the longest path
Our Simple Control Structure
Clock cycle
State
element
1
Combi national logic
State
element
2
27
An Abstract View of the Critical Path
Register file and ideal memory:
The CLK input is a factor ONLY during write operation
During read operation, behave as combinational logic:
Address valid => Output valid after access time.
Critical Path (Load Operation) =
PCs Clk-to-Q +
Instruction Memorys Access Time +
Register Files Access Time +
ALU to Perform a 32-bit Add +
Data Memory Access Time +
Setup Time for Register File Write +
Clock Skew
Clk
5
Rw Ra Rb
32 32-bit
Registers
Rd
A
L
U
Clk
Data
In
Data
Address
Ideal
Data
Memory
Instruction
Instruction
Address
Ideal
Instruction
Memory
C
l
k
P
C
5
Rs
5
Rt
16
Imm
32
32
32
32
A
B
N
e
x
t
A
d
d
r
e
s
s
Single Cycle Implementation
Calculate cycle time assuming negligible delays except:
memory (2ns), ALU and adders (2ns), register file access (1ns)
MemtoReg
MemRead
MemWrite
ALUOp
ALUSrc
RegDst
PC
Instruction
memory
Read
address
Instruction
[310]
Instruction [2016]
Instruction [2521]
Add
Instruction [50]
RegWrite
4
16 32
Instruction [150]
0
Registers
Write
register
Write
data
Write
data
Read
data 1
Read
data 2
Read
register 1
Read
register 2
Sign
extend
ALU
result
Zero
Data
memory
Address
Read
data
M
u
x
1
0
M
u
x
1
0
M
u
x
1
0
M
u
x
1
Instruction [1511]
ALU
control
Shift
left 2
PCSrc
ALU
Add
ALU
result
28
A Real MIPS Datapath (CNS T0)
Summary
5 steps to design a processor
1. Analyze instruction set => datapath requirements
2. Select set of datapath components & establish clock methodology
3. Assemble datapath meeting the requirements
4. Analyze implementation of each instruction to determine setting of control
points that effects the register transfer.
5. Assemble the control logic
MIPS makes it easier
Instructions same size
Source registers always in same place
Immediates same size, location
Operations always on registers/immediates
Single cycle datapath => CPI=1, Clock Cycle Time => long
