1. ECE 445 – Computer Organization
The Single-Cycle Datapath and Control Unit
(Lecture #10)
The slides included herein were taken from the materials accompanying
Computer Organization and Design, 4th Edition, by Patterson and Hennessey,
and were used with permission from Morgan Kaufmann Publishers.

2. Chapter 4: Sections 1 – 4 Appendix D: Sections 1 – 3, 5
Material to be covered ...
Chapter 4: Sections 1 – 4
Appendix D: Sections 1 – 3, 5
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ECE Computer Organization

3. ECE 445 - Computer Organization
Introduction
§4.1 Introduction
CPU performance factors
Instruction count
Determined by ISA and compiler
CPI and Cycle time
Determined by CPU hardware
We will examine two MIPS implementations
A simplified version - single-cycle
A more realistic version - pipelined
Simple subset, shows most aspects
Memory reference: lw, sw
Arithmetic/logical: add, sub, and, or, slt
Control transfer: beq, j
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ECE Computer Organization

4. Instruction Execution
PC  instruction memory, fetch instruction
ALL instructions must be fetched
Register numbers  register file, read registers
ALL instructions must be decoded
Depending on instruction class
Use ALU to calculate
Arithmetic result
Memory address for load/store
Branch target address
Access data memory for load/store
PC  target address or PC + 4
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ECE Computer Organization

5. CPU Overview (Datapath)
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ECE Computer Organization

6. ECE 445 - Computer Organization
Multiplexers
Can’t just join wires together
Use multiplexers
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7. CPU Overview (Control)
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ECE Computer Organization

8. ECE 445 - Computer Organization
Logic Design Basics
Information encoded in binary
Low voltage = 0, High voltage = 1
One wire per bit
Multi-bit data encoded on multi-wire buses
Combinational element
Operate on data
Output is a function of input
State (sequential) elements
Store information
§4.2 Logic Design Conventions
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9. Combinational ElementsY I0 I1
M u x S
Multiplexer
Y = S ? I1 : I0 +
ALU F
Adder
Y = A + B
Arithmetic/Logic Unit
Y = F(A, B)
AND-gate
Y = A & B
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ECE Computer Organization

10. ECE 445 - Computer Organization
Sequential Elements
Register: stores data in a circuit
Uses a clock signal to determine when to update the stored value
Edge-triggered: update when Clk changes from 0 to 1 D
Clk Q
What if the D Flip-Flop must retain its current value (rather than store a new one)?
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ECE Computer Organization

11. ECE 445 - Computer Organization
Sequential Elements
Register with write control
Only updates on clock edge when write control input is 1
Used when stored value is required later D
Clk Q
Write
How is the Write (enable) signal implemented?
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ECE Computer Organization

12. ECE 445 - Computer Organization
Clocking Methodology
Combinational logic transforms data during clock cycles
Between clock edges
Input from state elements, output to state element
Longest delay determines clock period
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ECE Computer Organization

13. ECE 445 - Computer Organization
Building a Datapath
Datapath
Elements that process data and addresses in the CPU
Registers, ALUs, mux’s, memories, …
Control Unit
Controls the behavior of the elements that comprise the datapath
We will build a MIPS datapath incrementally
Refining the overview design
§4.3 Building a Datapath
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ECE Computer Organization

14. Instruction Fetch Stored Program Computer
32-bit register
Increment by 4 for next instruction
Stored Program
Computer
Program Counter (aka. Instruction Address Register)
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ECE Computer Organization

15. R-Format Instructions
Some examples:
add $s0, $t1, $t0
sub $s3, $s1, $t1
and $t2, $t3, $s1
or $t5, $s6, $s7
sll $t1, $t0, 3
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ECE Computer Organization

16. R-Format Instructions
Read two register operands (rs and rt)
Perform arithmetic/logical operation
Write register result (rd)
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ECE Computer Organization

17. Load/Store Instructions
These are I-Format instructions
Some examples:
lw $s0, 32 ($t1)
sw $s1, 40 ($t2)
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ECE Computer Organization

18. Load/Store Instructions
Read register operands
Calculate address using 16-bit offset
Use ALU, but sign-extend offset
Load: Read memory and update register
Store: Write register value to memory
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ECE Computer Organization

19. ECE 445 - Computer Organization
Branch Instructions
These are I-Format instructions
Some examples:
beq $s2, $t3, L1
bne $t3, $t4, Else
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ECE Computer Organization

20. ECE 445 - Computer Organization
Branch Instructions
Read register operands
Compare operands
Use ALU
Subtract and check Zero output
Calculate target address
Sign-extend displacement
Shift left 2 bits (word displacement)
Add to PC + 4
Already calculated by instruction fetch
What does the
Zero output indicate?
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ECE Computer Organization

21. Branch Instructions Just re-routes wires Sign-bit wire replicated
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ECE Computer Organization

22. Composing the Elements
Simple datapath does an instruction in one clock cycle
Each datapath element can only do one function at a time
Hence, we need separate instruction and data memories
Read instruction memory (every instruction must be fetched)
Read or Write data memory (for lw and sw, respectively)
Use multiplexers where alternate data sources are used for different instructions
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ECE Computer Organization

23. R-Type/Load/Store Datapath
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24. ECE 445 - Computer Organization
R-Type Instructions rs
[25..21] rt
[20..16]
[31..0] rd
[15..11]
Control
Unit
[31..26]
control signals
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ECE Computer Organization

25. ECE 445 - Computer Organization
Example: add $s0, $t0, $t1
add
$t0
[$t0]
$t1
[$t1]
$s0
[$t0] + [$t1]
[$t0] + [$t1]
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ECE Computer Organization

26. ECE 445 - Computer Organization
Example: sub $s3, $s2, $s1
sub
$s2
[$s2]
$s1
[$s1]
$s2
[$s2] - [$s1]
[$s2] - [$s1]
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ECE Computer Organization

27. ECE 445 - Computer Organization
Example: and $t0, $s1, $s2
and
$s1
[$s1]
$s2
[$s2]
$t0
[$s1] & [$s2]
[$s1] & [$s2]
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ECE Computer Organization

28. ECE 445 - Computer Organization
Example: or $s5, $t3, $s3 or
$t3
[$t3]
$s3
[$s3]
$s5
[$t3] | [$s3]
[$t3] | [$s3]
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ECE Computer Organization

29. ECE 445 - Computer Organization
Example: sll $s3, $t1, 2
sll
$t1
$s3
Can the shift operations be implemented with this datapath?
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30. R-Type/Load/Store Datapath
Which datapath components are not required for R-type instructions?
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ECE Computer Organization

31. ECE 445 - Computer Organization
Questions?
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ECE Computer Organization