1. CSCI206 - Computer Organization & Programming
Single Cycle Datapath
zyBook: 11.3

2. The Basic MIPS Datapath
Needs extra wires for jal / jr
How is next pc selected? (control)
How fast can it be clocked?

3. Instruction Decoding
Examine the bit pattern in the instruction to determine the exact instruction, data registers, and the immediate value, if any.
R-type instruction
I-type instruction
J-type instruction
R-type: 31 25 20 15 5 op rs rt rd
funct
shamt 10
I-type: 31 25 20 15 5 op rs rt
immediate 10
J-type: 31 25 20 15 5 op
destination address 10

4. Executing R Format Operations
R format operations (add, sub, slt, and, or)
perform operation (op and funct) on values in rs and rt
store the result back into the Register File (into location rd)
R-type: 31 25 20 15 5 op rs rt rd
funct
shamt 10
Instruction
Write Data
Read Addr 1
Read Addr 2
Write Addr
Register
File
Read
Data 1
Data 2
ALU
overflow
zero
ALU control
RegWrite
Fetch
PC = PC+4
Decode
Exec
Note that Register File is not written every cycle (e.g., sw), so we need an explicit write control signal for the Register File
Since writes to the register file are edge-triggered, we can legally read and write the same register within a clock cycle – the read will get the value written in an earlier clock cycle, which the value written will be available to a read in a subsequent cycle.

5. Executing Load and Store Operations
Load and store operations involves
compute memory address by adding the base register (read from the Register File during decode) to the 16-bit signed-extended offset field in the instruction
store value (read from the Register File during decode) written to the Data Memory
load value, read from the Data Memory, written to the Register File
Instruction
Write Data
Read Addr 1
Read Addr 2
Write Addr
Register
File
Read
Data 1
Data 2
ALU
overflow
zero
ALU control
RegWrite
Data
Memory
Address
Read Data
Sign
Extend
MemWrite
MemRead 16 32
store
load
Note there are separate read and write controls to the memory – only one of which may be asserted on any given clock cycle. The memory unit needs a read signal, since, unlike the register file, reading the value of an invalid address can cause problems as we will see later. (Standard memory chips actually have a write enable signal that is used for writes.)

6. Executing Branch Operations
Branch operations involves
compare the operands read from the Register File during decode for equality (zero ALU output)
compute the branch target address by adding the updated PC to the 16-bit signed-extended offset field in the instruction

7. Executing Branch Operations
Instruction
Write Data
Read
Data 1
Data 2
ALU
zero
ALU control
Sign
Extend 16 32
Shift
left 2
Add 4 PC
Branch
target
address
(signal “zero” sent to branch control logic)
Read Addr 1
Register
File
Read Addr 2
Write Addr

8. Executing Jump Operations
Jump operation involves
replace the lower 28 bits of the PC with the lower 26 bits of the fetched instruction shifted left by 2 bits
Add 4 4
Jump
address
Instruction
Memory
Shift
left 2 28
Read
Address PC
Instruction 26

9. Composing the Elements
Morgan Kaufmann Publishers
13 November, 2018
Composing the Elements
First-cut data path 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
Use multiplexers where alternate data sources are used for different instructions
Chapter 4 — The Processor

10. R-Type/Load/Store Datapath
Morgan Kaufmann Publishers
13 November, 2018
R-Type/Load/Store Datapath
Chapter 4 — The Processor

11. Morgan Kaufmann Publishers
Full Datapath
13 November, 2018
Chapter 4 — The Processor