ELT3047 Computer Architecture
Lecture 7: Single Cycle Processor Design Hoang Gia Hung
Faculty of Electronics and Telecommunications
University of Engineering and Technology, VNU Hanoi Last lecture review ❑ ISAs support arithmetic
➢ Signed and unsigned integers
➢ Floating-point approximation to reals
❑ Arithmetic operations have finite range and precision
➢ Operations can overflow and underflow
➢ Need to account for this in programs ❑ MIPS ALU ISA
➢ Core instructions: 54 most frequently used
▪ 100% of SPECINT, 97% of SPECFP
➢ Other instructions: less frequent
❑ Today’s lecture: Building a single-cycle processor
➢ Additional logic for datapath & control
Building a Processor: Datapath & Control ❑ Components of a processor Datapath
• Collection of components that process data
• Performs the arithmetic, logical and memory operations Control
• Tells the datapath, memory and I/O devices what
to do according to program instructions
❑ Two MIPS implementations …
➢ A simplified version (today)
➢ A more realistic pipelined version (later)
❑ … based on a simple MIPS ISA subset ➢ Memory reference: lw, sw
➢ Arithmetic/logical: add, sub, and, or, slt ➢ Control transfer: beq, j Instruction Execution 1. Fetch: Instruction
▪ Get instruction from memory Fetch
▪ Address is in Program Counter (PC) Register n Instruction 2. Decode: Decode ctio
▪ Find out the operation required Operand 3. Operand Fetch: Instru t Fetch x
▪ Get operand(s) needed for operation Ne Execute 4. Execute:
▪ Perform the required operation Result Write
5. Result Write (Store):
▪ Store the result of the operation MIPS Instruction Execution
❑ Show the actual steps for 3 representative MIPS instructions
➢ Fetch and Decode stages not shown, the standard steps are performed add $3, $1, $2 lw $3, 20($1) beq $1, $2, ofst Fetch standard standard standard Decode
Operand o Read [$1] as opr1
o Read [$1] as opr1
o Read [$1] as opr1 Fetch
o Read [$2] as opr2
o Use 20 as opr2
o Read [$2] as opr2
o MemAddr = opr1 + opr2
Taken = (opr1 == opr2 )? Execute
Result = opr1 + opr2 o Use MemAddr to read
Target = (PC+4) + ofst4 from memory Result if (Taken)
Result stored in $3
Memory data stored in $3 Write PC = Target ◼ opr = operand ◼ ofst = offset ◼
MemAddr = Memory Address
Refined MIPS Instruction Execution ❑ Refined design
➢ Merge Decode and Operand Fetch – Decode is simple for MIPS
➢ Split Execute into ALU (Calculation) and Memory Access add $3, $1, $2 lw $3, 20( $1 ) beq $1, $2, ofst Fetch Read inst. at [PC] Read inst. at [PC] Read inst. at [PC] Decode &
o Read [$1] as opr1
o Read [$1] as opr1
o Read [$1] as opr1 Operand
o Read [$2] as opr2
o Use 20 as opr2
o Read [$2] as opr2 Fetch
Taken = (opr1 == opr2 )? ALU
Result = opr1 + opr2
MemAddr = opr1 + opr2
Target = (PC+4) + ofst4 Memory Use MemAddr to read Access from memory Result if (Taken)
Result stored in $3
Memory data stored in $3 Write PC = Target Building MIPS datapath
❑ We will build a MIPS datapath incrementally: Fetch
➢ Refining the overview design ❑ What we are going to do: Decode n
➢ Look at each stage closely, figure out the requirements and processes ALU
➢ Sketch a high level block diagram, then zoom in for each elements Instructio t
➢ With the simple starting design, check whether x Memory
different type of instructions can be handled: Ne Access
▪ Add modifications when needed
❑ Study the design from the viewpoint of a Result
designer, instead of a "tourist" Write Fetch Stage: Requirements
❑ Instruction Fetch Stage:
1. Use the Program Counter (PC) to fetch the instruction from memory
▪ PC is implemented as a special register in the processor
2. Increment the PC by 4 to get the address of the next instruction:
▪ How do we know the next instruction is at PC+4?
▪ Note the exception when branch/jump instruction is executed
❑ Output to the next stage (Decode):
➢ The instruction to be executed 1. Fetch 2. Decode 3. ALU 4. Memory 5. RegWrite Fetch Stage: Block diagram Increment by 4 for next instruction Decode Stage 32-bit register Element: Instruction Memory
❑ Storage element for the instructions Instruction
➢ It is a sequential circuit Address
▪ Uses a Clk signal to determine when to Instruction update the stored value
▪ Edge-triggered: update when Clk Instruction
changes (edges of the Clk pulses) Memory
➢ Has an internal state that stores information
➢ Clock signal is assumed and not shown Memory
❑ Supply instruction given the address
➢ Given instruction address 𝑀 as input, the ………..
memory outputs the content at address 𝑀 2048 add $3, $1, $2
➢ Conceptual diagram of the memory layout is 2052 sll $4, $3, 2 given on the right 2056 andi $1, $4, 0xF …… ……….. The Idea of Clocking
❑ It seems that we are reading and updating the PC at the same time: ➢ How can it works properly? ❑ Magic of clock
➢ PC is read during the first half of the clock period and it is updated with PC+4
at the next rising clock edge Add Time 4 PC Read Clk address In Instruction PC 100 104 108 112 Instruction In 104 108 112 116 memory Decode Stage: Requirements
❑ Instruction Decode Stage:
➢ Gather data from the instruction fields:
1. Read the opcode to determine instruction type and field lengths
2. Read data from all necessary registers
▪ Can be two (e.g. add), one (e.g. addi) or zero (e.g. j)
❑ Input from previous stage (Fetch):
➢ The instruction to be executed
❑ Output to the next stage (ALU):
➢ Operation and the necessary operands 1. Fetch 2. Decode 3. ALU 4. Memory 5. RegWrite Decode Stage: Block Diagram Register Fetch Stage numbers AL 5 Read Data Read 32 register 1 data 1 U Inst. 5 Read register 2 Register Operands Stag File 5 Write register Read 32 data 2 e Operation Collection of registers, known as register file Element: Register File 5 Read Read 32 register 1 data 1 Register 5 Read numbers register 2 Register File 5 Write Data register 32 Read 32 data 2 Write Data data RegWrite
❑ A collection of 32 registers:
➢ Each 32-bit wide; can be read/written by specifying register number
➢ Read at most two registers per instruction
➢ Write at most one register per instruction
❑ RegWrite is a control signal to indicate: ➢ Writing of register
➢ 1(True) = Write, 0 (False) = No Write
Decode Stage: R-Format Instruction add $8, $9, $10 opc 31: 000 Notation: ode 26 000 Inst [Y:X] = bits X to Y in Instruction 25: 010 rs Inst [25:21] 21 01 5 Read Read 32 content of register 1 data 1 register $9 20: 010 5 Read rt register 2 Register 16 10 File 5 Write 15: 010 register Read 32 content of rd data 2 11 00 32 Write register $10 data sha 10: 000 mt RegWrite 6 00 Result to be stored fun 100 into register $8 5:0 (produced by later ct 000 stage)
Decode Stage: I-Format Instruction addi $21, $22, -50 op 00 31: c 1 od 00 26 e 0 25: 101 rs Inst [25:21] 21 10 5 Read Read 32 content of register 1 data 1 register $22 20: 101 5 Read rt register 2 16 01 Register 5 Write File register 111 Read 32 data 2 Write 1 1 data Imm 11 15: edi 1 110 RegWrite 0 at e 0 1 Problems: 110
- Destination $21 is in the "wrong position"
- Read Data 2 is an immediate value, not from register
Decode Stage: Choice in Destination opc 001 addi $21, $22, -50 31: ode 000 26 25: 101 rs Inst [25:21] 21 10 5 Read Read 32 content of register 1 data 1 register $22 20: 101 5 Read rt register 2 16 01 Register Write File 5 register 111 M Read 32 data 2 Write 1 1 U data Imm X 11 Inst [15:11] 15: edi 1 110 RegWrite 0 at RegDst e 0 1 110
Solution (Write Reg. No.): RegDst:
Use a multiplexer to choose the A control signal to choose correct write register number
either Inst[20:16] or Inst[15:11] as the write register number based on instruction type Element: Multiplexer ❑ Function: control
➢ Selects one input from multiple input lines m ❑ Inputs: in0 ➢ 𝑛 lines of same width . M . U out . ❑ Control: X in ➢ 𝑚 n-1 bits where 𝑛 = 2𝑚 ❑ Output: Control=0 → select in to out ➢ 0
Select 𝑖𝑡ℎ input line if control = 𝑖 Control=3 → select in to out 3 Decode Stage: Choice in Data 2 opc 001 31: addi $21, $22, -50 ode 000 26 25: 101 rs Inst [25:21] 21 10 5 Read Read 32 content of register 1 data 1 register $22 20: 101 5 Read rt register 2 16 01 Register 5 Write File register 111 M Read 32 data 2 M Write 1 1 U data U Imm Inst [15:11] X 11 X 15: edi 1 110 RegWrite 0 at RegDst e 0 ALUSrc Inst [15:0] 16 Sign 32 1 110 Extend A control signal to choose either
Solution (Rd. Data 2): "Read data 2" or the sign extended
Use a multiplexer to choose the correct operand 2. Inst[15:0] as the
Sign extend the 16-bit immediate value to 32-bit second operand
Decode Stage: Load Word Instruction lw $21, -50($22) opc 100 31: ode 011 26 Do we need any modification? 25: 101 rs Inst [25:21] 21 10 5 Read content of Read 32 register 1 data 1 register $22 20: 101 5 Read rt register 2 16 01 Register 5 Write File register 111 M Read 32 data 2 M Write 1 1 U data U Imm Inst [15:11] X 11 X 15: edi 1 110 RegWrite 0 at RegDst e 0 ALUSrc Inst [15:0] 16 Sign 32 1 110 Extend