Single cycle CPU¶
How to execute an instruction¶
Instruction Fetch (IF)¶
- Fetches the instruction pointed to by PC from instruction memory
Note
Also calculates PC + 4 for the next instruction (PC + \(x\)) where \(x\) is the size (in bytes) of current instruction
Instruction Decode (ID)¶
- Decode and read register operands (rs2, rs1)
Execute (EX)¶
- Execute according to instruction type
Memory Reference (MEM)¶
- Memory access
Note
For instruction sd, it will store rs2 to memory and write PC + 4 into PC
Register Write Back (WB)¶
- Write register back to RF and PC
Components¶
- ALU
- PC: Program counter, points to the next instruction to be executed
- Adder: increments or decrements the PC
- Register file: where the registers live
- Instruction memory: stores instructions 啊不然勒
- Data memory: stores data 啊不然勒
- Control signals: control behaviour of ALU and read write paths
- Sign extension unit: ALU needs unsigned 32 bit numbers, whether to do subtraction is controlled by ALUOp
Control Signals¶
BranchMemReadMemtoRegALUOpMemWriteALUSrcRegWrite
RISC-V and MIPS architecture difference
MIPS: needs additional RegDst control signal to indicate which register to write to
- R-type: the destination is called
rd - I-type: the destination is called
rt