Processor design: the single-cycle datapath and control

You now know what RV32I instructions do and how they are encoded, and lesson 4 supplied the building blocks: registers, multiplexers, adders, an ALU and a register file. This lesson puts them together into a working processor. The design here, a single-cycle implementation, executes each instruction completely in one long clock cycle. No real high-performance processor works this way, but every real processor contains the same datapath, and pipelining, the subject of lesson 10, is easiest to understand as a way of overlapping the steps of this design. By the end of the lesson you will be able to trace any RV32I instruction through the datapath, state the value of every control signal, and work out the clock period from component delays.

The method follows the way processors are actually designed. First list what each class of instruction needs to do. Then assemble a datapath that can do all of it, adding multiplexers wherever two instructions need different inputs at the same point. Finally design the control unit, the combinational logic that looks at each instruction's opcode and function fields and sets every multiplexer and write enable accordingly.

What each instruction needs

Every instruction begins the same way. The processor fetches the instruction from memory at the address in the PC, and computes PC + 4, the address of the next instruction in sequence. After that, the classes diverge. R-type instructions read two registers, pass them through the ALU and write the result back to a register. I-type arithmetic does the same with an immediate in place of the second register. Loads compute an address with the ALU from a register and an immediate, read data memory and write the loaded value to a register. Stores compute an address the same way and write a register's value to data memory. Branches compare two registers and, if the condition holds, replace PC + 4 with PC plus a sign-extended offset. jal writes PC + 4 to rd and jumps to PC + offset; jalr writes PC + 4 to rd and jumps to rs1 + offset. lui writes an upper immediate to rd, and auipc writes PC plus an upper immediate.

Reading down the table shows where the datapath needs choices. The ALU's second input is sometimes a register and sometimes an immediate, so a multiplexer selects it. The value written back to the register file is sometimes the ALU result, sometimes data from memory and sometimes PC + 4, so a three-input multiplexer selects it. The next PC is sometimes PC + 4, sometimes PC plus an offset and sometimes the ALU result, so another multiplexer selects that. Each choice becomes a control signal.

The datapath

The single-cycle datapath has these components. The PC is a 32-bit register. The instruction memory is read at the PC's address and outputs the 32-bit instruction; in this simple design it is separate from the data memory, so an instruction fetch and a data access can happen in the same cycle, an arrangement called a Harvard architecture at this level and approximated in real processors by separate instruction and data caches. The register file has two read ports, addressed directly by the rs1 and rs2 fields of the instruction, and one write port, addressed by rd. An immediate generator extracts the immediate from whichever format the instruction uses and sign-extends it to 32 bits. The ALU computes the arithmetic or logic result. The data memory is read or written at the address the ALU produces. Two adders compute PC + 4 and PC + immediate, and multiplexers route values between the units.

Because the register fields sit in the same place in every format, as lesson 5 emphasized, the register file can start reading rs1 and rs2 as soon as the instruction arrives, before the control unit has decided what the instruction is. If the instruction turns out not to use rs2, the value read is simply ignored. Reading registers speculatively like this costs nothing and saves time, and it is the payoff for the slightly awkward S and B formats.

The immediate generator is a good example of how the encoding shapes the hardware. It needs a multiplexer for each of the 32 output bits, choosing which instruction bit, or the sign bit, goes there according to the format. Because the RISC-V formats keep immediate bits in the same instruction positions whenever possible, and always take the sign from instruction bit 31, most output bits have only two or three possible sources, which keeps the generator small and fast.

Control

The control unit is combinational logic that takes the opcode, funct3 and funct7 fields and produces the control signals. In the design here, they are: RegWrite, which enables writing the register file; ALUSrc, which selects a register or the immediate as the ALU's second input; ALUOp, which tells the ALU what to compute; MemRead and MemWrite, which enable data memory; WBSel, which selects the value to write back; and PCSel, which selects the next PC. The branch decision combines a Branch signal from the control unit with the comparison result: the PC takes the branch target only when the instruction is a branch and its condition holds.

# Main control signals for each class of instruction (x = don't care)
SIGNALS = ['RegWrite', 'ALUSrc', 'ALUOp', 'MemRead', 'MemWrite', 'WBSel', 'PCSel']
CONTROL = {
    'R-type': (1, 'reg', 'funct', 0, 0, 'alu', 'pc+4'),
    'I-ALU':  (1, 'imm', 'funct', 0, 0, 'alu', 'pc+4'),
    'load':   (1, 'imm', 'add',   1, 0, 'mem', 'pc+4'),
    'store':  (0, 'imm', 'add',   0, 1, 'x',   'pc+4'),
    'branch': (0, 'reg', 'cmp',   0, 0, 'x',   'pc+imm if taken'),
    'jal':    (1, 'x',   'x',     0, 0, 'pc+4', 'pc+imm'),
    'jalr':   (1, 'imm', 'add',   0, 0, 'pc+4', 'alu'),
    'lui':    (1, 'imm', 'pass',  0, 0, 'alu', 'pc+4'),
}
print(f'{"class":<8}' + ''.join(f'{s:>9}' for s in SIGNALS[:-1]) + '  PCSel')
for cls, vals in CONTROL.items():
    print(f'{cls:<8}' + ''.join(f'{str(v):>9}' for v in vals[:-1]) + f'  {vals[-1]}')

class    RegWrite   ALUSrc    ALUOp  MemRead MemWrite    WBSel  PCSel
R-type          1      reg    funct        0        0      alu  pc+4
I-ALU           1      imm    funct        0        0      alu  pc+4
load            1      imm      add        1        0      mem  pc+4
store           0      imm      add        0        1        x  pc+4
branch          0      reg      cmp        0        0        x  pc+imm if taken
jal             1        x        x        0        0     pc+4  pc+imm
jalr            1      imm      add        0        0     pc+4  alu
lui             1      imm     pass        0        0      alu  pc+4

Two features of the table matter. First, a write enable must never be a don't-care: a store with RegWrite accidentally set to 1 would corrupt a register, so RegWrite and MemWrite always have definite values. Second, many signals are don't-cares for some classes, and designers exploit them to simplify the logic, just as the sum-of-products simplification in lesson 4 did. In practice the main control unit produces a short ALUOp code, and a second, smaller block, the ALU control, combines ALUOp with funct3 and funct7 to choose the exact ALU operation: for an R-type instruction, funct3 = 000 with funct7 = 0000000 means add, and with funct7 = 0100000 means subtract. Splitting control into levels keeps each block small, a standard technique.

A single-cycle simulator

The behaviour of the whole datapath fits in a short program. The simulator below executes real RV32I machine words, the encodings produced by the LLVM assembler for a nine-instruction program, and prints the value on each important wire in every cycle: the PC, the instruction, the register values read, the immediate, the ALU result, any memory access and the value written back. Each loop iteration is one clock cycle, and every state change, to registers, memory and the PC, happens together at the end of the iteration, exactly as at a clock edge.

PROGRAM = [0x00500293, 0x00700313, 0x006283B3, 0x00702023, 0x00002E03,
           0x406E0EB3, 0x005E8463, 0x00100F13, 0x00200F93]
NAMES = ['addi t0,zero,5', 'addi t1,zero,7', 'add t2,t0,t1', 'sw t2,0(zero)', 'lw t3,0(zero)',
         'sub t4,t3,t1', 'beq t4,t0,8', 'addi t5,zero,1', 'addi t6,zero,2']
ABI = {5: 't0', 6: 't1', 7: 't2', 28: 't3', 29: 't4', 30: 't5', 31: 't6'}
sext = lambda v, b: v - (1 << b) if v >> (b - 1) else v

def immediate(w, op):
    if op in (0x13, 0x03): return sext(w >> 20, 12)                       # I-type
    if op == 0x23: return sext(((w >> 25) << 5) | ((w >> 7) & 31), 12)    # S-type
    if op == 0x63:                                                         # B-type
        return sext(((w >> 31) << 12) | (((w >> 7) & 1) << 11) | (((w >> 25) & 63) << 5) | (((w >> 8) & 15) << 1), 13)
    return 0

reg, mem, pc = [0] * 32, {}, 0
while pc // 4 < len(PROGRAM):
    w = PROGRAM[pc // 4]
    op, rd, f3 = w & 0x7F, (w >> 7) & 31, (w >> 12) & 7
    rs1, rs2, f7 = (w >> 15) & 31, (w >> 20) & 31, w >> 25
    a, b, imm = reg[rs1], reg[rs2], immediate(w, op)             # register file read
    alu_b = b if op in (0x33, 0x63) else imm                      # ALUSrc multiplexer
    alu = a - alu_b if (op == 0x33 and f7 == 32) else a + alu_b   # ALU (add or sub here)
    wb, note, next_pc = None, '', pc + 4
    if op in (0x33, 0x13): wb = alu
    elif op == 0x03: wb = mem.get(alu, 0); note = f'read M[{alu}]={wb}'
    elif op == 0x23: note = f'write M[{alu}]={b}'
    elif op == 0x63:
        taken = a == b                                            # beq comparison
        note = 'taken' if taken else 'not taken'
        if taken: next_pc = pc + imm                              # PCSel multiplexer
    print(f'PC={pc:2d} {NAMES[pc // 4]:<15} rs1={a:3d} rs2={b:3d} imm={imm:3d} '
          f'ALU={alu:4d} {note:<14}' + (f'{ABI[rd]}<-{wb}' if wb is not None else ''))
    # clock edge: all state updates happen together
    if op == 0x23: mem[alu] = b
    if wb is not None and rd: reg[rd] = wb
    pc = next_pc
print('final:', {ABI[i]: reg[i] for i in ABI})

PC= 0 addi t0,zero,5  rs1=  0 rs2=  0 imm=  5 ALU=   5               t0<-5
PC= 4 addi t1,zero,7  rs1=  0 rs2=  0 imm=  7 ALU=   7               t1<-7
PC= 8 add t2,t0,t1    rs1=  5 rs2=  7 imm=  0 ALU=  12               t2<-12
PC=12 sw t2,0(zero)   rs1=  0 rs2= 12 imm=  0 ALU=   0 write M[0]=12
PC=16 lw t3,0(zero)   rs1=  0 rs2=  0 imm=  0 ALU=   0 read M[0]=12  t3<-12
PC=20 sub t4,t3,t1    rs1= 12 rs2=  7 imm=  0 ALU=   5               t4<-5
PC=24 beq t4,t0,8     rs1=  5 rs2=  5 imm=  8 ALU=  10 taken
PC=32 addi t6,zero,2  rs1=  0 rs2=  0 imm=  2 ALU=   2               t6<-2
final: {'t0': 5, 't1': 7, 't2': 12, 't3': 12, 't4': 5, 't5': 0, 't6': 2}

Follow the beq row: t4 = 12 − 7 = 5 equals t0, so the branch is taken and the PC jumps from 24 to 32, skipping the instruction that would have set t5. The final register values confirm it: t5 is still 0 and t6 is 2. Note also that in the beq row the ALU result is computed but nothing uses it; in the single-cycle design every unit computes something every cycle, and control decides which results matter.

Timing: why single-cycle is slow

In a single-cycle design, every instruction must finish within one clock period, so the period must be long enough for the slowest instruction. The slowest is the load, which uses every unit in sequence: instruction memory, register read, ALU, data memory, write-back multiplexer and register setup. An add, which skips data memory, could finish much sooner, but it still has to wait for the clock. The table of delays below is illustrative but realistic in its proportions, with memory accesses the slowest components.

# Illustrative component delays in picoseconds
D = {'imem': 250, 'regread': 150, 'mux': 25, 'alu': 200, 'dmem': 250, 'regsetup': 20}
paths = {
    'R-type': ['imem', 'regread', 'mux', 'alu', 'mux', 'regsetup'],
    'load':   ['imem', 'regread', 'mux', 'alu', 'dmem', 'mux', 'regsetup'],
    'store':  ['imem', 'regread', 'mux', 'alu', 'dmem'],
    'branch': ['imem', 'regread', 'mux', 'alu', 'mux'],
    'jal':    ['imem', 'mux', 'regsetup'],
}
times = {k: sum(D[c] for c in v) for k, v in paths.items()}
for k, t in times.items():
    print(f'{k:>6}: {t} ps')
period = max(times.values())
print(f'single-cycle clock period = {period} ps ({1e3 / period:.2f} GHz)')

mix = {'R-type': 0.44, 'load': 0.24, 'store': 0.12, 'branch': 0.16, 'jal': 0.04}
avg = sum(mix[k] * times[k] for k in mix)
print(f'average time actually needed per instruction = {avg:.0f} ps')
print(f'time wasted by the fixed clock = {100 * (1 - avg / period):.0f}%')

R-type: 670 ps
  load: 920 ps
 store: 875 ps
branch: 650 ps
   jal: 295 ps
single-cycle clock period = 920 ps (1.09 GHz)
average time actually needed per instruction = 736 ps
time wasted by the fixed clock = 20%

The single-cycle design has a CPI of exactly 1, but its clock period is set by the load, and on average a fifth of every cycle is spent waiting. Worse, the problem grows with richer instruction sets: a floating-point divide would stretch every cycle for every instruction. Two classic responses exist. A multicycle design breaks each instruction into steps of one short cycle each, letting simple instructions take fewer cycles than complex ones, at the cost of a CPI above 1. A pipelined design, lesson 10's subject, also breaks instructions into steps but overlaps them, so that a new instruction starts every cycle. Pipelining wins, which is why essentially every processor since the late 1980s is pipelined.

Beyond the basic datapath

The design in this lesson omits several things a real processor needs. It has no way to handle exceptions, such as an illegal instruction or a misaligned access, or interrupts from devices; RISC-V handles these by saving the PC in a control and status register called mepc (or sepc in supervisor mode), recording the cause in mcause, and jumping to a handler address, as lesson 12 describes. It supports only word loads and stores; byte and halfword accesses need extra logic to select and extend bytes. And it treats memory as if every access took one cycle, which real memory, as lesson 11 shows, does not. Each extension adds multiplexers, control signals and states, but none changes the basic structure: fetch, decode and read registers, execute, access memory, write back.