Datapath

The Datapath is where all data computations take place

• Usually registers, computational stuff, and wires and muxes.
• A big “schematic” that the control unit actually controls.

• The ALU and the Storage Unit
  • Registers, computers, things that connect em
• The Control Unit
  • The “mind” of the processor that decides what’s going on.
  • Basically a big FSM
    • Going between states that invoke different control signals.
  • Synced to system-wide signals (clock, reset)
  • The outputs are just (datapath) control signals
    • These are invoked to do things in the data path; I like to think of the data path as a schematic
    • That would be SelxA, SelAB, ALUop, LdRA, and LdRB (possibly done as well, to say “the FSM is done!“)
      • When done is 1, the output assumes its “meaning”
      • Also go is another way to control the processor (“hey, processor, do things now” or “don’t do anything please”)

Datapath + Control

Example: Make a processor that calculates $x^2+2x$

• This is the schematic given. The inputs and whatnot come from the FSM!
• Only 1 input: X
  • SelxA, SelAB, ALUop, LdRA, and LdRB are all control signals
• LdRA = load register A. Same with LdRB
  • If on, then RA is stored on the rising edge. Otherwise its not
• The sequential parts are the MUX’s are possibly the ALU (depends on how we’re multiplying)
  • We assume we’re using the fat, combinational-circuit-type multiplier
• Implicitly everything is connected by the clock signal.

When creating the algorithm to do this, note that

• Algorithm (would this be the datapath?todo)
  • On the first step, X gets stored into RA and RB. In the second step, we multiply X with itself via the two MUX’s
    • Remember that the ALU has a “pass-through” operation!
  • Now X is in RA and RB and the input. Multiply X by itself using any of these sources to get X^2 and store it into RA
    • RA has x^2 now
    • RB still has x (we did NOT toggle LdRB)
  • We can use the X source and RB and add those together to get 2x and store it into RB.
    • But in other scenarios, the source X could be gone!
    • In that case, we can do x^2 + x, and then x^2 + x + x (i.e. we’re adding from RB twice into RA) Here is the final table; running these in order gets the required result!
• For the state table
  • SelAB is “X” for “dont care” since we are not using B when “copying” A
  • The rest are obvious

The Storage unit thing

• We know of registers