The Processor Andreas Klappenecker CPSC321 Computer Architecture

The Processor: Datapath and Control We want to implement portions of MIPS memory-reference instructions: lw, sw arithmetic-logical instructions: add, sub, and, or, slt control flow instructions: beq, j We ignore multiply, divide, and other integer and floating point instructions.

Goals Explain main design principles of datapath control Simplicity Short exposition of Verilog

Implementation of Instructions Fetch instruction Send the PC to the memory location containing the next instruction Read registers fetch registers using fields of the instruction lw needs just one register most other instructions need two registers Next steps depend on instruction class

Implementation of Instructions II Once register operands have been fetched, they can be operated on to compute a memory address (lw and sw) to compute an arithmetic result (int ops) to compare (for a branch) Use output of the ALU

Implementations of Instructions III Output of ALU is written to a register in the case of arithmetic-logical instructions used as an address in the case of load and store instructions to determine the next instruction address in the case of branch instructions

Abstract, simplified view Two types of functional units: elements that operate on data values (combinational) elements that contain state (sequential) More Implementation Details

Combinatorial and Sequential Elements The ALU is a combinatorial element Other elements of the design are not combinatorial, but contain a state An element with some internal storage is called a state element State elements have at least two inputs: data clock (determines when data is written)

Clocking Methodology We need to decide when signals can be read and written We need to specify timing behavior For simplicity, we assume an edge-triggered clocking strategy (synchronous design) All storage elements are updated on either raising edge or falling edge: cycle time rising edge falling edge

Synchronous Design Typical execution: – read contents of some state elements, – send values through some combinational logic – write results to one or more state elements

Refresh you memory! Read Appendix B about Logic Design Keywords latch D-flip flop gates clock …

Two inputs: the data value to be stored (D) the clock signal (C) indicating when to read & store D Two outputs: the value of the internal state (Q) and it's complement D-latch

D flip-flop Output changes only on falling clock edge

Conclusions  We will use D-flip flops to build the register file  We gradually build up the datapath  Simple components will allow us to do this  We add the control logic a little later  You will need a firm understanding of logic design  Study Chapter 4, read Appendix B