# The Datapath --- CS 130 // 2025-10-29 <!--=====================================================================--> # Continuing our Processor Design <!-- .slide: data-background="#004477" --> <!--=====================================================================--> ## Downloads if you need them [Logisim Register File and ALU](https://ericmanley.github.io/Fall2025/CS130/assets/RegisterFileAndALU.circ) <!--=====================================================================--> ## 12:30 section progress  * I was rushed to show you this working, but we forgot to hook up the *clock* and the register file's *write* enable <!--=====================================================================--> ## 2:00 section progress  <!--=====================================================================--> ## What's next? We have enabled `addi` instructions ```mips addi $8, $0, 5 ``` but what about a regular `add`? ```mips add $10, $8, $9 ``` <img src='/Fall2025/CS130/assets/images/decodezoom.png' height='300'> <!--=====================================================================--> ## Group discussion (12:30) Where would you draw wires to support an `add`?  <!--=====================================================================--> ## Group discussion (2:00) Where would you draw wires to support an `add`?  <!--=====================================================================--> # Course Themes <!-- .slide: data-background="#004477" --> <!--=====================================================================--> ## Overarching Theme - Learning how a high-level program is actually executed on your computer's processor <!---------------------------------->  <!----------------------------------> ## Trajectory of the Course 1. Assembly language programming 2. Digital logic 3. **Processor architecture** 4. The C Programming Language <!--=====================================================================--> # Review <!-- .slide: data-background="#004477" --> <!--====================================================================--> ## Review - **CPU:** Central Processing Unit - the circuits that execute instructions from a program - AKA "the processor" - Processor architecture - logical design of CPU - how do memory, the ALU, Register File, etc. all fit together? <!----------------------------------> <!--  -->  image from https://www.scientecheasy.com/2024/09/laptop-components.html/ <!---------------------------------->  <!---------------------------------->  <!--====================================================================--> # The Datapath <!-- .slide: data-background="#004477" --> <!--=====================================================================--> ## The Datapath - What is a "datapath" and what does it consist of? + <!-- .element: class="fragment"--> It is the main component of the CPU that executes instructions + <!-- .element: class="fragment"--> Fetches instructions and data from memory, performs arithmetic, etc. + <!-- .element: class="fragment"--> Consists of ALUs, a register file, RAM, and various supplementary logic gates <!--====================================================================--> ## Instruction Execution 1. Uses the PC to fetch the next instruction to be executed from memory 2. <!-- .element: class="fragment"--> Identifies instruction type and registers involved and fetches their contents from the register file 3. <!-- .element: class="fragment"--> Depending on the instruction: - <!-- .element: class="fragment"--> Uses the ALU to do appropriate arithmetic - <!-- .element: class="fragment"--> Read/write RAM for load/store - <!-- .element: class="fragment"--> Update PC to PC + 4 or the jump target address <!--====================================================================--> # Building a Datapath <!-- .slide: data-background="#004477" --> <!--====================================================================--> ## Instruction Fetch  <!----------------------------------> ## Review: Handling Instruction Fetch - We started this in Logisim  <!----------------------------------> ## Decoding the Instruction - You can set your splitter to have custom fan-out ranges <div class="twocolumn"> <div>  </div> <div>   </div> </div> <!--====================================================================--> ## R-Format Instructions - Read two register operands, perform ALU operation, write result to target register  <!----------------------------------> ## Inputs to the Register File   <!----------------------------------> ## Connecting it to the ALU   <!--====================================================================--> ## Load/Store Instructions - Read register operands, calculate address, read/write to memory  <!--====================================================================--> ## Branch Instructions - Read register operands, compare operands, calculate target address <!---------------------------------->  <!--====================================================================--> ## The Full Datapath <img src='/Fall2025/CS130/assets/images/COD/full_datapath.png' height='500'>