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CIT593_M5 Module Summary & Key Takeaways - Google Docs
Key Takeaways from Module 4: Datapath
Module Learning Objectives
● Describe how a basic von Neumann model CPU/microprocessor can be implemented using combinational and sequential logic devices
● Describe the datapath and control pathways present in a CPU
● Write “machine language” that can program a CPU
● Describe the purpose and implementation of a program counter in a CPU
● Understand and analyze the hardware components needed to allow for conditional programming statements (if/else and loops) to be supported by a CPU
Module Glossary
● Central processing unit (CPU): the electronic circuitry within a computer that carries out the instructions of a computer program by performing the basic arithmetic, logic, controlling, and input/output operations specified by those instructions; also called a central processor or main processor
● Program counter(PC): a register in a computer processor that contains the address (location) of the instruction being executed at the current time
● ALU: a combinational digital electronic circuit that performs arithmetic and logical operations
● Datapath: pathway that the data flows through in a CPU; set up by control signals specified by an instruction to the CPU
● Register file: contains small amount of data our processor operates upon
● Control memory: holds control signals for our processor; essentially holds the “program” we want our CPU to execute
● TEST box: allows us to alter the value of the PC by testing what comes out of the ALU for a particular condition
● von Neumann model: computer architecture based on a 1945 description by the mathematician/physicist John von Neumann and others. It contains the following 4 pieces:
○ Processing unit (contains ALU and register file)
■ Performs all calculations (“overworked clerk”)
○ Memory ( for instructions and data memory)○ Control unit (contains PC)
■ Serves as index into program memory to indicate which instruction
must be carried out by the processing unit○ Input/output
■ Provides a means for information to be loaded into or read from the internals of the machine
● von Neumann loop: 4 steps to be performed by the von Neumann model machine to carry out an instruction on the machine:
1) Fetch instruction from program memory
2) Decode instruction from program memory (determine control signals)
3) Execute instruction in ALU (commit data to register file if needed)
4) Update program counter repeat loop
Study Tips From Your TAs
● Building a basic CPU
○ 4 main pieces:
1) Memory2) Data and instructions3) PC4) ALU
○ CPU as a finitestate machine
■ In general, an FSM consists of 3 basic parts:
● An Nbit register which stores the state of the machine
○ In our CPU, this would be the memory
● A block of logic that computes the next state as a function of the current state and any inputs
○ In our CPU, this would be the adder/subtractor (input = control mem)
● A block of logic which computes the output based on the current state
○ We don’t have this in our CPU yet, but we will!
● NZP Tester
○ Two internal parts to the TEST component
■ 1) Determine if incoming # is negative/zero/positive■ 2) Compare incoming condition from user to output of part 1