Building Data Path and Control Implementation Scheme - Processor And Control Unit - Computer Science Engineering (CSE) PDF Download

BUILDING DATA PATH AND CONTROL IMPLEMENTATION SCHEME

Datapath

• Components of the  processor that perform arithmetic operations and holds data.

Control

• Components of the processor that commands the datapath, memory, I/O devices according to the instructions of the memory.

Building a Datapath

•  Elements that process data and addresses  in the CPU - Memories, registers, ALUs.
• MIPS datapath can be built incrementally by considering only a subset of instructions
• 3 main elements  are

Fig. 3.1 Datapath

• A memory unit to store instructions of a program and supply instructions given an address. Needs to provide only read access (once the program is loaded).- No control signal is needed
• PC (Program Counter or Instruction address register) is a register that holds the address of the current instruction
•  A new value is written to it every clock cycle. No control signal is required to enable write
•  Adder to increment the PC to the address of the next  instruction ​
• An ALU permanently wired to do only addition.  No extra control signal required

Fig. 3.2 Datapath portion for Instruction Fetch

Types of Elements in the Datapath

State element:

• A memory element, i.e., it contains a state
•   E.g., program counter, instruction memory Combinational element:
• Elements that operate on values
• Eg adder ALU E.g. adder, ALU

Elements  required by the different classes of instructions

• Arithmetic and logical instructions
• Data transfer instructions
• Branch instructions

R-Format ALU Instructions

• E.g., add $t1, $t2, $t3
• Perform arithmetic/logical operation
• Read two register operands and write register  result

Register file:

• A collection of the registers
• Any register can be read or written by specifying  the number of the register
• Contains the register state of the computer

Read from register

• 2 inputs to the register file specifying the numbers
• 5 bit wide inputs for the 32 registers
• 2 outputs from the register file with the read values
• 32 bit wide
• For all instructions. No control required.
• Write to register file
• 1 input to the register file specifying the number  5 bit wide inputs for the 32 registers
• 1 input to the register file with the value to be written  32 bit wide
• Only for some instructions. RegWrite control signal.

ALU

• Takes two 32 bit input and produces a 32 bit output
• Also, sets one-bit signal if the results is 0
• The operation done by ALU is controlled by a 4 bit control signal input. This is set according to the instruction .