ROHINI COLLEGE OF ENGINEERING AND TECHNOLOGY
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. 3.2.1 Datapath
[source : V. Carl Hamacher, Zvonko G. Varanesic and Safat G. Zaky, ―Computer Organization]
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
EC - 8552 COMPUTER ARCHITECTURE AND ORGANIZATION
ROHINI COLLEGE OF ENGINEERING AND TECHNOLOGY
An ALU permanently wired to do only addition. No extra control signal required
Fig. 3.2.2 Datapath portion for Instruction Fetch Types of Elements in the
Datapath
[source : V. Carl Hamacher, Zvonko G. Varanesic and Safat G. Zaky, ―Computer Organization]
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
EC - 8552 COMPUTER ARCHITECTURE AND ORGANIZATION
ROHINI COLLEGE OF ENGINEERING AND TECHNOLOGY
• 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
• inputs to the register file specifying the numbers
• bit wide inputs for the 32 registers
• 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 .
EC - 8552 COMPUTER ARCHITECTURE AND ORGANIZATION
