R-format Datapath

DATAPATH FOR R-FORMAT

R type instructions (e.g. ADD $t1, $t2, $t3)

---> steps:

    • Read two registers

        – Register file

    • Perform an ALU operations

        – ALU

    • Write the result into a register

        – Register file

Datapath component

--->Register file

• A collection of registers in which any register can be read or written by specifying the number of register (register address) in the file.  

• Needs a write control signal “RegWrite”.

• How many ports are required?

---> ALU

R- FORMAT INSTRUCTION

• Read two register operands (each 5 bits)
• Perform arithmetic/logical operation (6 bits)
• Write register result (5 bits)

R- FORMAT INSTRUCTION DATAPATH

TABLE 2

Table 2 shows the operation of the datapath for R-format instruction, such as add $t0, $t1, $t2. The operation:

1.The instruction is fetched, and the PC is incremented

2.Two registers, $t1 and $t2, are read from the register file; also RegDst, RegWrite and ALUOp is set.

3.The ALU operates on the data read from the register file, using the function code (bits 5:0, in the funct field) to generate the ALU function


4.The result from the ALU is written into the register file using bits 15:11 of the instruction to select the destination register ($t0)


LOAD / STORE INSTRUCTION

---> Read register operands

---> Calculate address using 16-bit offset

         ◦Use ALU, but sign-extend offset

---> Load: Read memory and update register


---> Store: Write register value to memory

LOAD / STORE DATAPATH

TABLE 3

Table 3 illustrate the execution of load word such as lw $t1, 4($t2) :

1.Instruction is fetched from the instruction memory, and PC is incremented.

2.Value of register $t2 is read from the register file.

3.The ALU computes the sum of the value read from the register file and the sign-extended, lower 16 bits of the instruction (offset = 4).

4.The sum from the ALU is used as the address for the data memory.

5.The data from the memory unit is written into the register file; the register destination is given by bits 20:16 of the instruction ($t1)

 

 

BY AINI KHAIRANI BT AZMI