Register file

Need a group of registers to store operands (32 in MIPS)

Registers are very fast memory, part of the CPU

Why not use only registers for all data?

Costs more to make registers than RAM

More registers mean more circuits to control them, therefore slower

Consider 32 integer registers inside a black box called a register file

Set of registers with combinational logic to select

Instruction to run:

add $1, $2, $3  # R[1] <- R[2] + R[3]

Fetch operands:

Need values in registers 2 and 3

To tell the register file which registers to use:

Need a value from 0 to 31

How many bits? ceil( lg( 32 ) ) = 5 bits

Need to specify 5 bits for each register

Where to get the 10 bits for register numbers?

From the instruction itself

IR (instruction register)

Hidden register containing the instruction currently being executed