A register is a memory element that can hold a multibit datum of n bits (such as a byte or a word), and it can be read and written. It can be implemented using n D flip-flops, each storing one bit.
A register file consists of a set of registers that can be read and written by supplying a register number to be accessed. it can be implemented with a decoder for each read and write port, and an array of registers
In MIPS, a register file has 32 registers, each storing a 32-bit value, that is 322=1024 bits in total implemented 1024 D flip-flops where all connected to the same clock signal.
In MIPS, we define a special register called zero that always contains the value zero, and it cannot be written to. (we do not present its implementation here)
Register Read
read port: the read register number input connected as the selector of n-to-1 multiplexor with n registers as data inputs of the multiplexor that outputs the read data
for n registers of m bits, a ⌊logn⌋-bit Read Register Number input is used as the selector for n-to-1 multiplexor that outputs m-bit Read Data
In MIPS, Two 5-bit lines (read register 1 and read register 2) specify which registers to read (values between 0 and 31), using two 32-to-1 multiplexors.
If read register 1 and read register 2 are the same, both outputs will contain the same value.
Register Write
write port
A Clock signal is connected to the clock input of all flip-flops
TODO: does not appear in this diagram, but it is connected to the clock input of all flip-flops instead of the AND output, which will be connected to E in the actual implementation.
A Write Enable signal (will be called later RegWrite)
A Register Number input (⌊logn⌋=5 bits) is implemented using a using a ⌊logn⌋-to-n decoder (5-to-32) that each one of its n=32 outputs is connected with the Write Enable signal using an AND gate to the clock input of the corresponding register flip-flop.
A Register Data input (32 bits) is connected to the data input of the corresponding register flip-flop.
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