Why do we need to Bit Extend the J type instruction to 2 bits only?

Question

Please have a look at this Single Cycle Data Path in MIPS. The 26 bits of J type instruction are being Bit Extended to 28. I don't get the point. Shouldn't it be extended to 31 so it makes 32 bits overall. Please help me out to clear the concept. Thanks

Answer 1

This is really no sign extension. Recall that the instructions in MIPS are 4-byte aligned.

This means that you can start an instruction at addresses which are 0 modulus 4 (i.e. 0, 4, 8, 12, ...)

Now, doing a shift left of 2 two bits is like multiplying by 4, which yields numbers which are always 0 modulus 4.

The actual address will be formed with: - the 4 most significant bits of the nPC (that is PC+4) (lets call them PPPP) - the 26 bits of the address field specified in the instruction, (lets call them AAA....AA) - 00 as the two least significant bits (which yields the required instruction alignment)

Thus the address will be (binary) PPPPAAAAAAAAAAAAAAAAAAAAAAAAAA00