question about jump in MIPS

What does the PCGPRLEN-1..28 means here?? Where d开发者_如何转开发oes this 4 bit comes from?

alt text http://img208.imageshack.us/img208/3750/jumpv.jpg

Im guessing that you are wondering why theres a 28 bit field, when the OP-CODE is alredy taking 6 bits.

Well, it turns out that the 26 bit field is shifted left twice. Thus, the destiny address is 4 byte aligned.

So, its "like" a 28 bit field. With it you can jump inside 256 MB aligned regions (as your img states)

And to answer your question, straight from MIPS 32 Arquitecture for programmers, volume II (downloadable from mips.com),page 14.

GPRLEN The length in bits (32 or 64) of the CPU general-purpose registers