In MIPS, what are HI and LO

Question

I'm reading about division in MIPS and I've found that div

Divides $s by $t and stores the quotient in $LO and the remainder in $HI

https://web.archive.org/web/20201111203150/http://www.mrc.uidaho.edu/mrc/people/jff/digital/MIPSir.html

And Wikipedia says

HI and LO are used to access the multiplier/divider results, accessed by the mfhi (move from high) and mflo commands.

http://en.wikipedia.org/wiki/MIPS_architecture

Are HI and LO registers? What number registers are they?

Answer 1

These are special registers used to store the result of multiplication and division. They are separate from the $0 .. $31 general purpose registers, not directly addressable. Their contents are accessed with special instructions mfhi and mflo (Move From HI/LO).

They are present in the Multiply Unit and are 32-bits each. More info here. As a pair, they hold the 64-bit full result of a 32x32-bit integer mult.

---

Raymond Chen's blog article The MIPS R4000, part 3: Multiplication, division, and the temperamental HI and LO registers has some very good info about early MIPS's non-intuitive behaviours, including mtlo / mtlo invalidating the previous hi / lo (respectively).

The incomplete integer instruction-set reference (linked in the question) for early MIPS also has some details, http://www.mrc.uidaho.edu/mrc/people/jff/digital/MIPSir.html, or see MIPS's official PDF manuals, or PDFs of manuals for classic MIPS CPUs.

Answer 2

HI and LO are not numbered registers, IIRC. They are only used to store the results of operations that would not fit in a single register (e.g. multiplying two 32-bit integers could result in a 64 bit integer, so the overflow goes in HI).

edit: according to this class description, they are indeed special registers, so they are not numbered, and only accessible using special commands.