The MIPS R4000 can perform multiplication and division in hardware, but it does so in an unusual way, and this is where the temperamental HI and LO registers enter the picture.

The HI and LO registers are 32-bit registers which hold or accumulate the results of a multiplication or addition. You cannot operate on them directly. They are set by a suitable arithmetic operation, and by special instructions for moving values in and out.

The multiplication instructions treat HI and LO as a logical 64-bit register, where the high-order 32 bits are in the HI register and the low-order 32 bits are in the LO register.

    MUL     rd, rs, rt      ; rd = rs * rt, corrupts HI and LO
    MULT    rs, rt          ; HI:LO = rs * rt (signed)
    MULTU   rs, rt          ; HI:LO = rs * rt (unsigned)

The simplest version is MUL which multiples two 32-bit registers and stores a 32-bit result into a general-purpose register. As a side effect, it corrupts the HI and LO registers. (This is the only multiplication or division operation that puts the result in a general-purpose register instead of into HI and LO.)

The MULT instruction multiplies two signed 32-bit values to form a 64-bit result, which it stores in HI and LO.

The MULTU instruction does the same thing, but treats the factors as unsigned.

The next group of multiplication instructions performs accumulation.

    MADD    rs, rt          ; HI:LO += rs * rt (signed)
    MADDU   rs, rt          ; HI:LO += rs * rt (unsigned)
    MSUB    rs, rt          ; HI:LO -= rs * rt (signed)
    MSUBU   rs, rt          ; HI:LO -= rs * rt (unsigned)

After performing the appropriate multiplication operation, the 64-bit result is added to or subtracted from the value currently in the HI and LO registers.

Note that the U suffix applies to the signed-ness of the multiplication, not to whether the operation traps on signed overflow during addition or subtraction. None of the multiplication instructions trap.

The operation runs faster if you put the smaller factor in rt, so if you know (or suspect) that one of the values is smaller than the other, you can try to arrange for the smaller number to be in rt.

You might think that the division operations take a 64-bit value in HI and LO and divide it by a 32-bit register. But you’d be wrong. They divide a 32-bit value by another 32-bit value and store the quotient and remainder in in HI and LO.

    DIV     rd, rs, rt      ; LO = rs / rt, HI = rs % rt (signed)
    DIVU    rd, rs, rt      ; LO = rs / rt, HI = rs % rt (unsigned)

None of the division operations trap, not even for overflow or divide-by-zero. If you divide by zero or incur division overflow, the results in HI and LO are garbage. If you care about overflow or division by zero, you need to check for it explicitly.

Okay, that’s great. We’ve done some calculations and put the results into HI and LO. But how do we get the answer out? (And how do you put the initial values in, if you are using MADD or MSUB?)

    MFHI    rd              ; rd = HI "move from HI"
    MFLO    rd              ; rd = LO "move from LO"
    MTHI    rs              ; HI = rs "move to HI"
    MTLO    rs              ; LO = rs "move to LO"

The multiplication and division operations take some time to execute,¹ and if you try to read the results too soon, you will stall until the results are available. Therefore, it’s best to distract yourself with some other operations while waiting for the multiplication or division operation to do its thing. (For example, you might check if you need to raise a runtime exception because you just asked the processor to divide by zero.)

The temperamental part of the HI and LO registers is in how you read the values out.

Tricky rule number one: Once you perform a MTHI or MTLO instruction, both of the previous values in HI and LO are lost. That means you can’t do this:

    MULT    r1, r2          ; HI:LO = r1 * r2 (signed)
    ... stuff that doesn't involve HI or LO ...
    MTHI    r3              ; HI = r3
    ... stuff that doesn't involve HI or LO ...
    MFLO    r4              ; r4 = GARBAGE

You might naïvely think that the MTHI replaces the value in the HI register and leaves the LO register alone, but since this is the first write to either of the HI or LO registers since the last multiplication or division operation, both registers are lost, and your attempt to fetch the value of LO will return garbage.

Note that this applies only to the first write to HI or LO. The second write behaves as you would expect. For example, if you perform MTHI followed by MTLO, the MTHI will set HI and corrupt LO, but the MTLO will set LO and leave HI alone.

Tricky rule number two: If you try to read a value from HI or LO, you must wait two instructions before performing any operation that writes to HI or LO.² Otherwise, the reads will produce garbage. The instruction that writes to HI or LO could be a multiplication or division operation, or it could be MTHI or MTLO.

Tricky rule number two means that the following sequence is invalid:

    DIV     r1, r2          ; LO = r1 / r2, HI = r1 % r2 (signed)
    ... stuff that doesn't involve HI or LO ...
    MFHI    r3              ; r3 = r1 % r2 GARBAGE
    MULT    r4, r5          ; HI:LO = r4 * r5 (signed)

Since the MULT comes too soon after the MFHI, the MFHI will put garbage into r3. You need to stick two instructions between the MFHI and the MULT in order to avoid this.

(Tricky rule number two was removed in the R8000. On the R8000, if you perform a multiplication or division or MTxx too soon after a MFxx, the processor will stall until the danger window has passed.)

Okay, next time we’ll look at constants.

¹ Wikipedia says that latency of 32-bit multiplication was 10 cycles, and latency of 32-bit division was a whopping 69 cycles.

² Commenter David Holland explains that this weird rule is due to a pipeline hazard: The multiply or divide operation is not recalled if an exception occurs while the operation is in flight. If the MFLO and a subsequent multiply are both in flight and an interrupt occurs, the multiply will complete by the time the exception handler gets around to saving the HI and LO registers. When execution resumes at the MFLO, it will read the low result of the following multiplication, rather than the preceding one. That’s why you have to wait two cycles: You have to make sure that the MFLO has cleared the pipeline before initiating any new operations that may write to HI and LO.