The MIPS R4000, part 2: 32-bit integer calculations

The MIPS R4000 has the usual collection of arithmetic operations, but the mnemonics are confusingly-named. The general notation for arithmetic operations is

    OP      destination, source1, source2

with the destination register on the left and the source register or registers on the right.

Okay, here goes. We start with addition and subtraction.

    ADD     rd, rs, rt      ; rd = rs + rt, trap on overflow
    ADDU    rd, rs, rt      ; rd = rs + rt, no trap on overflow
    SUB     rd, rs, rt      ; rd = rs - rt, trap on overflow
    SUBU    rd, rs, rt      ; rd = rs - rt, no trap on overflow

The ADD and SUB instructions perform addition and subtraction and raise a trap if a signed overflow occurs. The ADDU and SUBU instructions do the same thing, but without the overflow trap. The U suffix officially means "unsigned", but this is confusing because the addition can be performed on both signed and unsigned values, thanks to twos complement. The real issue is whether an overflow trap is raised.

There are also versions of the addition instructions that accept a 16-bit signed immediate as a second addend:

    ADDI    rd, rs, imm16   ; rd = rs + (int16_t)imm16, trap on overflow
    ADDIU   rd, rs, imm16   ; rd = rs + (int16_t)imm16, no trap on overflow

Note that the U is double-confusing here, because even though the U officially stands for "unsigned", the immediate value is treated as signed, and the addition is suitable for both signed and unsigned values.

There are no corresponding SUBI or SUBIU instructions, but they can be synthesized:

    ADDI   rd, rs, -imm16   ; SUBI   rd, rs, imm16
    ADDIU  rd, rs, -imm16   ; SUBIU  rd, rs, imm16

(Of course, this doesn't work if the value you want to subtract is −32768, but hey, it mostly works.)

The next group of instructions is the bitwise operations. These never trap.¹

    AND     rd, rs, rt      ; rd = rs & rt
    ANDI    rd, rs, imm16   ; rd = rs & (uint16_t)imm16
    OR      rd, rs, rt      ; rd = rs | rt
    ORI     rd, rs, imm16   ; rd = rs | (uint16_t)imm16
    XOR     rd, rs, rt      ; rd = rs ^ rt
    XORI    rd, rs, imm16   ; rd = rs ^ (uint16_t)imm16
    NOR     rd, rs, rt      ; rd = ~(rs | rt)

Note the inconsistency: The addition instructions treat the immediate as a signed 16-bit value (and sign-extend it to a 32-bit value), but the bitwise logical operations treat it as an unsigned 16-bit value (and zero-extend it to a 32-bit value). Stay alert!

The last group of instructions for today is the shift instructions. These also never trap.

    SLL     rd, rs, imm5    ; rd = rs <<  imm5
    SLLV    rd, rs, rt      ; rd = rs <<  (rt % 32)
    SRL     rd, rs, imm5    ; rd = rs >>U imm5
    SRLV    rd, rs, rt      ; rd = rs >>U (rt % 32)
    SRA     rd, rs, imm5    ; rd = rs >>  imm5
    SRAV    rd, rs, rt      ; rd = rs >>  (rt % 32)

The mnemonics stand for "shift left logical", "shift right logical" and "shift right arithmetic". The V suffix stands for "variable", and indicates that the shift amount comes from a register rather than an immediate.

Yup, that's another inconsistency. Following the pattern of the addition and bitwise logical groups, these instructions should have been named SLL for shifting by an amount specified by a register and SLLI for shifting by an amount specified by an immediate. Go figure.

There are no built-in sign-extension or zero-extension instructions. You can get zero-extension in one instruction by explicitly masking out the upper bytes:

    ; zero extend byte to word
    ANDI    rd, rs, 0xFF    ; rd = ( uint8_t)rs

    ; zero extend halfword to word
    ANDI    rd, rs, 0xFFFF  ; rd = (uint16_t)rs

Sign extension requires two instructions.

    ; sign extend byte to word
    SLL     rd, rs, 24      ; rd = rs << 24
    SRA     rd, rd, 24      ; rd = (int32_t)rd >> 24

    ; sign extend halfword to word
    SLL     rd, rs, 16      ; rd = rs << 16
    SRA     rd, rd, 16      ; rd = (int32_t)rd >> 16

And I'm going to mention these instructions here because I can't find a good place to put them:

    SYSCALL imm20           ; system call
    BREAK   imm20           ; breakpoint

Both instructions trap into the kernel. The system call instruction is intended to be used to make operation system calls; the breakpoint instruction is intended to be used for software breakpoints. Both instructions carry a 20-bit immediate payload that can be used for whatever purpose the operating system chooses.

Here are some more instructions you can synthesize from the official instructions:

    SUB     rd, zero, rs    ; NEG     rd, rs
    SUBU    rd, zero, rs    ; NEGU    rd, rs
    ADDU    rd, zero, rs    ; MOVE    rd, rs
    OR      rd, zero, rs    ; MOVE    rd, rs
    NOR     rd, zero, rs    ; NOT     rd, rs
    SLL     zero, zero, 0   ; NOP
    SLL     zero, zero, 1   ; SSNOP

There are many possible ways of synthesizing a MOVE instruction, but in order to be able to unwind exceptions, Windows NT requires that register motion in the prologue or epilogue of a function must take one of the two forms given above.

Similarly, there are many ways of performing a NOP. Basically, any non-trapping 32-bit computation that targets the zero register is functionally a nop, but the two above are treated specially by the processor.

  • NOP = SLL zero, zero, 0 is special-cased by the processor as a nop that can be optimized out entirely. Use it when you need to pad out some code for space.
  • SSNOP = SLL zero, zero, 1 is special-cased by the processor as a nop that must be issued, and it will not be simultaneously issued with any other instruction. Use it when you need to pad out some code for time. (The SS stands for "super-scalar".)

The encoding of SLL zero, zero, 0 happens to be 0x00000000, which I'm sure is not a coincidence. I'm not convinced that it's a good idea, though. I would have chosen 0x00000000 to be the encoding of a breakpoint or invalid instruction.

Okay, those are the 32-bit computation instructions. Next time, we'll look at multiplication, division, and the temperamental HI and LO registers.

¹ Alas, there is no NORI instruction. You think I'm joking, but I'm not. Be patient.

Comments (14)
  1. Antonio Rodríguez says:

    In the 6502, the BRK instruction’s opcode is 0x00 ($00 in 6502 notation). This has the side effect of dropping into the debugger whenever there is a jump to a position in memory that doesn’t contain code (0 is pretty common among data or uninitialized memory, so there is a big chance that the processor will hit a zero pretty soon if something goes bad).

  2. calvin says:

    Just be careful: SLL 0, 0, 0 might not work properly on some processors:

  3. Having all-bits-zero be a valid instruction is definitely not a good idea… having it be NOP is worse. It’s easy to have a bug in your virtual memory system that results in getting pages of zeros where you intended to have code; if that happens, and you jump to that page, the processor silently executes the NOPs. Then it either (if you’re lucky) runs off the end of that chunk of memory and triggers a fatal trap, or (otherwise) gets to a non-zeroed page, where it will continue executing in some entirely unrelated code. When that code then crashes it can be very difficult to figure out what happened or how you managed to get a clearly impossible call stack.

    1. Petteri Aimonen says:

      Another not-good idea is how some microcontrollers such as ARM Cortex-M have vector table at 0x0000. Having null pointer be a mapped address is bad enough, having it be a mapped address that always contains lots of valid pointers is even worse. It’s easy to end up in a very confusing state when calling e.g. a virtual function on a null pointer to C++ object.

      1. Joshua says:

        The x86 has that in real mode. I’m 2 steps from the guy who formatted his hd by it.

      2. Patrick says:

        x86 realmode has the same issue, with the IVT living at address 0:0.
        In fact, if I’m not totally mistaken, there were special hacks in Win95 (using expand-down segments) to protect it from misbehaving applications.

    2. Scarlet Manuka says:

      0x00 was the NOP instruction on the Z80 as well (the first processor I ever got down to machine code with). Of course, not many people were putting a virtual memory system onto one of those.

      1. R P (MSFT) says:

        I had a Z80-based S-100 box (made by Digitex) that sported 256KB of RAM and a 20MB hard drive. It was also a multi-user system, so it’s more than likely that it wasn’t using virtual memory as we know it now, but just switching between static pages on context switches.

    3. Medinoc says:

      The Motorola 68k had both a 16-bit NOP with a specific code, and a 0x00000000 instruction that decoded to “ORI.b #0, D0” (or in C terms, “D0 |= 0”) which is which is, in fact, also a no-op.

      1. Falcon says:

        Close to a no-op, but not quite – it affected condition codes.

      2. Medinoc says:

        Well thinking about it, it might not be a complete no-op: It’s possible it sets the status bits (zero, negative etc.) according to the D0 register.

  4. Evan says:

    > The U suffix officially means “unsigned”, but this is confusing because the addition can be performed on both signed and unsigned values, thanks to twos complement. The real issue is whether an overflow trap is raised.

    Not that this makes them a good name, but I wonder if the names come from C. C signed int overflow is undefined behavior, meaning a trap is an acceptable response (and I actually think is a /good/ response, though I’m sure that’s controversial), so the trapping ADD/SUB instructions can be used. Unsigned overflow* /is/ defined though, so it can’t trap — or rather, it could, but then it would just have to continue execution, so that’s slow — and you have to use the non-trapping ADDU/SUBU instructions.

    I don’t know what MIPS compliers actually do, and Compiler Explorer doesn’t seem to have a MIPS option. It’d be interesting to know.

    * Technically, the C and C++ standards say that, because it’s defined, unsigned arithmetic /can’t/ overflow, because overflow is UB. But I’m not convinced anyone uses “overflow” in that sense who doesn’t read the C/C++ standards for fun. :-)

    1. Hubert Lamontagne says:

      I’ve definitely seen C++ code that overflows signed integers on purpose. In an emulator for the sound chip of the SNES, the calculations are done in 16-bit 2’s complement, and to get the original wraparound behavior (needed to get the wind sound in Chrono Trigger), you have to do essentially this:
      int32_t value = adpcm_decode();
      value = value <> 16;

      1. Voo says:

        No you don’t “have to do” any such things. Undefined behavior is exactly that: undefined. Writing the correct code is a negligible extra amount of work and means your code will actually work.

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