The SuperH-3, part 7: Bitwise logical operations

Raymond Chen

August 13th, 20192 0

The SH-3 has a rather basic collection of bitwise logical operations.

    AND Rm, Rn          ; Rn &= Rm
    AND #imm, r0        ; r0 &= unsigned 8-bit immediate

    OR  Rm, Rn          ; Rn |= Rm
    OR  #imm, r0        ; r0 |= unsigned 8-bit immediate

    XOR Rm, Rn          ; Rn ^= Rm
    XOR #imm, r0        ; r0 ^= unsigned 8-bit immediate

    NOT Rm, Rn          ; Rn = ~Rm

Nothing fancy. No nor or nand or andnot or other goofy bitwise operations. Just plain vanilla stuff. Do note that the 8-bit immediate is unsigned here.

There is also an instruction for testing bits wthout modifying anything other than the T flag.

    TST Rm, Rn              ; T = ((Rn & Rm) == 0)
    TST #imm, r0            ; T = ((r0 & signed 8-bit immediate) == 0)

The test instruction performs a bitwise and and compares the result with zero. In this case, the 8-bit immediate is signed.

But wait, there’s something goofy after all: Load/modify/store instructions!

    AND.B #imm, @(r0, GBR)  ; @(r0 + gbr) &= 8-bit immediate
    OR.B  #imm, @(r0, GBR)  ; @(r0 + gbr) |= 8-bit immediate
    XOR.B #imm, @(r0, GBR)  ; @(r0 + gbr) ^= 8-bit immediate
    TST.B #imm, @(r0, GBR)  ; T = ((@(r0 + gbr) & 8-bit immediate) == 0)

These .B versions of the bitwise logical operations operate on a byte in memory indexed by the r0 and gbr registers. Okay, so TST.B is not a load/modify/store; it’s just a load, but I included it in this group because he wants to be with his friends.

In practice, the Microsoft compiler does not generate these instructions.

Finally, we have this guy, the only truly atomic instruction in the SH-3 instruction set.

    TAS.B @Rn              ; T = (@Rn == 0), @Rn |= 0x80

The test-and-set instruction reads a byte from memory, compares it against zero (setting T accordingly), and then sets the high bit and writes the result back out. This was clearly designed for building low-level synchronization primitives, but I’m not sure anybody actually uses it.

I say that it is the only truly atomic operation because it holds the data bus locked for the duration of its operation. The load/modify/store instructions we saw above do not lock the bus, so it’s possible for a coprocessor to modify the memory out from under the SH-3.

That’s it for the logical operations. Next up are the bit shifting operations.

Raymond Chen

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2 comments

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  • Yukkuri Reimu 0

    If it is the only atomic instruction and nobody uses it how do you construct locks? Do you just never have DMA or multiprocessing of any kind?

    • Fabian Giesen 0

      The SH-3 is from the mid-90s and targeted at embedded applications. I don’t think it has any SMP support. (For that matter, I don’t think the versions of Windows CE that ran on it had SMP support either.)
      IO with peripherals that have DMA capability usually has explicit hand-offs of some kind or other, not locks in shared memory. And between processes or threads, you don’t need hardware-level atomic operations on a single-processor system, just some way of guaranteeing that the scheduler doesn’t context-switch out of a thread at the wrong time. There are several ways of accomplishing this but the important part is that the OS can do it by itself.

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