The ARM processor (Thumb-2), part 15: Miscellaneous instructions

Raymond Chen

There are far more ARM instructions than I’m going to cover here. I’ve skipped over the floating point instructions, the SIMD instructions, and some other specialty instructions that I haven’t yet seen come out of the compiler.

Here are a few that are still interesting, even if I haven’t seen the compiler generate them.

    ; count leading zeroes (high order bits)
    clz     Rd, Rm          ; Rd = number of leading zeroes in Rm

    ; reverse bits
    rbit    Rd, Rm          ; Rd = Rm bitwise reversed

    ; reverse bytes
    rev     Rd, Rm          ; Rd = Rm bytewise reversed

    ; reverse bytes in each halfword
    rev16   Rd, Rm          ; Rd[31:24] = Rm[23:16]
                            ; Rd[23:16] = Rm[31:24]
                            ; Rd[15: 8] = Rm[ 7: 0]
                            ; Rd[ 7: 0] = Rm[15: 8]

    ; reverse bytes in lower halfword and sign extend
    revsh   Rd, Rm          ; Rd[31:8] = Rm[ 7:0] sign extended
                            ; Rd[ 7:0] = Rm[15:8]

A few miscellaneous bit-fiddling instructions. The reversal instructions are primarily for changing data endianness.

The next few instructions provide multiprocessing hints.

    ; yield to other threads

    ; wait for interrupt

The YIELD instruction is a hint to multi-threading processors that the current thread should be de-prioritized in favor of other threads. You typically see this instruction dropped into spin loops, via the intrinsic __yield().

The WFI instruction instructs the processor to go into a low-power state until an interrupt occurs. There are other instructions related to “events” which I won’t bother going into.

The next few instructions are for communicating with the operating system:

        svc     #imm8       ; system call
        bkpt    #imm8       ; software breakpoint
        udf     #imm8       ; undefined opcode¹

The system call and breakpoint instructions both carry an 8-bit immediate that the operating system can choose to use for whatever purpose it desires. The breakpoint instruction breaks the rules and always executes even if an encompassing IT instruction would normally cause it to be ignored. In other words, bkpt overrides IT.

The undefined opcode is a block of 256 instructions from 0xde00 through 0xdeff that are architecturally set aside as undefined instructions and which will not be given meaning in future versions of the processor.

But just because the processor leaves them undefined doesn’t mean that operating system can’t give them special meaning. Windows defines custom artificial instructions in the undefined space.²

    __debugbreak            ; udf #0xFE
    __debugservice          ; udf #0xFD
    __assertfail            ; udf #0xFC
    __fastfail              ; udf #0xFB
    __rdpmccntr64           ; udf #0xFA
    __brkdiv0               ; udf #0xF9

Most of these are special ways of manually generating specific exceptions.

Opcode Exception Notes
__debugbreak STATUS_BREAKPOINT The “real” breakpoint instruction.
__debugservice STATUS_BREAKPOINT Communicate with debugger, r12 is function code.
__fastfail STATUS_STACK_BUFFER_OVERRUN Misleadingly-named.

The __brkdiv0 instruction is emitted by the compiler if it detects a zero denominator.

    cbnz    r0, @F          ; jump if denominator is nonzero
    __brkdiv0               ; oops: manually raise div0 exception
@@: bl      __rt_sdiv       ; software divide/remainder
                            ; (r0, r1) = (r1 ÷ r0, r1 mod r0)

The last artificial instruction is __rdpmccntr64, which reads a 64-bit cycle counter. This special instruction has a dedicated fast path through the trap handler, so it can produce the result in around 60 cycles.

There is also an instruction to access coprocessor registers.

    ; move register from coprocessor
    mrc (a bunch of stuff)

The coprocessor registers are encoded in a totally wacky way. There’s no point learning what each of the values means. All that matters is that they represent the register you want to read.

There are a few coprocessor registers named software thread ID register which are not used by the processor, but are provided with the intention that operating systems use them to record per-thread information. The two available from user mode are named TPIDRURW and TPIDRURO; the first is read-write and the second is read-only. Windows uses TPIDRURW to hold the thread information.

And of course, we have this guy:


Actually, there are two of this guy, a 16-bit NOP and a 32-bit NOP. The NOP instruction does nothing but occupy space. Use it to pad code to meet alignment requirements, but do not use it for timing because processors are allowed to optimize it out, or even run faster.

Now that we have the basic instruction set under our belt, we’ll look at the calling convention next time.

Bonus chatter: Why doesn’t Windows use udf #0xff? The gcc toolchain uses udf #0xff as its “We should never get here” trap instruction. Putting an artificial instruction there would cause such a program to continue executing after it thought it had triggered a fatal exception.

¹ Although the ARM documentation provides the udf mnemonic for the undefined instruction, not all assemblers recognize it, so you may be forced to encode the hex value directly into your code if that’s what you want.

² I don’t know why Windows chose the udf space for these artificial opcodes instead of using the svc space. Maybe there’s some fine print in the processor manual that makes svc unsuitable for this sort of thing. We know that bkpt is a bad choice for an artificial opcode because bkpt executes even if an encompassing IT instruction would have skipped it.

Then again, use of udf to create artificial instructions is explicitly listed in the processor architecture manual as a valid use of the udf instruction, so at least it’s not breaking any unwritten rules.


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  • Oliver Atkins 0

    Thanks for this fun series. It reminds me of dipping my toe into ARM assembler on my Archimedes A410 as a kid. You could access an ARM assembler through BBC BASIC, all of which was in ROM, so it was very easy to get started.

    • Neil Rashbrook 0

      Although that would have been an ARM-2, so somewhat different to the Thumb ARM that’s being described here. For instance, in classic ARM, you have the luxury of 16,777,216 different software interrupts! (I don’t know how many the Archimedes actually used though.)

      • Simon Farnsworth 0

        RISC OS splits the 24 bit SWI numbers into a set of bitfields. The OS itself reserves all SWI numbers below 0x40000 for its own use, and thus the SWI table was very sparse.

  • Jonathan Duncan 0

    ARM NOP facts:

    1. Prior to ARMv6 ARM didn’t really have a NOP instruction, if you wrote “NOP” you’d actually get “mov r0, r0″(ARM) or “mov r8, r8″(THUMB)
    2. In ARM mode `00 00 00 00` encodes to `andeq r0, r0, r0` which is effectively a NOP, which is handy to remember.
    3. In THUMB mode `00 00` encodes to `movs r0, r0` which is almost a NOP (but does corrupt flags).

    The last two things means in case of data corruption causing a jump to uninitialised data you’ll sometimes see access violation at the first byte after a executable region. This means the CPU has fallen off the end of a mapped region after sliding through pages of uninitialised `00` bytes.

  • Jonathan Duncan 0

    I’m not sure what the windows ARM syscall convention is, but on Linux despite the fact that the `svc` instruction provides a handy #imm8 (or #imm24 in ARM mode), this isn’t used to hold the syscall number.

    This is not just because they might need more than 256 syscalls, but is also to do with the fact that the ARM CPU doesn’t provide any easy way of accessing the immediate of this instruction once the syscall is trapped into supervisor mode.

    The old Linux OABI syscall convention did use to use the immediate but this required reading and (essentially) disassembling the last instruction at the userspace PC to extract the immediate.

    This had issues with being fairly slow, and required having additional code in place to handle the fact the read could potentially page fault.

    As far as I can tell everyone has decided that the `svc` immediate was all-around a bad idea and resorted to putting the syscall number in a register.

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