Motorola 68k Application Binary Interface (ABI)

https://m680x0.github.io/doc/abi.html

by mshockwave • 5 years ago

63 24 5 years ago

m680x0.github.io

It's great to see the 68k backends of GCC and LLVM getting love again.

In many respects the 68000 is the oldest most "retro" architecture for which you can still run a modern compiler suite. Processor introduced in 1979 for which you could compile a Rust binary today. That's pretty amazing.

jagrsw3• 5 years ago

The ISA is really beautiful (maybe with the exception of BCD additions). I have a m68k disk image/kernel which works with qemu-system-m68k, out of historical sentiment (used to have A500, now I have A4000 - sadly no time to play with it, so in my basement).

As for abi/syscalls. I noticed that strace reports hundreds of calls to get_thread_area() under Linux/m68k. I presume under x86, arm etc. some CPU register is used to store pointer to the TLS, but under m68k "they" decided to invoke syscall every single time. I don't think it'd be very efficient, and also it pollutes strace output, unfortunately, given how frequently TLS is accessed these days.

  $ uname -a
  Linux ds-m68k 5.16.0-rc1-g02e0dceb9e3b #7 Fri Jan 14 01:27:22 CET 2022 m68k GNU/Linux
  $ strace -c /bin/ls
  % time     seconds  usecs/call     calls    errors syscall
  ---- ----------- ----------- --------- --------- ----------------
  0.00    0.000000           0         6           read
  0.00    0.000000           0         1           write
  0.00    0.000000           0        10           close
  0.00    0.000000           0         1           execve
  0.00    0.000000           0         2         2 access
  0.00    0.000000           0         3           brk
  0.00    0.000000           0         3           ioctl
  0.00    0.000000           0         1           munmap
  0.00    0.000000           0         2         2 statfs
  0.00    0.000000           0         1           uname
  0.00    0.000000           0        10           mprotect
  0.00    0.000000           0        14           mmap2
  0.00    0.000000           0         2           getdents64
  0.00    0.000000           0         8           openat
  0.00    0.000000           0       285           get_thread_area
  0.00    0.000000           0         1           set_thread_area
  0.00    0.000000           0         9           statx
  ---- ----------- ----------- --------- --------- ----------------
  100.00    0.000000           0       359         4 total

jandrese2• 5 years ago

Maybe that is a form of exploit mitigation? Maybe they can't cache the TLS pointer because the kernel may change it out from under you without warning? I do agree that it smells like a possible performance issue and it seems like there must be a better way to do it.

windenntw• 5 years ago

Could you post a bit more info about that binary? (eg: run ldd to dump the used libs, etc)

Could you get some kind of stack trace at the time of the get_thread_area call?

I'd like to be able to pinpoint the call(s) somewhere in

https://codesearch.debian.net/search?q=get_thread_area&perpk...

Findecanor• 5 years ago

I have been playing with defining my own toy ABI, and I came up with a trick for thread-local storage:

Allocate the segment for TLS and the thread stack together so that both start (stack grows down, TLS up) at a 2*n - aligned address, with n large enough so that the stack size never exceeds 2*n. Then, you could always get TLS-1 from SP by setting the lower bits to 1.

To avoid an odd size for the segment, you could put the TLS block at negative offset from the end of the segment.

I had intended mine for machines with 64-bit pointers, but data structures tend to be smaller on 32-bit machines, so they can cope with smaller stacks. On the M68K Amiga the stack size was fixed and often relatively small: recommended minimum was 16000 bytes, and for large programs recommended size was 80000. Modern operating systems for CPUs with MMUs grow the stack dynamically but still have to set aside a part of the address space for the maximum it could grow into.

pcwalton• 5 years ago

I wouldn't say anything with separate register files for addresses and data is "beautiful", but it's certainly miles better than other microprocessors at the same time like the 65816.

gmueckl• 5 years ago

The 68k series are rather simple designs by today's standard and should be old enough to have lost patent protection. How hard would it be to reimplement the ISA as a Verilog/VHDL softcore?

gpderetta2• 5 years ago

It has been done. Search for the Apollo 68080 for example.

edit: also relevant is the the PiStorm, a software emulated 680x0 running on a RaspberryPi that plugs into a 680x0 socket.

edit2: the MiST/MiSTer are also well known 680x0 open source FPGA implementations.

herio• 5 years ago

The AmigaOS ABI was really designed for the m68k, it leverages registers as much as possible. Pass variables in data registers, pointers in address registers. A4 for local data, A6 for the library pointer, results in D0.

That said, most library routines would then store the register contents on the stack and restore on return so not sure it's more efficient for memory access etc.

The m68k is the most CISC of ISAs I've seen, you have plentiful addressing modes and register orthogonality. It had adressing modes to do auto postincrements/predecrements, indirect indexed accesses and all sorts of fun.

mrlonglong3• 5 years ago

It would be nice to see the m68k get some 64 bit love to bring it right up to date. I've been considering what such a beast could look like compared to the cruddy x86 64 bit ISA.

cmrdporcupine3• 5 years ago

The moment you start doing that you are going to lose backwards compatibility anyways, so may as well go back to the drawing board and revisit other compromises made in the ISA (like separate data and address registers, for example. Or big endian instead of little endian) and you end up with something else, IMHO.

Maybe something closer to a 64-bit version of the NS32k ISA.

KerrAvon• 5 years ago

Those changes would lose the character of the M68k, though.

jerrysievert2• 5 years ago

I really miss my copy of the m68k programmers reference manual. when I finally had to learn x86, I remember being so frustrated at how fractured it was compared to the beauty that was m68k at the time.

mshockwave• 5 years ago

I think you're talking about this one? https://m680x0.github.io/doc/official-docs

jerrysievert2• 5 years ago

I was actually talking about this one (sorry about the amazon link): https://www.amazon.com/M68000-16-bit-microprocessor-Programm...

much more blue cover - I think I kept it until about 10 years ago, when I did a purge of physical books that didn't have a fairly deep sentimental attachment. of course, that still ended up with hundreds of pounds of paper.

Motorola 68k Application Binary Interface (ABI)

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