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0 pointsbetterunix12y ago0 comments

All of those features are already in SBCL, in one form or another. I think the issue is a little deeper than that; for example, the difficulty of producing a small executable in SBCL, one which does not come with the entire CL runtime attached to it. There is also the difficulty of debugging low-level code in SBCL.

In other words, the problem is not with Lisp, nor with a lack of support for low-level features, but with tooling. The tool support for writing a kernel in Common Lisp is pretty bad at the moment, and there are not many people trying to improve on that.

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zhemao12y ago· 2 in thread

It does? Could you point to some documentation? I'd be really interested in that.

betterunixOP12y ago

Those features are mostly in SBCL's FFI, which is (somewhat) documented here:

http://www.sbcl.org/manual/#Foreign-Function-Interface

You can disable the garbage collector with without-gcing or gc-off. To manually allocate and deallocate you need to go beyond SBCL, at least as far as I know; you can call malloc and free directly if you want (via the FFI) or you can write your own allocator using the low-level memory access facility.

Hiding in the low-level code in the FFI and in SBCL itself are "system area pointers:"

http://www.sbcl.org/manual/#index-int_002dsap

Finally, inline assembly. This is the least documented of the three features you mentioned, but here is an example of how to do it with the VOP facility:

http://pvk.ca/Blog/Lisp/hacking_SSE_intrinsics-part_1.html

Unfortunately, all of the above are extremely hard to use due to the lack of good tooling. Manual memory management is possible, but going from a pointer to a block of memory to a Lisp object is not at all straightforward (it is possible with a lot of low-level hacking using the sb-vm and sb-sys packages). You can define VOPs, but it involves a lot of steps; you are probably better off just using the FFI, but debugging across the FFI boundary is hard, and the FFI imposes a performance hit that might outweigh the benefit of your assembly language code.

Like I said, the problem here is tooling. I suspect that there is a lack of demand for these things among SBCL and CMUCL users, and in the Lisp community in general.

zhemao12y ago

Ahh, yeah, hmm. That's not exactly what I meant. A language for writing an OS on a conventional CPU would need to allow straightforward control of the memory for its own data structures, not just foreign memory. In lieu of that, I suppose you could turn just off the GC when doing time-sensitive things (like responding to interrupts).

That's sort of besides the point, though, because the main problem with SBCL and other CL implementations is that they are written for use in userspace programs and depend on having access to kernel system calls and library functions like malloc(). So if you were to write a kernel in CL, you'd first have to write an implementation for CL that can run on bare metal. At that point, you might as well just implement a different Lisp dialect that only has the features you need in order to write the kernel.

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