What is a ``reflective'' system?
Sun, 11 May 1997 12:44:28 -0500 (CDT)
Bill House wrote:
> Jordan Henderson wrote:
> >Perhaps the Lisp assembly could assemble down to the LispVM. But, what
> >will the LispVM be written in? C? This might work well with Flux OS,
> >which seems to define it's interfaces in C language terms. Also, if
> >we're assembling down to a LispVM, then we could influence the LispVM
> >design to make sure there are good subroutine and exception mechanisms
> >built into it at a low level, and then make the assembler a very high
> >level assembler, complete with all kinds of syntactic sugar and
> >syntactic support for exceptions. Maybe I need to be posting this to
> >the LispVM list. I hope they are looking at Dis from Inferno.
> >There's reason to believe that a register based VM like Dis is orders
> >of magnitude simpler to implement as a JIT compiler and much-much
> >smaller (I understand that Acer and other's have chosen Inferno to build
> >NetPCs on instead of Java because they estimate the VM and support will
> >be 1 MB of RAM instead of 4 MB of RAM required for JVM).
> I like these ideas. LispVM is in all likelihood going to be written in C,
> although I suppose there's some merit to the idea of writing it in Scheme using
> FWIW, the AgentBase VM runs its SmartLisp test suite in 2MB.
The big problem with this idea (making a Lisp Assembler which assembles to
a LispVM) is that the system is not fully 'reflective', the LispVM could
not be defined in terms of it. Using a Scheme/Lisp->C compiler to try to
write the lowest level stuff still means that the GC, Virtual Memory and
some other low-level stuff will be written in C.
I think that perhaps the goal of full reflectivity is not best. If we could
define a LispVM separately, perhaps with several implementations (C, JIT,
interpreted Lisp for prototyping) then the rest of the system could be
fully reflective assembling/compiling to this. It would require that a
LispVM backend be written for CMU-CL eventually. I can see objections
already. We have people who want to use CMU-CL with native compilation
for everything, without regard to whether this is a good idea or not. I
think these people have "come in late from the picture show" and didn't
hear that we were going to have a LispVM.
> Bill House
> The views expressed are mine alone,
> unless you agree with me.