function calls (long)

[Albert van der Horst]

In article <12350 at goofy.megatest.UUCP
> djones at megatest.UUCP (Dave Jones) writes:
> ( In a follow up on Tim Olson)c
>What I would like to see is a register-set that acts as the cache for
>a virtual stack in a machine with true stack machine instructions. That
>would be my idea of a really good time, although it might put some graph
>colorers out of business, having no registers to allocate. Does anybody
>know if such a scheme was proposed for SPARC, and if so, why it was
>rejected? Seems like such a win, and not all that big a step
>beyond the register-window approach.

This idea is not new.
It is about time for this idea to provide the most ideal machine
to write compilers for.

 There is a weird language that you may not know about. It is called
FORTH, and it is based on a virtual machine. This virtual machine is 
IMHO exactly right. 
  It has two stacks, one for the return addresses, and one for
the calculation; the latter is visible to the FORTH programmer.
The designer of FORTH, Charles Moore, has cast this idea into silicon.
This resulted in a RISC chip (NOVIX 4000) with some amazing properties.
For instance, a one cycle call, and mostly a FREE return from a 
subroutine. It also allocates registers as needed.  
  I was the president of a small group in the Netherlands that explored
this idea independantly. We got far enough to create a working emulator
(written in FORTH...) for our chip, baptized FIETS (Fast implementation
of Enhanced Translators and Systems, by sheer accident also the dutch
word for bycicle). Then came Moore and interest in the project faded.
Two of the group members gained hands on experience with the NOVIX,
resulting in for example a program that generates a video signal by
toggling an output pin, showing a picture with the name "NOVIX".

  Despite some coverage by BYTE the NOVIX did not become a succes at
all. In my opinion, real mistakes were the lack of a C-compiler for it
- the FIETS contained 16 stack based general purpose registers, 
that would have made such a C-compiler viable. The NOVIX registers were
much less in number, caused by a too narrow Forth based scope. Also
Charles Moore still insists that programs needing more than a 16 bit 
address range are desing errors.... Marketing was directed towards the
incrowd of FORTH programmers. Probably very few of you would
have noted the value of the chip from the BYTE article. Even to a FORTH-
literate the article in BYTE was hard to digest. The end was: NOVIX went
out of business. 

   Fortunately, the basic idea's have been bought by Harris Semiconductor's
and they know better what makes a processor sing. They have already
produced a few practically successful designs, and are still developping
it further. These chips are optimized for real time signal processing, and
have extremely fast context switches. (switching stacks, no registers
to save)

I do hope we will arrive at the processor bottom line.
  The bottom line is this.
You have the choice between your code being inline, or a single cycle
call/return overhead.
All instructions (let's say almost) do something usefull, like addition,
putting the result in a new register "on the fly" if required.
Or on the other hand adding an offset to a base address will drop the
offset from the stack automatically.

I think " the register caching via stacks" is much easier to implement than
the register window stuff. And as far as I can see, it is easier on the
compiler optimizer too. 
                              
                              
N.B. Those interested (Harris?) may mail me for the FIETS instruction set.
N.B. I am currently exploring transputers, that also have an "allocate
     register as needed" mechanism, without overflowing however and very shalllow.
                              
                              
View expressed are my own { not all FIETS members would agree.
                          { my boss does not know what I am talking about

Albert van der Horst               {eunet!mcvax!hp4nl!bsovax!albert}
P.O. 8052                                                                    
3053 RB   Utrecht            " It's pitch dark. You might be eaten by a grue" 
The netherlands              " I hope you got more light than from a lamp"