Re: Mixing virtual and real machine code in an interpreter

"Steven D. Majewski" <sdm7g@elvis.med.virginia.edu>
Wed, 23 Mar 1994 02:12:00 GMT

          From comp.compilers

Related articles
Mixing virtual and real machine code in an interpreter graham@pact.srf.ac.uk (1994-03-16)
Re: Mixing virtual and real machine code in an interpreter sastdr@unx.sas.com (1994-03-21)
Re: Mixing virtual and real machine code in an interpreter pardo@cs.washington.edu (1994-03-22)
Re: Mixing virtual and real machine code in an interpreter nickh@harlequin.co.uk (1994-03-22)
Re: Mixing virtual and real machine code in an interpreter sdm7g@elvis.med.virginia.edu (Steven D. Majewski) (1994-03-23)
Re: Mixing virtual and real machine code in an interpreter sosic@kurango.cit.gu.edu.au (1994-03-30)
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Newsgroups: comp.compilers
From: "Steven D. Majewski" <sdm7g@elvis.med.virginia.edu>
Keywords: interpreter, forth
Organization: University of Virginia
References: 94-03-039
Date: Wed, 23 Mar 1994 02:12:00 GMT

Graham Matthews <graham@pact.srf.ac.uk> wrote:
>I am interested in the problem of mixing sections of real and virtual machine
>code. I have an interpreter for a language L that compiles L into a virtual
>machine code and executes this. For certain critical sections, I would like to
>compile into native assembler. I envisage having some sort of virtual machine
>instruction that jumps to a section of real machine instructions, and
>similarly jump back ...


FORTH words are entries in a dictionary, with a Code Field Address (CFA)
and a Parameter Field Address (PFA)


The CFA is the address of the code that interprets the Parameter Field.


When the entry is a Forth defined word, the Parameter Field contains
threaded code ( Tokens or addresses ) and the CFA points to a routine
typically called DOCOL ( for "do colon" - a Forth definition begins with a
":" ) which is the threaded code interpreter.


Forth systems usually have an assembler to code new primitives - which are
coded the same as the builtin predefined words: the CFA points to the PFA
- the parameters field contains machine code - and it jumps to that code.


A Forth code optimizer could be constructed which creates a new version of
a word by recursively inlineing the threaded code until it has been
expanded to primitives ( assuming PIC primitives )




---


To reiterate some old threads from comp.arch:


Interpreter virtual machines are typically stack-machines. Running a
virtual stack machine on a register-rich RISC processor loses the
capability to use all those RISC registers. Also, the inner interpreter in
Forth and other interpreted languages are usually tight loops around
indirect jumps - just the type of code that typically doesn't pipeline
well. Interpreted code usually has a higher RELATIVE disadvantage running
on a RISC processer. ( This is inferring a pattern from numerous published
benchmarks. ) So *avoiding* interpreted code, and inlineing on a RISC
should be that much *more* of a win.




- Steve Majewski (804-982-0831) <sdm7g@Virginia.EDU>
- UVA Department of Molecular Physiology and Biological Physics
--


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