[virtmach] bytecodes and stacks

[Stephen Pelc]

Date sent:      	Tue, 30 Nov 1999 20:45:16 +0000
From:           	Ceri Storey <cez@nomorespam.freeserve.co.uk>
To:             	virtmach@iecc.com
Subject:        	Re: [virtmach] bytecodes and stacks
Send reply to:  	virtmach@iecc.com

Ceri Storey said:
> personally, i can't see why why two stacks would be a better solution, 
> since when the machine returns from a function, the arguments bound
> in that procedure will be lost forever, and if a two stack model were
> used, the virtual machine/ programmer would need to pop the excess data
> from the data stack.
The advantage of a two stack VM is that it allows any number of 
results to be returned by a procedure. The traditional C style 
"stack" is more properly a list of frames in which each frame is 
built and released as a whole. In a stack machine each item on 
the stack can be dealt with individually. Having a separate 
return stack avoids having the return address amongst procedure 
parameters and results, and thus permits more flexible data 
handling.

However, when we designed two stack VMs to run C efficiently 
(see the OTA specifications at www.europay.com) we added 
additional VM support for frames. The canonical Forth stack 
machine is not suitable for frame construction in which it is 
assumed (as C and many other languages do) that the frames are 
in main memory.

Stephen

--
Stephen Pelc, sfp@mpeltd.demon.co.uk
MicroProcessor Engineering Ltd - More Real, Less Time
133 Hill Lane, Southampton SO15 5AF, England
tel: +44 1703 631441, fax: +44 1703 339691
web: http://www.mpeltd.demon.co.uk