Virtual Machines

"7 Register-Based Machines (S. 157-158)

7.1 Introduction

The virt ual machines described so far have all had th e same property: they use at least one stack and a collection of special-purpose regist ers. This has been taken as the way to const ruct virtu al machines. It is certainly a relatively quick and easy way to do it and it is a target machine for which it is particularly easy to generate code. There are, however, alternatives, the primary being that based on the Register- Transfer Model (RTM) . This is the model upon which most hardware processors are based.

There have been many machine simulato rs over the years and there are programming environments , particularly for real-time and embedded systems, that allow programs for one processor to be executed using a simulated processor. However, t he use of the RTM for virt ual machines is relat ively new; the largest implementation to date (2004) is the Parrot virt ual machine for Perl6. This chapte r is concerned with this alternative organisation for virt ual machines.

Section 7.2 is concerned with the arguments surrounding registerbased machines, both pro and con. Section 7.3 contains a description of one way to organise a regist er-based virt ual machine. Since the only really public register-based virtual machine of any st rength is t hat for Parrot , Section 7.4 is a description of Parrot s general organisation , while Section 7.5 contains a description of Parrots instruction set (the description is only partial because th e published documentation is, as yet , incomplete- th e best way to understand t he Parrot VM is to read its code). In Section 7.6, a DIY register machine is presented.

In Section 7.7, it is shown how the two-stack code for the simple ALEX programming language can be converte d (macro processed, in essence) into code for the DIY register-t ran sfer machine. The following section contains examples of such translat ions using a simple function. The correct ness of the translat ion from two-stack to register-machine code is the subject of Section 7.9 In Section 7.10 a more natural compilation of ALEX to register-machine code is presented. In the last section, Section 7.11 some extensions to the register machine are considered. 7.2 The Register-Transfer Model Almost all work on abstract machines has concentrated on stack-based architectures.

The SECD machine is the classic of this form, of course. The Pascal-S , Pascal P4, UCSD Pascal, Smalltalk and Java abstract machines are also stack-based. One good reason for constructing stack-based machines is that expressions can be directly evaluated on the stack . Stacks are also used to represent scope, thus making procedures and block structures easy to implement. Stacks are required to implement recursion , a feature of ISWIM, Algol60 and most modern programming languages . Stack-based architectures are good for compiler writers, therefore."