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Virtual Machines

:	Iain D. Craig
:	Virtual Machines
:	Springer-Verlag
:	9781846282461
:	1
:	CHF 141.10
:

:	Hardware
:	English

:	269
:	Wasserzeichen/DRM
:	PC/MAC/eReader/Tablet
:	PDF

I love virtual machines (VMs) and I have done for a long time.If that makes me 'sad' or an 'anorak', so be it. I love them because they are so much fun, as well as being so useful. They have an element of original sin (writing assembly programs and being in control of an entire machine), while still being able to claim that one is being a respectable member of the community (being structured, modular, high-level, object-oriented, and so on). They also allow one to design machines of one's own, unencumbered by the restrictions of a starts optimising it for some physical particular processor (at least, until one processor or other). I have been building virtual machines, on and off, since 1980 or there abouts. It has always been something of a hobby for me; it has also turned out to be a technique of great power and applicability. I hope to continue working on them, perhaps on some of the ideas outlined in the last chapter (I certainly want to do some more work with register-based VMs and concur rency). I originally wanted to write the book from a purely semantic viewpoint.

"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."

	Copyright Page	225
	4	225
	Preface	6
	Acknowledgements	8
	Table of Contents	8
	9	8
	1 Introduction	8
	14	8
	1.1 Introduction	14
	1.2 Interpreters	16
	1.3 Landin's SEeD Machine	16
	1.4 The Organisation of this Book	18
	1.5 Omissions	20
	2 VMs for Portability: BCPL	20
	24	20
	2.1 Introduction	24
	2.2 BCPL the Language	25
	2.3 VM Operations	28
	2.4 The OeODE Machine	30
	2.5 OCODE Instructions and their Implementation	30
	31	30
	2.5.1 Expression Instructions	31
	2.5.2 Load and Store Instructions	33
	2.5.3 Instructions Relating to Routines	33
	2.5 .4 Control Instructions	35
	2.5.5 Directives	36
	2.6 The Intcode/Cintcode Machine	36
	37	36
	3 The Java Virtual Machine	36
	40	36
	3.1 Introduction	40
	3.2 JVM Organisation: An Overview	41
	3.2.1 The stack	42
	3.2.2 Method areas	43
	3.2.3 The PC register	43
	44	43
	3.2.4 Other structures	45
	3.3 Class Files	45
	3.4 Object Representation at Runtime	53
	3.5 Initialisation	55
	3.6 Object Deletion	57
	3.7 JVM Termination	58
	3.8 Exception Handling	58
	3.9 Instructions	59
	3.9.1 Data-manipulation instructions	61
	3.9.2 Control instructions	64
	3.9.3 Stack-manipulating instructions	67
	3.9.4 Support for object orientation	67
	69	67
	3.9.5 Synchronisation	72
	3.10 Concluding Remarks	72
	4 DIY VMs	72
	73	72
	4.1 Introduction	73
	4.2 ALEX	74
	4.2.1 Language Overview	74
	4.2.2 What the Virtual Machine Must Support	77
	4.2.3 Virtual Machine-Storage Structures	78
	4.2.4 Virtual Machine-Registers	80
	4.2 .5 Virtual Machine-Instruction Set	82
	4.2.6 An Example	91
	4.2.7 Implementation	93
	4.2.8 Extensions	97
	4.2.9 Alternatives	100
	4.2.10 Specification	105
	4.3 Issues	108
	4.3.1 Indirect and Relative Jumps	109
	4.3.2 More Data Types	110
	4.3.3 Higher-Order Routines	118
	4.3.4 Primitive Routines	118
	4.4 Concluding Remarks	119
	5 More Stack-Based VMs	119
	121	119
	5.1 Introduction	121
	5.2 A Simple Object-Oriented Language	122
	5.2.1 Language Overview	122
	5.2 .2 Virtual Machine-Storage Structures	123
	5.2.3 Virtual Machine-Registers	125
	5.2.4 Virtual Machine-Instruction Set	125
	5.2.5 Extensions	128
	5.2.6 Alternatives	128
	5.3 A Parallel Language	129
	5.3.1 Language Overview	129
	5.3.2 Virtual Machine-Storage Structures	131
	5.3.3 Virtual Machine-Registers	133
	5.3.4 Virtual Machine-Instruction Set	134
	5.3.5 Implementation	136
	5.3.6 Extensions	138
	5.3.7 Alternatives	140
	5.3.8 Issues	141
	5.4 Concluding Remarks	141
	5.4.1 Some Optimisations	141
	5.4.2 Combining the Languages	142
	6 Case Study: An Event-Driven Language	142
	143	142
	6.1 Introduction	143
	6.2 The Structure of Rules	145
	6.3 Events	148
	6.4 Execution Cycle	148
	6.5 Interpretation Rules	150
	6.6 VM Specification	153
	6.6.1 States and Notational Conventions	154
	6.6.2 Infra-Rule Transitions	157
	6.6.3 Extra-Rule Transitions	160
	6.6.4 VM-Only Transitions	162
	6.6.5 Introspective Operations	163
	6.7 Rule Equivalences	165
	6.8 Concluding Remarks	166
	7 Register-Bas