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Yet another virtual machine

Posted by Aardvajk, 12 September 2011 · 212 views

Hey. It's been a month since my last journal. That's long. Lots of us seem to be posting like this these days.

Work continues well. It has been a big shift from being a solo hobbyist developer to working on a real project as part of a (admittedly small) team but I was kind of prepared for the differences through listening to guys on here over the years and seem to be adjusting okay.

Since I'm working on something real now, I decided that I wanted a home project that was just purely for fun and had no real purpose. I've always been interested in compilers and virtual machines and have written a plethora of scripting languages, but never written a VM that supported dynamic allocation in a heap and a language that took advantage of it.

So that's what I'm doing. The following assembler script calculates the length required to concatenate two strings, allocates it, copies the data in the outputs the result plus the size of the new buffer, then frees it. The contents of the memory location labelled 'name' dictates which second string to use.


	setax string0

	strlen

	push

	setax name

	get

	movra

	strlen

	push

	add

	pop

	incrx

	alloc

	movar

	movad

	setax store

	put

	setax string0

	strlen

	copy

	push

	setax store

	get

	push

	add

	pop

	movrd

	setax name

	get

	movra

	strlen

	incrx

	copy

	setax store

	get

	movra

	outs

	strlen

	outn

	setax end

	outs

	setax store

	get

	movra

	free

	exit



store: 0

name: string2

string0: "hello world from " 0b

string1: "paul" 10b 0b

string2: "eric" 10b 0b

end: 10b 0b



Horrible, isn't it? :) But it isn't really designed to be human written, this is just testing the VM works properly. It's really more designed to target a compiler at. I've decided though that this compiler is going to compile via text assembly like the above so I can check the output more easily, plus it looks cool.

It's basically a three register VM - a value register and two address registers. Memory is just a contiguous block of chars that contains the program, then the stack, then the heap. Heap management is currently just a case of storing free blocks in a vector and merging adjacent free blocks together but more than ample for what I need.

So all good fun, largely because it is pointless and complicated.




Fun stuff! You should try writing a simple reference-counted garbage collector to go with it.
Yeah, was thinking about that. More interested in writing a compiler that tracks when things go out of scope and deletes them, more like RAII, first though.
Or, you know...

Squishy...

;)
Secret hint: implement your RAII-style deterministic destruction to make use of reference counting under the hood. Think of it like all your pointers being shared_ptr internally.

In the trivial case, the reference just goes from 0 to 1 to 0 and you destruct the object. In the complex case, it can be really convenient to have transparent reference counting and deterministic destruction.
@ApochPiQ True, need it anyway for built in COW strings.

@FLeBlanc Thanks for the vote of confidence in my game. I've sadly currently lost all interest but I may pick it up again at some point.
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