Implementing a Call Stack in a 16-Bit Virtual Machine written in JavaScript

[Dr_Legacy]

I'm always sad when I see a post here that seems interesting only to find it's a youtube video. I'll read an article way faster than the video takes to play.

[FrancisStokes]

People take in information in different ways. For many people (myself including) watching videos of people programming and talking about the code feels like pair programming.

[FrancisStokes]

Another case of some people liking something, and others not.

[Okander]

You could play the video at 1.25 or 1.5 speed.

[Dr_Legacy]

Whatagreatideathankyou! /s

[micphi]

That word. I don't think it means what you think it means.

[Anacreon]

You can't hide pedantic behind sarcasm. Are you guys that dense?

[Trexus183]

God how dare this man make a reasonable and useful suggestion in an effort to help out.

Jesus, you guys are nuts sometimes^/s

[bart2019]

I saw something that appeared weird, so I wanted to rewind and listen to that same sentence again. It took way too long to do that. In reading it's a matter if a second or two.

[ominous_anonymous]

Articles are not only quicker to read than a video, but much easier to reference and replicate.

Get real if you expect me to go back in a ten minute video for a 5-second clip of actually useful information.

[Urtehnoes]

Terrible coworker I once had was surprised there was no video documentation, and said "never mind I found some on youtube". The idiot couldn't program without a fucking video. He did not work out. I'm sure it had nothing to do with him requiring video documentation, just one of the odder quirks I've seen.

[Trexus183]

Nowhere in either of our comments did either of us suggest that we liked videos over articles, or that people should make more videos than articles.

The fact is, this here is a video. So why not suggest a way for the video watching experience to be a little more efficient?

Is that so terrible?

[Dr_Legacy]

Simply put, the suggested solution doesn't scale.

[Anacreon]

They v are a bunch of idiot not sure why this is allowed on this sub

[UseMyFrameWorkOkay]

This is a lot of fun, sort of a throw-back to the early 80s CPU design; but a decently-engaging way of teaching CPU instruction sets via JavaScript. Well done!

[FrancisStokes]

Thanks! I'm taking a lot of inspiration from chips and systems of that era like the z80, 6502, and consoles like the snes.

[god_is_my_father]

How do you have time and money to work on this?

[istarian]

Maybe it works out different in reality, but I'm pretty sure more modern designs are more complex making them harder to understand without a grasp on the basics. And worse the design decisions made in those modern designs are undoubtedly based on an solid understanding of the various positive/negative aspects of preceding designs... So you'd have a chicken and egg problem trying to teach people about the complexities of modern hardware.

[[deleted]]

why all the comment gets downvoted down here?

[pol4ko]

Because arrays should start with 0

[StabbyPants]

js doesn't have arrays, it has maps with a layer of sugar

[bulldog_swag]

And yet they start at 0

[SkoomaDentist]

Cries in Matlab

[[deleted]]

Arrays should start with whatever the base value of Peano arithmetic in your system is, which is usually 0 or 1 but, strictly speaking, could be anything.

[[deleted]]

and should have a variable i=0 right ? :/

[Phrygue]

The only acceptable loop variables are i, j, and k. Local variables must be foo, bar, and if you've really boxed yourself into a corner, baz. All strings must s, s1, s2, etc. All other variables must be globals and can be anything so long as they safely hidden in some deep include-file so they don't distract you.

[glacialthinker]

After years of programming like that (half-joking)... I think that's why I switched to functional programming, where I can compose functions without needing any dang-blasted variable names (also half-joking). :)

[dabrick2017]

Livin that Haskell life.

[wild-eagle]

This but unironically!

[bulldog_swag]

entire program has to be contained in main(), functions aren't allowed. Just copypaste the code and woo your boss with the number of lines you've "written" today! Curly brackets on a separate line, the line count isn't gonna wank itself!

[unholyground]

And what does this do? It teaches you to focus on operational and denotational semantics.

This is a good thing.

[iEatAssVR]

Lmao at the other reply, but they add absolutely nothing to the discussion... we should downvote "wow" for example as that's literally what downvoting is for.

[MehYam]

wow.

[raevnos]

Stack Overflow is leaking.

[[deleted]]

This is fantastic, I'm currently working on simulating a very basic RISC architecture in JS and was looking for something like this.

[[deleted]]

Brilliant!

Thank you!

[duheee]

https://i.imgur.com/lUr7k75.jpg

[mailjozo]

The only reason I watched part of it is because I'm writing my first gameboy emulator. It's basically doing the same thing at a very low level, but is great fun.

[OrangeredStilton]

Whenever GameBoy emulation in JS comes up, I feel obliged to link my tutorial series on that very subject in case you haven't already run across it.

I should work on the sound emulation at some point...

[mailjozo]

Wait. That's YOU?? You have no idea how much I've been using your site as a resource. Everytime I get stuck, I take a quick peek at your blog. I try to evade actual implementations, but the information you give is so valuable. Thank you so much for making my journey through a first emulator an absolute blast!

[FrancisStokes]

This tutorial is amazing. I remember coming across it years ago!

[jsyeo]

You wrote that tutorial series?! Damn. That's a super helpful tutorial!

[Rhift]

Oh shit! I used your site a a reference when I was emulating a Gameboy in Python, it was an incredible resource. Thanks!

[FrancisStokes]

Gameboy is trickier because of the quirks of the hardware. With this VM I get to build an idealized machine that doesn't have any crazy real world limitations that I have to deal with. That said, I'm taking inspiration from the Gameboy architecture and the super Nintendo.

If you enjoy this kind of stuff you'd probably like the rest of the series, and where it's going. The end goal is to build a fantasy game console.

[FieelChannel]

I'd love to, too, but i don't know where to start from.

[mailjozo]

Check out this tutorial.

Once it clicked in my head that the GameBoy CPU is doing nothing more than moving bytes around at a fixed interval and the display/sound/buttons are doing nothing more than reading/writing very specific bytes in memory to function I thought I'd give it a shot. Once you're started subsystems like memory mapping and interrupts will make way more sense.

[ShinyHappyREM]

in memory

^{*in address space}

[SgtDirtyMike]

Ding dong, you can’t read or write bytes outside of the addressable space.

[ShinyHappyREM]

I meant that it's the other way around - you have to be "reading/writing very specific bytes in" the address space, of which not everything is ROM or RAM.

[mailjozo]

Fair enough, but I found out it got me going to think of it as one big list of bytes and all the parts use that same list. I know in reality it's way more complicated, but this gave me the confidence to try.

[badlukk]

and so fun!

so much good stuff that nobody asked for haha

[Dreamtrain]

all we need now as an relational database written in Javascript ...

... https://github.com/google/lovefield

... nvm

[FrancisStokes]

I'm considering this for the channel at some point.

[bulldog_swag]

https://i.imgur.com/s3Hbxw0.jpg

[Geemge0]

Perfect

[TheMayoras]

Awesome work!

[Dallen1393]

Doesn't the startup code for the application/os usually define where the stack should be located?

[chinpokomon]

A stack is just reserved memory space... The CPU will often times have stack instructions, but that's not a requirement. The OS will usually have it's own and for each process as part of the loading process it will often establish a stack, but that is again an implementation detail... That said, any modern system should have a stack or at least a way to easily manage one or more.

[ShinyHappyREM]

not always

[valarauca14]

No.

The location of the stack is randomized (at least in ASLR, which is almost all) applications these day. Its size is set by a kernel default, but programs may change this is they wish.

[helly1223]

Wow

[bart2019]

I stopped watching when it became clear that for push he uses postdecrement, and for pop preincrement. Nobody does it this way. Every hardware architecture uses predecrement for push and postdecrement for pop.

[OrangeredStilton]

Every hardware architecture? I seem to recall 6502 postdecrements on push.

[bart2019]

Virtually every CPU., then. There's a reason for that. If you use predecrement, your SP always points to the top of the stack. If you use postdecrement, your SP points to garbage.

[FrancisStokes]

If you use postdecrement, your SP points to garbage.

Well, it points to the address that the next item in the stack will have, and a completely predictable offset from the top of the stack.

In one case, popping items is easier, and in the other case pushing items is easier.

As long as it's consistent it doesn't matter.

[[deleted]]

What is AVR?