Written and Maintained by Gregory Nacu

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February 20, 2020 — #97Programming Theory

Stack Manipulation, Some Gotchas

The 6502/6510 has a fairly limited stack. This is one reason why the processor is really not designed to do pre–emptive multi–tasking. The CPU is 8–bit, meaning all of its internal registers, (besides the program counter,) are 8–bits wide. The stack pointer, too, is only 8–bit. Which means the stack can only be 256 bytes deep.

There are two points here. On the one hand, that's way more than enough. Most 6502 programmers realize, after they've been programming for a while, that they rarely if ever use more than around one quarter of the total available stack. In normal 6502 code, the return address of a JSR is what the stack is most often used for. But that's only 2 bytes. You could call 120 subroutines each one nested within the previous one, and still not run out of stack. This is a huge amount of stack. It's so huge things like BASIC string conversion use the end of stack memory as temporary working space, because almost never is anything using that memory.

On the other hand, if you program in C, it's a different story. The nature of C is that all the arguments of a function, and all the additional local variables of a function are passed via and stored on the stack. This has the advantage that C functions are re-entrant by default, and recursion is therefore a breeze. This is particularly handy in a multi–tasking environment where those functions may be being shared concurrently by more than one process. And this technique is helped along by the fact that other more powerful CPUs have more flexible stack–relative addressing modes.

However, as long as we're not writing in C, or some other stack heavy high–level language, we do have a lot of stack available.

February 20, 2020Help This Site Succeed

Dear reader and Commodore 64 enthusiast,

You've come here to read one of my high–quality, long–form, weblog posts. Thank you for your interest, time and input. It would be a lonely world without loyal and friendly readers like you.

I'm creating C64 OS and documenting my progress along the way, to give something to you and contribute to the Commodore community. Please consider purchasing one of the items I am currently offering or making a small donation, to help me continue to bring you updates, in–depth technical discussions and programming reference. Your generous support is greatly appreciated.

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February 4, 2020 — #96Technical Deep Dive

I2C Serial Bus in 6502

Let me give some context for why this project exists. Then I'll go into some detail on what I discovered, and we'll explore how the i2c bus and protocol works and dive deep into my 6502 implementation.

I'm working on an operating system, C64 OS, for the Commodore 64. An operating system can almost always benefit by knowing the current date and time. At the very least because it's useful for setting the clock that appears at the end of the menu bar.

The C64 didn't originally ship with a built–in realtime clock, nor did it include a standard port for an RTC, like the later Amiga did with its clock port. With the arrival of more advanced operating systems, such as GEOS, several options for accessing an RTC became available, but there was no one standard. Creative Micro Designs, CMD, always eager to make the C64/128 productive, embedded RTCs into most of their products. The CMD HD, the CMD FD, the CMD RamLink and the CMD SmartMouse, all included RTCs. For the storage devices, the RTC is accessed as a standard DOS command and the response is read from the device's error channel. In other words, the communications with these RTCs is piggybacked on the IEC serial bus, for which there is already an implementation built into the C64's KERNAL ROM.

The SmartMouse must have employed a different and custom communications method.

SD2IEC, for which I have written a full user manual, has support for an RTC in the firmware, that is nearly 100% compatible with the CMD storage devices, but most SD2IEC hardware implementations don't actually include one.

The IDE64 has an RTC, but I haven't yet explored how to talk to it. The 1541 Ultimate II+ and the Ultimate 64 have an RTC built in, and it is accessible using Ultimate DOS commands which are sent over its proprietary Ultimate Command Interface. This requires some effort to actually support, but it's not too hard. C64 OS already includes an RTC driver for the Ultimate Command Interface.

The 1541 UII+, the Ultimate 64 and IDE64 are, however, outside the budget of many potential C64 OS users. And the CMD devices are no longer commercially available. In all likelihood a typical C64 OS user will gain access via the minimum hardware requirements of an SD2IEC. They are likely, however, to end up with one that does not include an RTC. It would therefore be convenient if they could acquire an affordable, stand–alone, RTC module. Of course, such modules exist for the Arduino. The DS3231, costs less than $2. A very affordable, convenient addition that makes your C64 running C64 OS better.

The DS3231 RTC module for the Arduino.
DS3231 RTC Module
January 31, 2020 — #95Software

WoC19 Video playback with 1541 Ultimate II+ (2/2)

This post is part 2 of a two–part follow up to my review of World of Commodore 2019. If you're interested and you haven't yet read part 1, WoC19 Introduction to C64 OS, please check out that post.

I had a one hour block of time to present at World of Commodore, but I asked them to split it into two half–hour blocks, because I wanted to give a talk about Video Playback on the C64 with 1541 Ultimate II+. It doesn't really need an introduction, because, that is the whole point of the presentation itself, which we'll get to shortly.

Additionally, I mentioned in part 1 that my presentations were given with slide presentations from my C64 Luggable. It is hard to tell in the YouTube videos exactly what is going on because the camera spends most of its time focused on the screen and only a few shots actually include me. And of those, you don't see much of my C64 and probably don't notice what's going on in my hands. However, if you check the thumbnail image for this post, above right, you'll see that I'm holding a NES controller.

I do love NES controllers for joysticks on my C64, as I've talked about here and here. But it turns out that a NES controller also makes for a really great, inconspicuous, one–handed slide clicker. The two red fire buttons make for easy back and forward controls that can be triggered with your thumb by holding the controller such that the d-pad is buried in your palm. Additionally, the long 6' cable provided me ample freedom of movement.

So, before we get into the presentation I wanted give a technical overview of how the presentation software works.

January 28, 2020 — #94Software

WoC19 Intro to C64 OS (1/2)

This post is part 1 of a two–part follow up to my review of World of Commodore 2019. If you're interested in getting an overview of the show and a taste of what you might get to enjoy first hand if you decide to come to World of Commodore in 2020, then I encourage you to check out that post.

It would have been too long to cover my own presentations from World of Commodore within that general review post. It also didn't seem right to give myself the royal treatment and just a paragraph about other presentations. In this two–part post, instead, I will go into what I presented.

The videos from the show have been uploaded to YouTube and are embedded below. I gave two talks, the first was an Introduction to C64 OS, and the second, which is covered in the second part of this post, was Video Playback with 1541 Ultimate II. Both talks were given with the assistence of a slide presentation for the first half followed by some live demos. The presentations themselves were done from my C64 Luggable with some software that I wrote prior to the show. A few people inquired about that and I'd love to give a brief description of how that works too.

So, let us not delay any further and get right into the juicy details.

January 15, 2020 — #93Editorial

World of Commodore '19

It's that time of year again, when we kick back, warm up behind our computer screens and read long reviews about cool Commodore and retro computer shows.

World of Commodore 2019 has come and gone, and it certainly didn't disappoint. This will be my fourth review of a World of Commodore event. Each year, after I digest for a bit, I try to come up with a theme to guide the narrative of my review. A couple of things though.

  1. This is the first year that I have given a talk at any retro computer event since at least 2008. It's been 11 long years, so I was kind of nervous to make my first return appearance.

  2. The second thing is that the show this year was shortened from a 2–day Saturday/Sunday event, to a 1–day Saturday–only event.

I was worried that the shortened timeframe was a prediction of lower attendence or decreased interest.

I am pleased to inform you, there was no shortage of interest or attendance. I've never seen the showroom floor so packed with people, machines and vendors. Even the number of PETs set up this year blew me away. There were PETs along all four sides of the room, and even on the inner rings of the room, and none of them was owned by the PET–master himself, Steve Gray, who wasn't there this year.

It's hard to pick just one theme this year, so I think I'll sum it up like this:

So many people, so many machines, so little time. Greg Nacu, "theme of the year," World of Commodore 2019

I feel like I say this every year, but I am always pleasantly surprised to find that so many of the people who show up are people I've never seen before. Obviously many of the fun and familiar regulars, Eric Kudzin and Jim Mazurek, Dave Ross, Joe Palumbo, Leif Bloomquist and Golan Klinger, and many others were there. But it isn't just the same group of people who attend, even the show floor presenters had many faces totally unknown to me.

The room was packed with people and machines, every table was full.
The room was packed with people and machines, every table was full.

In 2016, when I first returned to World of Commodore after an 8–year hiatus, one of the organizers of the show, in a moment of unbridled pessimism, said, "There's only one way that the membership of a Commodore club goes…" He left the word down to linger in our imaginations.

I can't speak for TPUG's actual membership, but as for World of Commodore? I was there in the mid 2000's. I remember when we were in a church basement made of mint green painted cinder blocks, adorned with Jesus fish. Okay, those were dark days. So, if these last few years are any indication, one thing is sure. Attendence and enthusiasm at this Commodore event isn't going down.

December 18, 2019 — #92Technical Deep Dive

1351 Mouse, and Mouse Driver

Lots of people now have asked me if C64 OS is open source. And, the answer is, no, it isn't.

Will it ever be? Who knows. My feeling about open source is that sometimes it's useful, and sometimes it's just a big pain in the butt. I don't want to negotiate with other developers about the direction of the project. I am planning to open a private beta, very soon. I like getting ideas, and in time I hope people will contribute by writing apps, utilities, and drivers for C64 OS and even sell those apps and utilities. That'd be good.

That said, I'm not putting in any copy protection, because I don't think there is a good way to do that in a way that doesn't hinder the software. We're now in an age of the C64 when the fine folk who want to support the work of others will pay to get a copy, and the people who don't want to never will. Copy protection basically just punishes the people who have paid for it, by making it harder for them to make legitimate copies and use it as they see fit.

I also believe that a project that gets open–sourced, especially if it becomes free, takes on the unfortunate stigma of being worthless. If you buy a copy, you've got some skin in the game. You're in the club. You're a user and a customer, and you're in a community. I think that's the way I want C64 OS to be positioned.

That said, from time to time I will publish the source to some aspects of what I'm working on. These are small stand–alone components that can be used by other people. And the first of these will be the 1351 mouse driver.

October 30, 2019 — #91Technical Deep Dive

Raster Interrupts and Split Screen

Welcome back. Hopefully this post will get technical with some useful information, but not so archane that it'll turn away more casual readers. A fine line to walk, but we'll see where we get.

Not too long ago I tweeted out my excitement about getting the split screen mode in C64 OS working. On the tails of that I put split screen to work in the NES Tester application and shortly after in Chess. Both of these applications have more than one purpose. Besides making use of split screen, working out its bugs and seeing what we can pull off, the NES Tester is useful for testing my NES Controller mods (see here and here) and also for experimenting with OS drivers for 2 and 4 player game controllers. Chess, as I outlined in my recent post C64 OS Networking and Chess, displays a graphical game board, but is also a place to experiment with networking, drivers, communications protocols and so on.

I want to dig in to what I've learned about the VIC–II, raster display updating, and more, and discuss what was necessary to get split screen working. But along the way, I've learned so much about the history of computers that I can't pass up the opportunity to talk about some of the details. I love the cake of computers, but I find their history to be a particularly tasty icing.

October 8, 2019 — #90Software

Utilities and the Utilities Menu

Last year I made a nice post introducing C64 OS Utilities. In which I discuss their advantages over GEOS Desk Accessories, and go into some detail about what they can do and how they will be used to augment the functionality of the system and its applications. That post is mostly still accurate, and is well worth a read. A few key points will be repeated in this post, but it should be considered a continuation of the earlier introduction.

Utilities are useful small apps that can be loaded, one at a time, concurrently with the main application. By relying on multiple smaller Utilities to do a chunk of work for your application you get several advantages:

  • Your application's main code can be made smaller
  • More main RAM can be freed up for use by your app's data
  • Your application will launch faster
  • The utilities you create can be used to enhance other applications
  • Using your application will be more consistent with the rest of the system

C64 OS's system folder is quite structured. Applications go in the applications folder. Drivers go in the drivers folder. And Utilities go in the utilities folder. The downstream benefits of even a simple level of organization like this are bigger than you would expect. Now to be fair, the opportunity to organize the many files of an OS like this often comes on the heels of hardware that enables it.

Let's recap GEOS for a moment. GEOS ran from a single side of a 1541 floppy disk. That's a 170K disk with a formatted capacity of just 166K. That's only 2.5X the total capacity of the C64's RAM. If you put the essential system files and the deskTop on a disk, you've already used just under half of that capacity. Then you add one application, maybe geoWrite, one document file and a couple of fonts, and the disk is full. Another thing to consider is that 1541 disks don't support subdirectories. But frankly, there's not much point in supporting subdirectories when the disk is full with just 10 or so of these files.

September 30, 2019 — #89Programming Theory

Drivers and Relocatable 6502 Code

Some quick housekeeping on my previous post, C64 OS Networking and Chess. I updated the post adding credit to Bo Zimmerman for the creation of the Zimodem firmware. And I had a chat with him about what I'd written. Turns out there are a couple of pieces of good news which I did not know.

The Zimodem firmware is entirely open source, in the update I linked to its GitHub Repository. But what I learned is that there is nothing particular to the C64NET hardware that the Zimodem firmware requires. What this means is that if you purchased a WiFi modem other than the C64NET, and you want to have access to this more advanced feature set, you can likely update the firmware on your existing modem.

Any wifi modem that uses either the ESP8266 or ESP32 or NodeMCU is likely to take the Zimodem firmware just fine. In fact, I got a Strikelink modem at one point and immediately blasted over it with my own firmware. Bo Zimmerman — 2019

For anyone who might ask, "Well then what's the point in getting the C64NET hardware itself?" It has some advantages over the other hardware as well. Most are powered directly off the C64's 5V lines. This can put stress on the power supply as it wasn't designed to drive the extra load. C64NET gets its power from the user port's 9V lines which it converts to 5V on the board. Additionally, the hardware handshaking lines, RTS and CTS are wired up on C64NET and are not always on other WiFi modems.

One more piece of good news. I said I was hopeful that we'd be able to get Zimodem's feature set on a highspeed RS-232 modem, as that would be the ultimate combination. Well, there are two options for this. It is likely that you could update the firmware on, say, the WiModem232. Or even easier yet, electronicsisfun.com has also made the GuruModem.

GuruModem. Highspeed RS-232 WiFi modem, running Zimodem firmware.
An early design. It's available now, but may look slightly different.

The GuruModem is a highspeed modem that connects over standard RS-232, so it can be connected to a C64 via GLINK232 or Turbo232, and can transfer data at up to 230Kbps! It is also based upon the Zimodem firmware and supports all of its features and commands, plus has some additional features that are enabled in the firmware's codebase when it targets the GuruModem's slightly larger and more capable microcontroller. If you ask me, this is the gold standard for high speed networking on the C64. I am so excited to get it working and supported in C64 OS, and to see what our old brown friend can accomplish.

Now on to the show, a technique for making relocatable code on 6502.

September 20, 2019 — #88Technical Deep Dive

C64 OS Networking and Chess

Welcome back dear readers. Let me start with some quick updates. I didn't put out a post in August, and I hate missing my deadlines. I took a week off in August to go off grid and spend some time with my family at a cottage. Rather than having a couple of smaller posts, I spent a lot of time working on a rather large reference document, the SD2IEC User's Manual. I worked on it for about 6 weeks, but just didn't have enough time to finish before the end of August. I posted the it to the blog on September 3, 2019. It's a doozy of a post. The original sd2iec github readme (upon which it is loosely based) was no slouch at 7,000 words. But at 28,000 words the SD2IEC User's Manual is 4X the size. I drew on the CMD HD User's Manual for inspiration in the syntax and BASIC and JiffyDOS examples. And I added a lot of descriptive content that was simply not present in the original. It is my hope that my work will become the go–to reference document for all SD2IEC users out there. Please link to the document and tell people about its existence.

It may seem odd that I post full user manuals to what is ostensibly a blog. However, I mentioned a couple of posts ago that I've introduced a new layout for navigating and finding the blog posts. The new View By Categories organizes the posts into categories based on how they've been tagged. This allows me, for example, to pull all the "Reference" documentation posts out and collect them together. The SD2IEC User's Manual can be found there now, side by side with other reference documentation posts, such as: C64 KERNAL ROM: Making Sense, the 6502/6510 Instruction Set, Commodore Hardware Information and reference tables for PETSCII, Screen Codes and key mappings.

Shortly after tweeting about the new SD2IEC user manual, Jérémie Marsin of Double–Sided Games recommended that to improve site SEO and general usability I should consider making the URLs of c64os.com friendly. Up until this point, posts have been referenced by their ID in a GET variable. Other parts of the site, the Buyer's Guide, C64 Luggable, and C64 OS documentation also exposed GET variables to access their subpages. I have plunged into Apache's MOD_REWRITE and manually updated a number of hardcoded links. The whole site now has more human readable and user friendly URLs. (The old urls, of course, continue to work. So any external links to my site won't break.)

In C64 OS news, I have recently got split screen mode actually working! Something I started talking about earlier this year. I've got it on my short list to write an in–depth technical post about what I learned and what I had to do to make a stable raster interupt that can be moved arbitrarily by dragging the status bar with the mouse. Look forward to reading about that.

I have also completed the work on getting the system–wide Utilities menu working. I had some original plans for how to implement it, thinking I didn't want to waste much memory. But I ended up with a solution that actually resulted in decreasing the memory footprint of the menu system in the process of adding support for the utilities menu. I've got another post planned to go into some detail on how this worked out. It's pretty cool.

And now on to the topic of today's post, C64 OS Networking and Chess.

Older Posts

Above are previews of the 10 most recent posts.
Below are titles of the 5 next most recent posts.

September 3, 2019 — #87Reference

SD2IEC User's Manual

July 19, 2019 — #86Hardware

Burning EPROMs with Promenade C1

July 12, 2019 — #85Programming Theory

Object Orientation in 6502 (Take 2)

June 13, 2019 — #84Technical Deep Dive

Versa64Cart and C64 ROMs

May 10, 2019 — #83Technical Deep Dive

The 6510 Processor Port



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