SymbOS: preemptive multitasking OS that can play mp3s, video on 8-bit Z80 PCs

A few MSX made it to the UK, they looked pretty fly, nice keyboards, but very few software titles compared to their peers, the vision of a shared OS with multiple hardware manufacturers was a great idea though and has persisted ;)

> I never came across any MSX machines when I was a kid and collecting weird computers. Did they ever make it across the pond to the US market?

No, they didn't. Despite being a Microsoft standard. They were big in Europe, Japan and Brazil.

The MSX were wonderful machines. The even had "plug and play" hardware – cartridges were designed for software, but they could have basically anything. Companies created disk drive interface, hard drive, lightpen, you name it. And they would all setup automatically, due to the included ROM.

Also had great audio and video capabilities for the time. Eg. from 1989 (during the IBM PS/2 era, with VGA and the pc speaker for sound) https://www.youtube.com/watch?v=gcZJ64PgtgA That's for MSX2(which still used the Z-80). An example for the original MSX in 1986: https://www.youtube.com/watch?v=sk5C2E90dGY

It is amazing what a <4 Mhz processor can do. It did have a GPU, which helped due to the sprites (see the zanac example above). However, much later, people discovered that the GPU was actually a bottleneck and the CPU would have been able to push pixels (patterns, actually) at a much faster rate if it had access to the video memory directly.

The MSX Standard was based on the Spectravideo design which was an American company (SV-318 and SV-328).

The Colecovision was also pretty close to the MSX and many games has been ported between both systems including some classic Konami games.

The only real MSX sold in USA as a far I know was made by Yamaha, CX5M, and sold as midi sequencer bundle in music stores.

Could you write more about living in a trailer? Sounds very interesting, especially considering that you have a lot of tech. If not a problem, couple pics would be nice too.

Thanks! I'm interested because I'm considering doing the same.

MP3 support requires a decoder card according to the page; I remember running into the same issues on my 486.

Page http://www.symbos.de/facts.htm, end of the third paragraph "Real preemptive Multitasking, a dynamic memory-management for up to 576K and more and a totaly MS-Windows-like GUI are the three most important things."

I'm also a little puzzled about real preemption on a Z80, but http://www.cpcwiki.eu/index.php/SymbOS#Technical_background states

"Even the Z80 8-bit CPU is able to run a pre-emptive multitasking environment. Missing mechanisms like memory protection, which are often called as a condition for multitasking, are system stability issues only. The MP/M operating system already proved, that multitasking on Z80 computers is possible.

While the MOS Technology 6502 can not move the stack, the Z80 can freely replace it to any position in the memory, which is more or less a condition for pre-emptive multitasking. The existance of an alternative register set accelerates the content switching between the tasks dramatically. The restriction of Z80 systems to a 64 kB address space can be solved with bank switching. In this way computers like the Amstrad CPC and PCW, MSX, Enterprise or SAM Coupé are able to access hundred or thousand of kilobytes of memory. "

I don't know how symbios does it but I don't think it's that hard to create something pretty close to preemptive multitasking on a z80. You have the vsync interrupt (50 times per second). Add to that some stack manipulation, some bookkeeping and a register store/restore procedure and you're done. It wouldn't be fast though, and it would also limit what a process is allowed to do (disable interrupt, write outside of its own memory region). The z80 provides no mechanism to enforce this, of course. But technically, it's a primitive kind of preemptive multitasking.

Preemption just means the task is stopped when its timeslice elapses, and a new task selected. You can do this on any CPU with a regular timer interrupt. On something like the ZX Spectrum you'll need to use the vblank interrupt; on something with a programmable timer, you can make the timing tighter.

(In either case, you'll probably want some mechanism for tasks to relinquish their timeslice early; no reason not to make multitasking run better when tasks cooperate, even if it doesn't absolutely require that.)

To do the actual task switch, do exactly what you do on any system:

- save register state in tasks's register area (the state will be available on entry to the interrupt routine, in the usual fashion. The task pointer will be a system global at a fixed address)

- set timer for one timeslice hence (timers probably count down to zero, so this will just be a question of putting a value somewhere into an I/O register)

- resumes task by jumping to the program counter value from the task's register area

- GOTO 10...

(The complications come on something like the 6502, which has a fixed-size, fixed-address stack - but this it hard task-switching difficult, not the preemption! How I imagine CLIPS gets around this: it requires CMD SuperCPU, which is a 65816. Like the Z80 and 6809, but unlike the 6502, the 65816 has a relocatable stack. So it will probably behave as I describe above.)

Or maybe a ti84+ port? :D

Not exactly a Z80 (more of a 8080/Z80 hybrid), but I'd love to see how a Gameboy port turns out.