Amiga Multitask

[itix]

I find it quite cool that Amiga in 1985 could multitask, with as low as 250 kb ram, not even Windows 3.1 could multitask, you could only use one program at the time. The Amiga was ahead of its time in 1985, and some years to come :-) I only wish i had an Amiga 1000 :-) it looks like a nice machine. Are any of you lucky enough to own an Amiga 1000?

It was impressive but I have to admit it was impossible to run Deluxe Paint and Kindwords same time even with 1024 kB RAM.

[itix]

it was not because of the OS but because of these programs. Kamelito

If it had virtual memory you could actually run paint program and word processor at the same time. So it is actually an OS limitation.

[itix]

Well, about stability you mix OS and applications, no memory protection forced serious Amiga programmers to write more robust programs, if you run a bad one the OS won't help you there. Kamelito

Neither did Windows 3.x. There Windows applications were running inside a single process and applications could share memory like Amiga does. What made it finally quite unstable piece of crap was trying to retrofit paging and virtual memory to this mess. Applications had to lock handles to convert them to real memory pointers but sometimes programmers were not careful. Sometimes they were using stale pointer that worked until memory paging kicked in.

[itix]

I'm trying to imagine a virtual-memory system on 8MHz and 28 KB/s floppy disks, but I can't think over the screaming in my brain. I think there may be a lesson in there somewhere.

Indeed. For proper multitasking you needed more memory and fast hard disks...

I remember when I was coding some application on my Amiga 500. It had 1MB RAM only and when asm source code was over 150 kB it was no longer possible compile it while keeping my editor running on a background. So I had to save my code to a floppy and close editor to run compiler. Only to find out I had made small typo and had to edit code again. Back then I didnt have an access to memory conservative compilers and had to use A68k and Blink. Those were times...

[itix]

Alternatively, you could just have efficient software that maximizes free memory for your actual work purposes and enough RAM to fit the task at hand, rather than churning data to and fro over a disk interface many orders of magnitude slower than the RAM which is itself likely not actually fast enough to keep up with the demands of the processor.

But, you know, that'd just be crazy.

C64 coders were totally crazy. They had only 64 kB and had to use all imaginable tricks to save few hundred bytes. But still, no matter how efficiently it is done, there is always upper limit...

But actually on Amiga the limit was in chip ram. Deluxe Paint was designed to use chip ram sparingly but code size was not important when it wasnt going to chip ram. It wasnt always important to optimize code for size and not even your data.

Obviously chip ram limitation crippled Amiga multitasking at some point. Even when there was enough (fast) ram there wasnt enough chip ram to store gfx and sound. Nothing is perfect, not even on Amiga :-)

[itix]

There are text editors now that take up 10-20MB just sitting idle with nothing open.

Only 20 MB? Must be efficient program. I just launched Word and it is taking over 40 MB right after launch. I am not sure if there is anything what good old Kindwords couldnt do? :-)

[itix]

Pre-emptive multitasking means you can still be run, even if some other software is so badly written that it never yields.

Why would you even want to use such badly written crap anyway?

It sort of degrades pre-emptive multitasking to co-operative if before executing application you have to figure out is application really multitasking or not. Many Amiga applications from golden era dont multitask at all.

[itix]

Since the OS didn't allow to share lots of resources (sound access was exclusive for example), it also limits what can be done using multitasking. No way to run two apps accessing paula for example...

It is not exclusive and two or more applications could play sound simultaneously. Audio.device has priority based channel allocation system where application with a higher allocation priority get an access to one or more audio channels. Applications which allocated a sound channel with lesser priority just wen silent.

But Amiga applications rarely had any use for audio and games and music software of course required all available channels.

That's where features like resource tracking come handy.

Without memory protection it is useless. It only causes the snowball effect.