Difference between Amiga and Atari 520?

[spirantho]

Something to note about the 520ST - the actual 520ST is a very limited machine - it had 512KB of RAM but not only no blitter and custom chips, but the AY-8912 sound chip as used in 8-bits of the time and it didn't come with a disk drive! The 520 was originally designed without a TV modulator too so you had to use a monitor with it. The monitor of course could be PAL/NTSC resolution in 16 colours or VGA resolution in black and white.... so if you wanted to run serious software and games you had to have two monitors! Most music software would balk at the medium-res monitor, IIRC.

I have such a 520ST from 1984 (serial number 5000-odd I think) at home - except it's been upgraded to 1MB of RAM. How was the upgrade done? The only way was to solder new RAM chips on top of the old ones, taking the high address line via a wire connected somewhere else on the motherboard. No slot expansions here!

Of course the STf came soon with a floppy built in, along with the STm with the TV modulator, and the most common the STfm. Then the STe which even had a blitter which most software never used, and then the TT and Falcon and things came along, which are a different story. The Falcon is a really nice machine, with a pre-emptive MultiTOS and stuff on it, along with SCSI and IDE onboard. More powerful than an A1200, but no software for it practically at all.

Here endeth AtariSTs 101.

[spirantho]

Speelgoedmannetje wrote:
The copper co-processor makes it possible to use multithreading natively. No extra software needed. No cpu cycles spilled waiting.

Erm... that's not quite accurate. The Copper is another processor, it doesn't affect the OS in any way to do with multitasking whatsoever.

Otherwise you could say that any PC with a WinTV card supports hardware multitasking because it's got a seperate RISC processor on-board which can wait for stuff without tying up the CPU.

Incidentally, the original 520ST may have been bundled with a disk drive but it didn't come out of the box with a disk drive. I know because my disk drive wouldn't fit in my 520ST's box.

[spirantho]

True, but there are other machines that'd be even better, hardware-wise. From what I recall the WAIT instruction waits for a vertical position of the electron beam, which is obviously nowhere near enough resolution to help with kernel level multitasking.

For instance, the scan rate of a PAL signal is 50Hz, yes? This means that given a vertical resolution of about 1,000 rows (including the blanking signal), that's an ability to raise an interrupt 50,000 times per second, i.e. 50KHz. Now we're using CPUs with a resolution of 50MHz, not KHz. If the multitasking resolution was limited to a signal from the copper, the CPU would be sitting around waiting most of the time we did a context switch.

As I see it, the copper has nothing to do with multi-tasking in the OS. It IS very useful in generating display lists because it takes the load off the CPU, but that's because it's a processor itself.

And if you want proof that AmigaOS doesn't use the copper, ask any Draco  owner if his machine a) has a copper and b) runs AmigaOS.