Why did the amiga use Zorro instead of ISA and PCI to begin with?

[mantisspider]

Hi guys,

something that always puzzled me when I used to read about this mysterious thing called Zorro in the AmigaFormat magazines, what is it?

Well I finally learnt what it was but was increasingly confused as to why Amiga would chose something that would make it totaly incompatible with 3rd party hardware already out there. Something I always complained about when I used to try and use different mice and joysticks on my amiga and spectrum 128k :-)

Does anyone know the story behind why the Amiga used zorro rather than have used an ISA-bus for their machines? From what I read the A2000 had both Zorro and ISA bus onboard but the ISA bus was only used when a PC-bridgeboard was installed.

Amiga History Guide:
Seven internal expansion slots (5x  100 pin Amiga Zorro II and 2 x 16-bit ISA slots). The ISA slots were disabled by default (only power and ground pins activated), but could be used when a Commodore bridgeboard was installed (a PC-on-a-card). Inactive slots can be used for non intelligent cards like TBCs or fan cards.

Couldnt the ISA-bus on the A2000 be used to work with ISA cards to run on the Amiga? From what I remember the ISA was not as reliable as the PCI-bus, but cards were cheap and everywhere.

Was it just that Amiga wanted to be seen as so different from the x86 PC machines that it would not switch to using a PCI-bus rather than develop the Zorro III?

This has always intreged me as it could have been one of those things that would have allowed the expansion and development of the Amiga to have kept up with the rapid growth of the x86 PC. There may even have been AGP port on the Amiga 4000.

Now wheres my time machine?

[Animagic]

I really don't know the reason of using the zorro slot but I can guess one:

PC's were way back these days, and perchaps a non-compatible slot could give them the "exclusive" of various cards (like TBC - Time Base Corrector - cards which most video professionals needed)

[Matt_H]

Seems simple to me: PCI didn't exist in 1985 when the Zorro concept was designed, and ISA was crap at that time. Bridgeboards did make ISA slots active on the Amiga side, but very few drivers were written.

[mantisspider]

I wouldnt say ISA was crap, my P133 still even had ISA cards when I got it, and I never actually had a problem with my Sound Blaster and my graphics card. They were aparently not as reliable as modern PCI cards, but its a bit harsh comparing an older piece of hardware to todays standards.

Anyone know the transfer rates for comparing the Zorro II to ISA?

[mantisspider]

@animagic,

efaristo re.
makes sense as it was picked up by the video industry. But still cant see anysense as to why they didnt allow both Zorro and the ISA-bus to be used as standard on the machine. hope these pointless questions dont become one of those that can never get anwsered. i need closure! :-S

[Animagic]

Hmmm....

Greek (or a sort of - is Efxaristo not efaristo na NOT
RE as it comes from MORE which comes from MOROS which means CRAZY :-)

I' m impressed!
 :-D

[bloodline]

mantisspider wrote:
@animagic,

efaristo re.
makes sense as it was picked up by the video industry. But still cant see anysense as to why they didnt allow both Zorro and the ISA-bus to be used as standard on the machine. hope these pointless questions dont become one of those that can never get anwsered. i need closure! :-S

Zorro was designed to allow access to the Amiga and it's special hardware... ISA has no ability to do that it lacks the custom chip signals etc...

The Zorro specification grew out of an Expansion slot on the A1000, that was designed to provide a far more flexable interface than ISA was designed to provide. (Zorro and Zorro II are basicly the same thing)

As for Zorro III, there was no PCI or 32bit ISA at the time so the Amiga team had to design their own 32 Bit bus. When PCI was developed the Amiga Design team planned to include it in the next Amiga, but Commodore folded before that ever happened.

[Amiga1200PPC]

Thanks to Bloodline for clarification.

[Animagic]

Yes, bloodline is very helpfull most of the time  :-)

[mantisspider]

lol,
 hmmm i always thought 're' came from 'koumpare' i think thats a nicer version

[bloodline]

Animagic wrote:
Yes, bloodline is very helpfull most of the time  :-)

That's my pleasure :-) I could bang on about Amiga Hardware until some one leads me back to my padded cell in the mental asylum :-D


Oh and they didn't include ISA as standard, simply becasue they didn't need to! The Zorro bus was fast, more flexable, and the ISA would have required extra hardware (an ISA controler chip etc..). Oh, and the ISA was not Autoconfig, where as Zorro is. AmigaOS (and amiga users) needs Autoconfig devices.

[mantisspider]

Bloodline, ur a star

[Tomas]

zorro was better than isa.. and pci did not exist yet in the beginning days of zorro

[downix]

@mantisspider

You forget that when the Amiga was created, ISA was limited to a max speed of 8Mhz and the majority of implimentations were 8-bit only.  Then, combine in the fact that ISA is little endian while the Amiga was big endian, the fact that ISA is really little more than a modularized 8088 local bus and the Amiga lacks an 8088 CPU, and that at that time the #1 expantion bus was the S-100 from the Apple II, there was no need nor desire for ISA expantion.  The primary reason ISA existed in the A2000 at all was for the 8088 Bridgeboard which gave an A2000 the ability to run IBM compatible software.  The A1000's sidecar did much the same thing.

Technologically, the Zorro bus was more flexible, faster, and was more widely used (since Zorro was a modified 68k local bus, and the majority of peripheral components were 68k-based, there were a wider selection of potential expantion components than one would have found with ISA) so there was no incentive to cripple the design with a hobbled 8-bit bus.

As for PCI, didn't exist then.  However, when PCI was unveiled in the summer of '93, Commodore had plans to switch to it for the next-generation "Acutiator" architecture.

[evilrich]

the #1 expantion bus was the S-100 from the Apple II

The S-100 bus was introduced with the Altair and then cloned and  used in many other early 8080- and Z80-based machines. I don't recall what bus the Apple II used, but, since the S-100 bus is basically just an externalized copy of the 8080 bus and the Apple II was a 6502-based machine, I serious doublt that it used the S-100 bus . . .