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

[Hammer]

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.

Precursor to VL-Bus and PCI slots (~1993), the 32bit slots in X86 PCs are either the EISA or MCA.  32bit ISA are known as EISA.

[Hammer]

bloodline wrote:

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

Oh, and the ISA was not Autoconfig, where as Zorro is. AmigaOS (and amiga users) needs Autoconfig devices.

Note that Plug’n’Play 16bit ISA (e.g. Yamaha Sonta S16 Sound card**) existed later.

**Plug’n’Play X86 OS is required.

[Hammer]

Hence Zorro. Designed for amiga, rather than atapted from some second rate 8088 crud...

In regards to 8088, the ISA bus would be XT ISA (8bits) as oppose to AT ISA(16bit).  8088 has an 8bit bus while the 80286 have a 16bit bus.

There are three generations of ISA
XT ISA(1981) = 8bit (with 8088)
AT ISA(1984) = 16bit (with 80286)
E-ISA(1988) = 32bit**(with 80386)

**completing against non-backward compatible  MCA.

"EISA systems also use an automated setup to deal with adapter-board interrupts and addressing issues. These issues often cause problems when several different adapter boards are installed in an ISA system. EISA setup software recognizes potential conflicts and automatically configures the system to avoid them. EISA does, however, enable you to do your own troubleshooting, as well as to configure the boards through jumpers and switches. This concept was not new to EISA; IBM's MCA bus also supported configuration via software. Another new feature of EISA systems is IRQ sharing, meaning that multiple bus cards can share a single interrupt. This feature has also been implemented in PCI bus cards. " - Upgrading & Repairing PCs Eighth Edition

[Hammer]

@downix

Quite true, but remember the time period we are discussing here is the '84-'86 era, 16-bit ISA was still underutilized and several clone manufacturers did not supply them with their machines. (My grandfathers clone, for example, ran with straight 8-bit ISA, and it was from '86)

ISA type usually corresponds to their processor i.e. it’s unlikely that the 8088/8086 based PC will have a 16bit ISA.

Note that, the PC AT standard states for 16bit ISA.  8bit ISA with the 80286 processor is just a substandard PC clone. EISA standard was developed primarily by Compaq.

PS; My parent’s old IBM PS/2  386 box has MCA instead.

[Hammer]

And note, I have a 286 PS/2 w/ 8-bit ISA.

What model was that? An IBM PS/2 Model 30 286 has 16bit ISA slots.  I don’t think 80286 processor can support 32bit EISA/MCA bus.

Back at that time, IBM has a knack of reserving high end stuff for high price market.  

His machine was an 8086.

That figures...

[Hammer]

From memory I think there were some machines that had a combination of 8-bit ISA & 8/16-bit ISA slots.

The reason for that and I quote....

"The extended 16-bit slots physically interfere with some 8-bit adapter cards that have a skirt--an extended area of the card that drops down toward the motherboard just after the connector. To handle these cards, IBM left two expansion ports in the PC/AT without the 16-bit extensions. These slots, which are identical to the expansion slots in earlier systems, can handle any skirted PC or XT expansion card. This is not a problem today, as no skirted 8-bit cards have been manufactured for years." -Upgrading & Repairing PCs Eighth Edition