1.3 Amiga Bus History
The original Amiga computer, the Amiga 1000, was introduced in 1985. While it had no built-in standard for expandability, the capability for some form of expansion was considered extremely important; personal computer history up to that date had shown several times that an open hardware expansion capability was often critical to a personal computer’s success and to its capability to adapt to new or unusual applications. The A1000 was designed with a connector giving access to the internal 68000 bus and a few other system signals. Shortly after introduction, the formal expansion specification for a card chassis that would connect to the A1000 was published. This bus became commonly known as the Zorro bus*. While the backplane specification was very easy to implement with 1985 PAL technology based on the existing 68000 signals, the specification did incorporate a number of advanced features. Far more sophisticated than the IBM-XT/AT and Apple II buses in common use at the time, the Zorro bus allowed any slot to master the bus, and it linked expansion cards with the system software. Addressing jumpers were eliminated, the card’s address instead being assigned by software, and cards could easily be identified by software and linked with appropriate driver programs, all with a minimum of user intervention.
With the introduction of the Amiga 2000 system, the Zorro bus was changed slightly. Additional discrete interrupt lines were added, replacing the encoded lines that couldn’t easily be used by any bus resident device. As it turns out, these additional encoded lines weren’t any more useful, as they couldn’t be disabled by software, and as such, they’re no longer considered an official part of the Zorro II bus specification (they are supported as part of Zorro III). Finally, the form factor was changed to match that of the IBM PC-AT card, acting as both a cost reduction and allowing the Zorro II bus to offer the PC-AT bus as one optional secondary bus extension. This modified specification became commonly known as the Zorro II bus, and it’s the Amiga bus standard that’s been in use for most of the Amiga’s life. And it’s a bus standard that will continue to be important.
