Q: I've heard some accelerated video cards can acclerate unaccelerated video cards. How does this work?
A: Some NuBus video cards, most notably the Apple Display Card 8•24GC, have the ability to take over the bus. This ability is known as being able to "master" NuBus block transfers.
What's that, you ask?
Well, allow me to provide some background first. Normal data transfer on NuBus is done in the form of long words, which are 32-bit chunks of data. NuBus architecture can also transfer data in the form of blocks (several long words at once). In these cases, the system takes over the bus and, instead of the bus telling itself how to transfer data (the usual state), the system tells the bus to wait while it transfers these big chunks of data. This reduces the time spent shuffling data back and forth on the bus, since the bus doesn't have to arbitrate its own data transfer and arbitration is done less often.
Macintosh computers have never had the ability to "bus master" in order to do block transfers from the NuBus cards to the CPU, but several NuBus video cards have been designed with this ability. The 8•24GC is probably the most commonly known example. In addition, several NuBus video cards have had the ability to act as "slaves," where a master card can take over and do block transfers to and from the slaved card. The Apple Display Card 4•8 and 8•24 are two great examples of cards that support slaved NuBus block transfers. This is how, when installed in the same system as an 8•24GC, the 4•8 and 8•24 will be accelerated as well.
On a related note, two 8•24GCs cannot be installed in the same system with acceleration enabled on both of them — each of these two "bus master" cards will try to take over, which will cause problems. If two are installed in the same system, the acceleration software (at least the official 8•24GC Control Panel) is smart enough to prevent this.
For a more on NuBus block transfers, see Tech Note HW 15.
copyright ©2000-2004 by Chris Lawson