VVS, BART, and the Power Macintosh

There are two commonly-encountered issues that involve the NuBus controller chip (known as BART) on the NuBus Power Macintoshes. The first concerns a bug in the earlier version of the chip. The second is the performance, and in particular its performance relative to the bus performance on the Quadras.

The Slave Mode Block Transfer Bug

There is a bug that appears in the earliest released version of the BART chip (BART4) that was fixed in a subsequent version (BART21).

When certain NuBus cards are installed in the BART4 Power Macintoshes with VideoVision Studio, playback of VideoVision movies is characterized by a window full of pink and green squares, usually followed by a system freeze. It does not matter if there is any software installed for that card or not.

Technical Details

The BART4 NuBus controller chip has a habit of disabling NuBus block transfers for EVERY slot if the declaration ROM of even one card does not explicitly state support for slave mode block transfers. This screws up VideoVision Studio.

It is fixed in BART21, although the card's manufacturers can work around it by updating the ROMs, as many have, including ATTO, DiaQuest, and Radius/SuperMac.

The most common card that still demonstrates this problem is the Digidesign AudioMedia II card, whose ROM is soldered on and was never updated. This card can only be used in BART21 Macs. There is no way to upgrade this earlier BART version without a complete logic board upgrade.

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Performance Relative to the Quadra

Due to the command set of the PowerPC chip, the BART chip had to use a different technique to move data around the NuBus that the ones used on the Quadras. For example, the move16 command used on the Quadras to move large blocks of data is unsupported by the PowerPC chip, and smaller blockmoves must be used on NuBus Power Macs, resulting in poor performance relative to, say, the Quadra 840AV.

PBBlockMoveInit

Initially, the 8100/80 was unable to exceed 1.7MB/sec with VideoVision Studio. Apple, however, provided an extension called PBBlockMoveInit that implements block transfers a little differently, resulting in performance that can reach about 5.5MB/sec with VideoVision Studio. This file is included on the third disk of the VideoVision Studio/Telecast disk set, and automatically installed by our installer on Power Macs.

This is not an issue on PCI Power Macintoshes, and this extension should not be used.

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The Quadra 840AV

The Quadra 840AV features the fastest NuBus Apple ever released. As such, it is ideal as a Telecast capture and playback station, as it can allow you to use data rates not achievable on Power Macs (particularly with Telecast), to maximize quality. The resulting files can be edited on Power Macintoshes (NuBus or PCI -- PCI is much faster) to speed up the creative process.

Curiously, the NuBus on the Quadra 840AV is so fast that processes that require moving a lot of data around, rather than number crunching, will run faster than a NuBus Power Mac. This includes common processes like crossfades, which can therefore render faster on the 840AV!

There are other issues that affect performance, but a career could be made from analyzing the reasons for the differences in behavior and performance between the NuBus of the Power Macs and the NuBus of the Quadras, we will not cover it here in any further detail.

We have determined that any summary of the elements that contribute to the performance characteristics of the PowerMac NuBus is a dangerous and misleading oversimplification. In short, it is an incredibly complex web of interdependencies involving much more than just the odd unsupported block transfer mode of the Motorola PowerPC 601 chip or the relationship between the motherboard and NuBus clock speeds.

And frankly, risking Apple's wrath by commenting here would be foolhardy.

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Power Macintoshes with BART4

The Power Macintoshes that include BART4 are as follows:

(The other pizza box CPU's are not supported by VideoVision.)

Power Macintoshes with BART21

The NuBus Power Macintoshes that include BART21 are as follows:

There was a rumor that the 7100/80 and some of the 80MHz Workgroup Servers were updated with BART21 in the later stages of manufacturing, but we have never seen any of these Macintoshes.

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