Adaptec and Western Digital made a different choice. The IBM AT bus introduced a new feature, the bus master signal. What this signal did was stop the microprocessor so that the IO card plugged into the AT bus could take complete control of the computer. In most cases what this meant was the bus master device could have unimpeded access to the memory, freezing out the processor or any other device. Adaptec and Western Digital marketed this feature as akin to the multiport or IO Processor of the mainframe computer, but it is obvious that it is not.

In the mainframe case both the main and IO processor have access to the memory, the most expensive version having 99%+ availability, the lower cost versions at least 50%+ availability. The Bus Master was nothing like this. This was exclusive access. The Bus Master could lock the bus as long as it liked, and the main processor was locked out of memory. If the Bus Master locked the bus for too long a time, memory could be corrupted since the memory refresh was suspended and other IO operations (like the keyboard example above) would fail. Adaptec and Western Digital engineers knew this and only locked the bus for to transfer a few bytes before relinquishing control back to the processor.

The reality was that Bus Master on the AT Bus, even in a multitasking environment provided limited, if any, performance enhancement. I have never seen a UNIX or Novell benchmark where the AHA-1540 has outperformed non-bus master controllers, given everything else equal. A good example of this was when Future Domain was bidding the TMC-830 against the Adaptec AHA-1540 at Tandy for a UNIX system. Tandy ran several multitasking benchmarks and conclude the TMC-830 outperformed the AHA-1540. Eventually Adaptec through driver tweaks and benchmarkmanship was able to equal our performance, but their card cost three times as much. However Tandy decided to go with Adaptec. When I met with the Tandy engineering manager he admitted that the only reason Tandy did this was because Marketing felt they could sell into the UNIX market more easily with a Bus Master SCSI adapter.