Western Digital and SCSI Bus Master Host Adapters

Future Domain had not developed a Bus Master SCSI host adapter, but had the opportunity to buy that product line from Western Digital in 1992.

A Tutorial on Bus Master – What is it?

One of the keys for a small company to be successful is to clearly define its mission or value proposition. After that is done the CEO must insist that the company focus on that mission like a laser beam. One of the things that I see kill small companies more than anything is trying to do everything. Small companies have limited resources and those resources have to be focused to be successful. When we decided to get in the SCSI adapter business we decided to focus on the PIO (programmed IO) adapter technology. This was based on my experience as a mainframe Computer systems engineer at Burroughs. The other directions we could have gone were Bus Master or DMA (Direct Memory Access). The main difference was the PIO adapter uses the main computer processor to do the IO operation; the Bus Master had its own processor and could do the IO operation itself. DMA is somewhere between Bus Master and PIO and I will discuss it later.

PC architecture of the 80’s and early 90’s

The IBM PC, XT, and AT and the Intel 8088.8086,80286 and 80386 microprocessors all had a single common data and address bus architecture. What this means is that the computer memory and all the IO devices share the same bus attachment to the microprocessor. This is not the only type of computer architecture. More expensive mainframe and some minicomputers of the day often had multiple bus architectures. The main bottleneck for processing was not the processor itself, but often it was the memory and/or IO subsystems.