Thread of a discussion of PAS16/OS2 CDROM Speed from Ken Nicholson of Mediavision. 7/7/93
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First of all, the PAS 16's SCSI hardware utilizes a polled I/O transfer
method.  That means that for each byte transferred, an I/O instruction must occur.

The current driver also polls the system for data available.  In the future, the OS/2 driver's data available indicator will be interrupt-driven.  This will free the CPU during the short time between data request and the time the data is ready for reading.  In a streamed video situation, that time should be negligible -- the data should always be available because of the drive's pre-buffering.

" That's why DMA was invented, to offload the CPU so something else could be  done during I/O."

It doesn't really work as well as you might hope.  While the DMA hardware is accessing the bus, the CPU is prevented from accessing it.  Also, sound
cards use DMA and they can't get access to the bus when a DMA-based SCSI
operation is using it. DMA is also used for RAM refresh.

Polled I/O has some advantages, though.  For example, no one has yet has a
problem with the PAS 16's SCSI interface being in conflict with another
device.  I've used many SCSI cards and I find the PAS 16's SCSI interface to be the easiest to install and get running.  Polled I/O can be very fast.
Adaptec has a high-end polled I/O SCSI chip that they claim can transfer
data at 3MB/sec!

We've established over several messages that the PAS 16's SCSI interface and driver are capable of exceeding 500Kb/sec transfer rates.  Let's look at the CD-ROM drive itself now.

To meet MPC level 1 specs, a drive must transfer 150Kb/sec with no more than 40% CPU usage.  The MPC level 1 spec says nothing about CPU use at 300Kb/sec and I suspect the problem lies here.  A drive may be able to transfer 300Kb/sec but if it uses 80% or more of the CPU bandwidth to do so, it's not going to allow smooth playback of 300Kb/sec video sequences.

The definitive test would be to compare these devices on the same SCSI
interface and check the performance playing a 300Kb/sec data rate movie
file:
   1) IBM CD-ROM drive (Toshiba 3401b)
   2) NEC 84jd CD-ROM drive
   3) External SCSI hard disk drive
   4) External removeable hard disk drive

My bet is that these tests will show the CD-ROM drive is the weak link in
the chain.

