Re: What does double buffer do for a modern SCSI drive?

From: Roger J.Hamlett - view profile
Date: Wed, Oct 13 1999 12:00 am
Email: R...@xxxxxxxxxxxxxxxxxxx (Roger J.Hamlett)
Groups: comp.periphs.scsi

'Double buffering', is only needed by ISA bus mastering controllers.
The problem is that the ISA bus can only address 16MB of memory, so a
card using BM, can only transfer data into this memory area. With a
modern machine with a minimum of (say) 64MB of memory, direct
transfers to the rest of the memory cannot be done, so a small
'intermediate' buffer in this lower memory area is used. The card
transfers to/from this buffer, and the CPU does the required transfer
to the higher memory address. It involves a small CPU overhead
(suprisingly little). The 'double buffer' driver in W95/98, is loaded
automatically, by adding the line 'doublebuffer=1', to the MSDOS.SYS
file (a text file in W95/8, containing such settings). The driver is
'smart', and will ask the device if it can transfer to higher memory,
and only load the actual driver if it cannot. Hence by default the
line is added for all SCSI cards, but on a PCI card, only a tiny
'stub' occupying about 2KB will be left. With a ISA card, the full
driver remains, and occupies more space. It is a suprisingly 'well
written' piece of code (for MS...), and has few problems. The program
is probably seeing this 'stub', and therefore complaining. You can
simply edit MSDOS.SYS to remove the test, if you are sure your card
does not need it.

Best Wishes

Louis Ohland wrote:
Why is dublbuffer needed? Assuming that you have a modern SCSI controller [tribble or later], and decent drives, why would you need to diddle with it?
.

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