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From: zhahai@gaia.UUCP (Zhahai Stewart)
Newsgroups: comp.sys.ibm.pc
Subject: Re: hard/floppy cntrllr f/IBM-PC
Message-ID: <129@gaia.UUCP>
Date: Tue, 11-Nov-86 19:30:03 EST
Article-I.D.: gaia.129
Posted: Tue Nov 11 19:30:03 1986
Date-Received: Wed, 12-Nov-86 20:23:33 EST
References: <2183@ecsvax.UUCP> <21900185@uiucuxc> <1179@tekigm2.UUCP>
Organization: Gaia Corp, Boulder, CO
Lines: 27
Summary: Small corrections

In article <1179@tekigm2.UUCP>, timothym@tekigm2.UUCP (Timothy D Margeson) 
writes:
> A few more notes, ALL ST506 or ST412 disk drives use an 8 bit data path, an
> most typically spin at about 3600 RPM. Data goes on and comes off the disk 
> at about 5 megabytes per second, WITH A GOOD CONTROLLER AND FAST MEMORY BUS!

Actually, these drives and almost all 3.5" and 5.25" Winchester drives [EXCEPT
THOSE USING RLL ENCODING LIKE THE SHUGART ST-238 WITH APPROPRIATE CONTROLLER]
use an instantaneous data rate of 5 Mbits/sec (625 KBytes/sec).  Counting the
address and data marks, error correcting codes, CRC, and inter sector gaps,
the sustained data rate cannot be much better than 1/2 MByte/sec within one
cylinder.  Most pc controllers cannot handle even that speed and skip one
or more sectors between each read/write - this is called interleaving. Tim's
point that a given hardware/software combination's best performance in terms
of interleave factor varies from other combinations, and is best not assumed
but tested.  His system worked best with a 6:1 interleave (skip 5 sectors
between accesses), which may be very dependent also on his use of VERIFY ON.
(Probably not tho - since his reccomended test used disk reads only, not 
disk writes; be sure your test includes both if that write performance also
matters to you).
Also, all of these disks use a bit serial (not 8 bit) data path.
Some controllers for the PC claim to be able to handle 1:1 interleaving,
ie: no sectors skipped; the pc's DMA should be able to handle that; I do
not know if a modification to DOS's disk drivers is needed.  If there is
some processing time needed for processing between sectors, this may not
operate some applications as fast as a slower interleave...
				zhahai/HiSystems