Copying DEC VAX set up disks rx50, help
cclist at sydex.com
Mon Mar 5 18:19:46 CST 2007
At sometime today, Allison wrote:
> It's not 300 data rate it's 250!
Well, let's see what the numbers say.
360 RPM is 6 revolutions per second or 0.167 second/revolution
300 RPM is 5 revolutions/sec or 0.200 seconds/rev.
Thus, the raw capacity of a 300 RPM drive at 250Kbps is 50,000 bits
or 6250 bytes. The raw capacity of a 360 RPM drive at 250kpbs is
41667 bits or 5208 bytes.
On the RX50, since there are 10 sectors of 5120 bytes, we have
6250-5120 = 1130 bytes for gaps and ID marks, CRCs, etc. on a 300 RPM
drive. On a 360 RPM drive, one has 5208-5120 = 88 bytes for all that
overhead. Not considering gaps yet:
There are 10 bytes for a IDAM: A1 A1 A1 FE cc hh rr nn cr cr, where
cc is the cylinder number, hh is the head number, rr is the sector
number, nn is the sector length code and cr cr is the CRC for the
IDAM. To that, add the data preamble of 4 bytes A1 A1 A1 FB and a 2
byte CRC and you have (without gaps) 15 bytes per sector or 150 bytes
for a 10-sector track--and we're out of space already at 360 RPM
before we can add the barest of gaps (e.g. 12 bytes of 00 preamble
for the IDAM, 12 bytes of 00 preamble before the DAM...
You just can't do it at 250Kpbs and 360 RPM. Heck, 10x512 is a
fairly tight fit at 300 RPM.
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