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From: herrickd@iccgcc.decnet.ab.com
Newsgroups: comp.arch
Subject: Re: Can old architectures run fast?
Message-ID: <4577.282fb5cd@iccgcc.decnet.ab.com>
Date: 14 May 91 15:02:53 GMT
References: <9105091145.AA04421@iecc.cambridge.ma.us>
Lines: 25

In article <9105091145.AA04421@iecc.cambridge.ma.us>, johnl@iecc.cambridge.ma.us (John R. Levine) writes:
> Traditional IBM disks allow variable length hardware disk blocks, and each
> block can have a key of up to 256 bytes.  You can have the disk controller
> search down a track or cylinder looking for a particular key.  This made
> perfect sense for ISAM on a 360/30, when the CPU stopped during disk I/O
> anyway, but it's pretty awful now.  IBM has for 20 years had more reasonable

a minor nit:  My 30 had a DASD controller that took channel programs
from the memory of the 30 and went off and did them.  If the 30 wanted
to wait, it could, but nothing compelled it to.

Having the DASD controller do the key lookup while the general purpose
computer goes on about its business makes good sense.  Finding the 
correct record on a track is a mechanical (as opposed to electronic)
process.  The 30 could continue doing things at electronic speeds
while the DASD controller watched the disk rotate.

DASD - Direct Access Storage Device.  We had three 2311 drives on that
system.  They looked like small washing machines with removable disk
packs under transparent covers.  The packs had ten recording surfaces
on six discs.  I believe the capacity was about two and a half million
bytes per pack.

dan herrick
herrickd@iccgcc.decnet.ab.com