Re: ancient OS history, ARM is sort of channeling the IBM 360

According to Stephen Fuld <SFuld@alumni.cmu.edu.invalid>:

understand why not for writes.  The no crossing block boundries is a
side effect of fixed block disks.  This couldn't happen in OS/360 with
CKD disks.

Actually, it did. OS had a Variable Block Spanned record format that
could split a logical record over several physical disk blocks.  I don't
think it was very widely used, but it's still there in z/OS:

https://www.ibm.com/docs/en/zos/3.1.0?topic=SSLTBW_3.1.0/com.ibm.zos.v3r1.cbcpx01/spanned.html

This doesn't mean it was a particularly good idea as implemented, of course.

I see that VSAM now has what they call linear datasets, which we would call
normal block files, which are used for Data in Virtual which we call memory
mapped files:

https://www.ibm.com/docs/en/zos/3.1.0?topic=guide-data-in-virtual
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