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

According to Lawrence D'Oliveiro  <ldo@nz.invalid>:

possible to request “locate mode”, where it returns the address of a
record directly within its internal buffers. But there are many
restrictions on this, among other things:
>
* It only works for reads, not for writes

It works fine for writes on IBM systems. PL/I even has a LOCATE
statement to get a buffer to write into.

* It doesn’t work for records crossing block boundaries

The PL/I programmer's guide has a note saying that if
you use spanned records, it does an internal copy so
don't bother.  It "works" but it doesn't help.

* It doesn’t work for compressed records

Indeed. 

In any event this was all decades ago. These days most systems do the
disk I/O through the pager so it's all locate mode all the time.
--
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