On Mon, 26 Nov 2012 10:01:38 -0600, Mark Zelden <[email protected]> wrote:

>. . .  Things
>like PSA overlays, UCB overlays etc.  Of course over the years IBM has added
>lots of code and features to repair critical control blocks and there are 
>captured
>UCBs etc., so it less frequent. . . .

The introduction of captured UCBs was a backward step from a system reliability 
point of view, as it increased the chances of a UCB being overlaid, and could 
make it very difficult to catch the culprit. The much later Captured UCB 
Overlay Protection restored the reliability status quo.

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