The PURIMP routine splits the page map into three parts: writable data
at the bottom, a large gap in the middle (which the GC will manage
later), and read-only code/data at the top. There's one CORBLK call for
each of these.
The final call, which gave pages BOT/2000 to 400 as the range, failed
with %EROPG when it hit the first unmapped page after the end of the
pure data. Experimentation shows that, at least with current ITS, this
is how CORBLK normally behaves.
Fix by calculating the final page from RHITOP rather than assuming 400.
3122c71eb2