System 51 released

[Moon at MIT-AI (David A. Moon)]

System 51.1 with microcode 705 is now tentatively installed.

The performance disoptimization in the paging code introduced in system 50
has been fixed.  This necessitates that you use the latest version of LCADR;QF
(which of course is included in system 51) when debugging a system 51 machine
with CC.  Also system 51 will NOT come up in microcode older than 705.

Most "ARGTYP" microcode errors (e.g. doing arithmetic on non-numbers) are now
recoverable by typing the control-C or Resume key and giving a replacement argument.

The "MAR" feature now works reliably.  It never did in the past.

The "alu" functions have been changed incompatibly; all functions and messages
that take an "alu" argument now use the same code numbers as the BOOLE function.
If you use the variables TV:ALU-XOR, etc., the change will be transparent.

BITBLT is faster in the easy case where it is just copying words from one
place to another.  This should make window-scrolling and most cases of window
switching faster.

The keyboard buffer is now 64 characters instead of 24.  This should make it
impossible to fill up the buffer by typing ahead, even when the machine is
paging hard.

As previously announced, STRING-REVERSE-SEARCH-SET and
STRING-REVERSE-SEARCH-NOT-SET now do what the manual has always
documented them as doing.  In previous systems they return 1+ what they
are documented to (except when they return NIL).

TRACE has been changed around somewhat.  It now works to redefine or recompile
a function that is traced (it remains traced).