MEMORY CLEAN FOR THE IBM PERSONAL COMPUTER

                             Version 1.00
                             May 3, 1983

                         Robert J. Beilstein
                        413 Wells Avenue, West
                    North Syracuse, New York 13212


One of the unfortunate "features" of the IBM Personal Computer is
that, while the 8088 processor will address 1024K of memory, the BIOS
(and, thus, DOS) will only recognize the first 640K (on "regular"
PC's, without the expansion box and ROM, 544K) of memory.

While there are a number of programs around which are able to ignore
the memory size passed from DOS, and make use of the additional
memory, there is still a fairly serious problem that needs to be
addressed.

When the machine is powered up, the contents of memory are
unpredictable.  In particular, there is probably only a 50%
probability that any given memory location has the proper parity.

The ROM BIOS "initial reliability tests" write to both planar and I/O
RAM during initialization.  This not only sets proper parity in the
memory locations (this happens automatically when the locations are
written into), it also tests the memory for proper operation (though,
curiously, it never attempts to write an odd-parity value).

Unfortunately, this setting and testing of memory only occurs in the
first 640K (544K) of memory.  Thus, any additional memory in the
system is left in an uninitialized state.

If a program attempts to reference a location in this uninitialized
memory before first writing to it (extremely poor programming
practice, it is true), then there is a 50% chance that the result will
be a "PARITY CHECK 2".

This seems like an awfully stiff penalty to pay for a program bug, as
it requires one to power down the machine in order to clear it.

MEMCLEAN is a program which will write an initial value (with good
parity) into all of the memory locations which IBM misses.

When called without additional parameters, the program will clear the
high memory from X'C0000' through X'EFFFF' (768K to 960K).

If a parameter of "544K" is specified, locations X'88000' through
X'9FFFF' (544K to 640K) will be cleared.  This is for compatibility
with older (non XT) PC's.

If "SEGA" is specified, then locations X'A0000' through X'AFFFF' are
included.  This area is defined to be "reserved" by IBM, but is (for
now) available for use.

Specifying "NOHIGH" will inhibit writing into the high memory area
(X'C0000' through X'EFFFF').

Note that writing into nonexistent memory locations causes no problems
(other than using up some time).  So there is no problem if there is
memory only up to (say) 576K.

The following examples illustrate the use of MEMCLEAN:

1.  A PC/XT with memory in the high area, but not in the "reserved"
area:

A>memclean


2.  A "regular" PC with 704K of contiguous memory (i.e. up through
X'AFFFF'):

A>memclean sega 544k nohigh

3.  A "regular" PC with every available memory location populated:

A>memclean 544k sega

4.  A PC/XT with 704K of contiguous memory:

A>memclean sega nohigh


Permission is given to use and copy this program and documentation
freely, as long as no charge is made for its distribution or use, and
as long as the notices and copyright statements in the program and
documentation are left intact.  For complete details of the terms and
conditions, please see the source code.

All other rights are reserved.  Program and documentation Copyright
(C) Robert J. Beilstein, 1983.