Tandy 2000 Service Manual page 222

Tandy@
Model
2000
Service
Manual
The
modified
address
test
has two
versions: the long test
will test the
R/\M
55,536
times per pass
(0000H-FFFFH) and
the short test wilt test the
RAl4
256
times per
pass
(0000-00FF). The
number
of tests per
pass
is
determined
by
a
16-bit
mask
register
which
is
incremented
by
one
for
each
wrLLe/read
cycle
through
the entire
memory
array (i.e.
'
00000H-TFFFFH
for
5l-2K).
The
data pattern written is
the
result of the
exclusive-OR
of the high
address
segment
register (16-bit) with the result of the
exclusive-OR
of
the
lower address
segment
or offset (16-bit)
and
the
mask
register. This data pattern is written
through
the
memory
array
and
then read
and
compared
to
check
for
accuracy.
Errors that
occur
will
be
listed individually in the error
table that specifies the data written, the data read,
the
exclusive-OR
of the
data
written
and Lhe
data read'
and the
address where
the error occurred. In
most cases,
this will
indicate
which
RAivl
chip in a particular
bank has
failed.
It is
recommended
that aII three
RAM
tests
should be used
to
verify correct operation of the
RAM
installed in the unit.
Although these
tests
do
not exercise
every combination of
bits that
can be
written throughout
the full
RAM
capacity,
they exercise
enough
wrLLe/read
operations to
achieve
a
fairly reliable test of
memory
f/0
and
data recovery to
isolate
most
common
memory
failures.
A complete
test that
exercises every
bit in
an
array is impractical
because
of
the
extreme
number
of bit
combinations,
especially-in larger
memory
arrays. For a l6-bit
system,
there are
I6Q^')
combinations, where
n
equals
the
memory
size (e.9.t
128K,256K,512K),
that
must be
written,
read
and
compared
to
complete
the full test. In
comparison,
the modified
address
method
reduces t.he
amount
of time it. takes to
complete
a
pass
but
even
the long modified
address
test
on
a 5I2f
memory
array will take approximately 30 hours
to
complete.
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