If
you shift the
data
and the mask
to
a new location and repeat the above three steps
over and over, the car will appear
to
move across the background (the buildings).
Blitter
Logic
Operations
-
Combining
Minterms
The blitter performs various
logic
operations, such as the one shown in the last section,
by combining
min
terms. A minterm
is
one of eight possible logical combinations of
data
bits from three different
data
sources.
For example, the following equation uses two minterms, ABC and ABC:
D=ABC+ABC
This means
that
the
logic
value of D
is
a 1 if either ABC = 1 or ABC = 1.
Another way of reading this equation is
that
D
is
true if and only if both A and
Bare
true. This is because the equation could be grouped
as:
D=AB(C+C)
However, since the term (C + C) is always true, this equation reduces
to
D =
AB.
Therefore, selecting the two minterms
ABC
and
ABC
will give the
logic
operation
D
=
AB.
These two minterms are selected with bits 7 and 6 of
BLTCONO.
The minterms
that
can
be
selected by
BLTCONO
control bits are as
follows:
MINTERMS: ABC ABC
ABC ABC ABC
ABC ABC
ABC
ENABLE BITS
(BLTCONO
LF7-LFO):
7 6 5 4 3 2 1 0
Since there are eight minterms, there are
256
possible equations
that
can be selected.
Table
of
Commonly
Used
Equations
For your convenience, table
6-1
contains a set of commonly used equations. The last one
in the table ( D
=
AB
+ AC ) is often referred
to
as the "cookie-cut" minterm selector.
Blitter Hardware
173
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