76543210
11000000
equals
$CO.
3.
If
you wish to use a function
that
is
"not"
one of the sources, such as A, you take
all of the min terms
not
enclosed by the circle represented by A on the figure.
4.
If
you wish to combine minterms, you need only
"or"
them together.
For
example,
the equation
AB
+ BC results
in:
AB=
BC=
Shifting
11001000
10001000
1 1 0 0 1 0 0 0 = $C8
When bit-plane images are stored with sixteen I-bit pixels
in
a memory word, situations
arise where a particular pixel must be
in
a different
bit
position within a word before
and after a block transfer.
For example, as described previously under "Logic Operations," the movement of a car
image
(B)
across a background (C) requires both the car
image
(B)
and the car outline
mask
(A)
to
be shifted to a new position each time the background is saved
(T
= AC),
the car
is
placed (C =
AB
+ A 0), and the background is restored (C = AT).
As
the
movement proceeds, the edge of the car image can,
in
general, land on any
bit
position
within a 16-bit word. This illustrates the need for a high-speed shift capability within
the blitter.
Accordingly, the blitter contains a circuit known as a barrel shifter
that
can be used
with both the A and the B
data
sources.
It
can shift these sources from 0
to
15
bits.
It
is a true barrel shifter; bigger shifts do not take more time than smaller shifts as they
would if performed by the microprocessor. This shifter allows movement of images on
pixel boundaries, even though the pixels are addressed
16
at
a time by each word
address of the bit-plane image.
Blitter Hardware
177
To Next Page
Loading...
