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TNL SN20-9844 (March 1987) to 75X0235
Operation
The Extended Monochrome Graphics Display Adapter is characterized by the following key
functional elements.
BAMDA
The display buffer (bit map
=
1024 X 1024) is a Bit Addressable Multidimensional Array
(BAMDA) which stores the picture elements (pels) for refreshing the monitor. The buffer is
arranged so that both horizontal and vertical accesses to the array are accomplished on a bit
addressable X , Y field. In addition, the bit length for write operations is variable (from 1 bit up
to 16 bits) in either the X or Y direction. Also logical operators can be specified on write
operations such as 'XOR', 'AND' and 'OR'.
Graphic Operation Processor
A graphic operation processor is used to:
•
Expedite transfers of arbitrary data block sizes from one location to another Bit wise Block
Transfer (Bit BL T) and in addition, perform Boolean operations on the data as it is being
moved
•
Draw geometric objects in the display buffer
•
Transfer image information stored in I/O channel memory.
Asynchronous Graphic Operation Queueing
A graphic operation queue mechanism is implemented in hardware and stored in the hidden
(nondisplayed) area of the bit map. Graphic operation commands are asynchronously linked to
one side of the queue list. The list acts as a first in - first out (FIFO) queue. The graphic
operation processor processes the commands one-by-one from the other side of the queue.
Interrupts, branching, and synchronization are additional control features of the command
queue.
Hardware Cursor
A 48 X 64 bit hardware cursor is combined simultaneously with the video output for each
displayed frame. There are 2 patterns which make up the cursor; the first is 'ANDed' with the
image whereas the second is 'XORed' with the image. These two images, when combined with
the video, allow for a programmable cursor pattern to appear on the screen in the cursor
position.
An X,Y pair of on-board registers enables cursor positioning at an X,Y pel location through
program control.
2 Extended Monochrome Graphics Adapter
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