M69000 Asiliant Technologies, M69000 Datasheet - Page 347

• Manufacturer Part Number: M69000
• Manufacturer: Asiliant Technologies

Specifications of M69000

• Lead Free Status / Rohs Status: Supplier Unconfirmed

E-12
BitBLT Operation
Contiguous vs. Discontiguous Graphics Data
Graphics data stored in memory, particularly in the frame buffer of a graphics system, has organizational
characteristics that often distinguish it from other varieties of data. The main distinctive feature is the
tendency for graphics data to be organized in multiple sub-blocks of bytes, instead of a single contiguous
block of bytes.
Figure E-6 shows an example of contiguous graphics data — a horizontal line made up of six adjacent pixels
within a single scanline on a display with a resolution of 640x480. If it is presumed that the graphics system
has been set to 8 bits per pixel, and that the first byte of frame buffer memory at offset 0h corresponds to
the upper left-most pixel of this display, then the six pixels that make this horizontal line starting at
coordinates (256, 256) would occupy six bytes starting at frame buffer offset 28100h, and ending at offset
28105h.
In this case, this horizontal line exists entirely within one scanline on the display, and so the graphics data
for all six of these pixels exists within a single contiguous block comprised of these six bytes. In this simple
case, the starting offset and the number of bytes are the only pieces of information that a BitBLT engine
would require to read this block of data.
Figure E-6: On-Screen Single 6-Pixel Line in the Frame Buffer
The simplicity of the preceding example of a single horizontal line contrasts sharply to the example of
discontiguous graphics data depicted in Figure E-7. The simple six-pixel line of Figure E-6 is now
accompanied by three more six-pixel lines placed on subsequent scan lines, resulting in the 6x4 block of
pixels shown.
Since there are other pixels on each of the scan lines on which this 6x4 block exists that are not part of this
6x4 block, what appears to be a single 6x4 block of pixels on the display must be represented by a
discontiguous block of graphics data made up of 4 separate sub-blocks of six bytes apiece in the frame
buffer at addresses 28100h, 28380h, 28600h, and 28880h. This situation makes the task of reading what
appears to be a simple 6x4 block of pixels more complex.
Two characteristics of this 6x4 block of pixels help simplify the task of specifying the locations of all 24 bytes
of this discontiguous block of graphics data. First, all four of the sub-blocks are of the same length. Second,
the four sub-blocks are separated from each other at equal intervals.
The BitBLT engine was designed to make use of these characteristics of graphics data to simplify the
programming required to handle discontiguous blocks of graphics data. For such a situation, the BitBLT
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69000 Databook
Subject to Change Without Notice
Revision 1.3 8/31/98