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Memory I/O and Organization
TVP4020
Programmers Reference Manual
32
4.3.3
Special Memory Modes
P
ERMEDIA
uses SGRAM to store data. SGRAM devices usually have
special features that are particularly useful for graphics.
Hardware Writemasks.
These allow writemasking in the framebuffer without incurring a
performance penalty. If hardware writemasks are not available, P
ERMEDIA
must be programmed to read the memory, merge the value with the new
value using the writemask, and write it back.
To use hardware writemasking, the required writemask is written to the
FBHardwareWriteMask register, the FBSoftwareWriteMask register
should be set to all 1's, and the number of framebuffer reads is set to 0
(for normal rendering). This is achieved by clearing the ReadSource and
ReadDestination enables in the FBReadMode register.
To use software writemasking (if hardware masks are not available), the
required writemask is written to the FBSoftwareWriteMask register and
the number of framebuffer reads is set to 1 (for normal rendering). This
is achieved by setting the ReadDestination enable in the FBReadMode
register.
Block Writes
Block writes cause consecutive pixels in the framebuffer to be written
simultaneously. This is useful when filling large areas but does have
some restrictions:
•
No depth or stencil testing can be done
•
All the pixels must be written with the same value so no color
interpolation, alpha blending, dithering or logical ops can be done
Block writes are not restricted to rectangular areas and can be used for
any trapezoid. Hardware writemasking is available during block writes,
but not software writemasking. The scissor tests and extent checking
operate correctly with block writes, and bitmask patterns can be applied.
The FBBlockColor register holds the value to write to each pixel. Note
that this register should not be updated immediately after a Render
command which performs a block write.
Sending a Render command with the PrimitiveType field set to
"trapezoid" and the FastFillEnable field set will then cause block filling of
the area. Note that during a block fill any inappropriate state is ignored
so even if stippling, color interpolation, depth testing and/or logical ops,
for example, are enabled they have no effect. However, scissor clipping
does function correctly with block writes.
P
ERMEDIA
always writes 32 pixels per block fill. It takes care of any partial
blocks at the beginning or end of spans.
Содержание TVP4020 PERMEDIA 2
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Страница 47: ...Memory I O and Organization TVP4020 Programmers Reference Manual 38 Texture address TextureBaseAddress T W S ...
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Страница 292: ...TVP4020 Programmers Reference Manual Register Tables 283 ...
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