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Memory I/O and Organization
TVP4020
Programmers Reference Manual
28
FBSourceOffse
t
is normally zero except during a copy operation
where data is read from one address and
written to another address. The FBSourceOffset
is held in the FBSourceOffset register.
W
is the screen width. Only a subset of widths are
supported and these are encoded into the PP0,
PP1 and PP2 fields in the FBReadMode
register. See the table in Appendix C for more
details.
These address calculations translate a 2D address into a linear address
so non power of two framebuffer widths (e.g. 640) are economical in
memory. The address is in pixels; this is translated to a physical byte
address by multiplying by the number of bytes in the pixel.
The width is specified as the sum of selected partial products which are
selected by the fields PP0, PP1 and PP2 in the FBReadMode register.
This is the same mechanism as is used to set the width of the
localbuffer, however the widths may be set independently. The range of
widths supported are tabulated in Appendix C, together with the values
for each of the PP fields. This table holds all the common screen widths.
For arbitrary screen sizes, for instance when rendering to 'off screen'
memory such as bitmaps the next largest width from the table must be
chosen. The difference between the table width and the bitmap width will
be an unused strip of pixels down the right hand side of the bitmap.
Note that such bitmaps can be copied to the screen only as a series of
scanlines rather than as a rectangular block, unless the Texture Read
unit is used. In this case the stride for the read can be set differently to
the write by means of the partial products However, windowing systems
often store offscreen bitmaps in rectangular regions which use the same
stride as the screen. In this case normal bitblts can be used
4.3.2
Framebuffer Color Formats
The contents of the framebuffer can be regarded in two ways:
•
As a collection of fields of up to 32 bits with no meaning or assumed
format as far as P
ERMEDIA
is concerned. Bit planes may be allocated
to control cursor, color look up tables (LUTs), multi-buffer visibility or
priority functions. In this case P
ERMEDIA
will be used to set and clear
bit planes quickly but not perform any color processing such as
interpolation or dithering. All the color processing can be disabled so
that raw reads and writes are done and the only operations are
writemasking and logical ops. This allows the control planes to be
updated and modified as necessary.
Summary of Contents for TVP4020 PERMEDIA 2
Page 1: ...Texas Instruments TVP4020 PERMEDIA 2 Programmer s Reference Manual Issue 4 ...
Page 284: ...TVP4020 Programmers Reference Manual A Gouraud Shaded Triangle 275 ...
Page 292: ...TVP4020 Programmers Reference Manual Register Tables 283 ...
Page 314: ...TVP4020 Programmers Reference Manual Index 305 ...
Page 315: ...Index TVP4020 Programmers Reference Manual 306 Index ...
Page 323: ...Index TVP4020 Programmers Reference Manual 314 ...