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Programming Model
TVP4020
Programmers Reference Manual
8
only reads these registers to determine the scissor clip extents. Most
registers are control registers.
Command Registers are those which, when written to, cause some
action to occur. Typically, the host will initialize the appropriate control
registers and then write to a command register to initiate drawing. Some
command registers such as ResetPickResult or Sync do not initiate
rendering. Apart from these, there are two types of command registers:
begin-draw and continue-draw. Begin-draw commands cause rendering
to start with those values specified by the control registers. Continue-
draw commands cause drawing to continue with internal register values
as they were when the previous drawing operation completed. Making
use of continue-draw commands can significantly reduce the amount of
data that has to be loaded into P
ERMEDIA
when drawing multiple
connected objects such as polylines. Examples of command registers
include the Render and ContinueNewLine registers.
For convenience in this document we often refer to "sending a Render
command to P
ERMEDIA
" rather than saying "the Render Command
register is written to, which initiates drawing".
Internal Registers are not accessible to host software. They are used
internally by the chip to keep track of changing values. Some control
registers have corresponding internal registers. When a begin-draw
command is sent and before rendering starts, the internal registers are
updated with the values in the corresponding control registers. If a
continue-draw command is sent then this update does not happen and
drawing continues with the current values in the internal registers. For
example, if a line is being drawn then the StartXDom and StartY control
registers specify the (x, y) coordinates of the first point in the line. When
a begin-draw command is sent these values are copied into internal
registers. As the line drawing progresses these internal registers are
updated to contain the (x, y) coordinates of the pixel being drawn. When
drawing has completed the internal registers contain the (x, y)
coordinates of the next point that would have been drawn. If a continue-
draw command is now given, these final (x, y) internal values are not
modified and further drawing uses these values. If a begin-draw
command had been used the internal registers would have been re-
loaded from the StartXDom and StartY registers.
For the most part internal registers can be ignored. It is helpful to
appreciate that they exist in order to understand the continue-draw
commands.
Efficiency Issues and Register Types
Software developers wishing to write device drivers for P
ERMEDIA
should
become familiar with the different types of registers. Some control
registers such as the StartXDom and StartY registers have to be
Содержание TVP4020 PERMEDIA 2
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