TVP4020
Programmers Reference Manual
Programming Tips
139
7.
Programming Tips
This chapter covers a variety of programming tips that make best use of
P
ERMEDIA
. The topics covered here are not exhaustive.
7.1
PCI Bus Issues
7.1.1
Improving PCI bus bandwidth for Programmed I/O and DMA
The simplest way to program P
ERMEDIA
is by writing data values into the
memory mapped registers. i.e. programmed I/O. This is appropriate for
primitives which require few set-up parameters such as 2D lines.
For more complex primitives such as Gouraud shaded triangles, where a
significant number of registers must be loaded for each primitive, it may
be more optimal to write directly to the P
ERMEDIA
FIFO input.
The advantage of this mechanism is that it is then possible to use DMA
burst transfers. The disadvantage of this method is that both the address
of the register and the data value to be loaded must be written,
apparently doubling the amount of data to be loaded.
However, to improve bus bandwidth utilization, the registers have been
grouped, into blocks which frequently all need to be updated together,
and an indexed addressing mode is supported which allows a single
"address" to be loaded, followed by the data for a whole set of registers.
An additional mode is supported which allows a large number of data
values to be loaded to the same register. This is useful for image
downloads.
For more detail, refer to section §3.2.
7.1.2
PCI burst transfers under Programmed I/O
PCI bus burst transfers typically allow up to four times the bandwidth of
individual transfers. However burst transfers are only initiated on the PCI
bus when successive addresses are being written to (i.e. the byte
address is incremented by 4). When using burst transfers to perform
programmed I/O to load the P
ERMEDIA
FIFOs, P
ERMEDIA
multiply maps
the FIFO input register throughout the range:
0x00002000 to 0x00002FFF in region 0
Thus when data is being loaded into the FIFO a software loop should be
written which starts by writing the first data item at the lower extreme of
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