Peritek VCD-V, User Manual

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Peritek
5-16 Programming On-board Devices and Memories
of the bits to be write-enabled is loaded into this register (i.e. bit set =
write
disabled ). The writemask register has no effect on system memory.
The 34020 actually maintains a writemask register internally which is
intended to be passed to VRAMs which support a "persistent writemask"
function. It depends on a special function of the VRAMs which involves
passing the contents of the 34020 writemask register into the VRAMs,
where the data is stored until changed by the 34020. Since regular VRAMs
only hold the writemask data for the current cycle, the standard 34020
writemask function won't work. Therefore, the graphics board has an
auxiliary writemask register. It is buried inside the 74BCT16652 registered
bus transceivers which pass data between the 34020 local data bus (LAD)
and the 34020 common memory and device data bus (MAD bus). The
74BCT16652s hold the writemask data (essentially acting as the VRAM
writemask holding register) and gate it onto the MAD bus at the beginning
of every VRAM cycle.
Writing the 74BCT16652 writemask register is a bit of a trick: When a
program writes into the 34020 writemask register, the 34020 executes a
special cycle to update the VRAM writemask holding register. A PLD
detects this cycle and stores this data in the 74BCT16652s.
5.3.3 VRAM Color Register and Block Fill Special Function
There are a few instructions (VBLT, VFILL, and VLCOL) which can only
be used with Video RAMs (VRAMs) which support so-called VRAM
special functions, which include block write and color register support.
Peritek PX Windows software automatically detects and uses special
functions if the board's VRAM can support them.
The Color Register is used in conjunction with the VRAM block fill mode
which allows up to 4 adjacent locations in the VRAM to be written in one
cycle. In this way, large areas with repetitive patterns can be written at a
considerable higher rate that normal.