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Programming Issues
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The Petra ASIC is functionally compatible with each of the components
that it replaces. In cases where functionality between ASICs is exclusive,
configuration switches or jumpers are provided to let you select the
desired functionality.
In several areas of functionality, the configuration switches provide
backward compatibility with earlier MVME167/177 implementations, but
you can override their settings in software if you wish. A "R/W" by the
corresponding register table entry in this manual denotes instances where
this override capability is present.
Where the older technology supported "fast page" or "EDO" DRAM
chips, the Petra memory controllers support SDRAM devices. The two
memory controllers modeled in Petra duplicate the functionality of the
"parity" memory controller found in the MC ASICs used on certain other
boards as well as that of the "single-bit error correcting/double-bit error
detecting" memory controller found in the MCECC ASICs used on the
MVME167/177.
This Programmer's Reference Guide describes the MCECC model (in
Chapter 4). In the MVME167/177 application, there is logic on the Petra
chip to prevent you from inadvertently enabling the MC memory controller
model.
The same SDRAM memory array serves both controller models. The
SDRAM array is 32 data bits wide with 7 checkbits. The array architecture
is a non-interleaved single bank for sizes below 32MB. For array sizes
above 32MB, additional physical memory banks are added but the
architecture remains non-interleaved.
A final note on the SDRAM implementation: The bandwidth between the
SDRAM and local bus is greater than it was with the earlier DRAM array.
As a result, software takes less time to execute. Applications that
incorporate elapsed-time functions which are dependent on code
execution may have problems.
For readers who need to know the ASIC-specific differences between the
previous MCECC and Petra/MCECC programming models in detail,
certain areas of the text in this manual are printed in italics and marked
with change bars (as is done here). Readers should compare those sections
to the corresponding sections of the first- and second-generation manuals.
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