Peritek
Software Summary
3-3
3.3 Write Posting
Features of many of the newer CPU designs include pipelining and write
posting. The CPU, which is much faster than the VMEbus interface, is
allowed to store (or post) a write operation to the CPU board's VMEbus
controller. The controller takes care of the write within the timing
requirements of the VMEbus. Pipelining is a procedure whereby the CPU
can process more than one instruction at a time. As a result, instructions
are not necessarily completed in the order that they were started.
In the case of sequential accesses to the VMEbus, which the Peritek baords
use, it can happen that the a write of the Peritek graphics board Line
Buffer can occur before a write to the Line Address Register (LAR) has
been completed. If you had wanted to change the LAR and then write, you
are not guaranteed that this has happened. This results in incorrect
operation. The way to get around this is to immediately read back the data
which has been written to the LAR, which flushes the pipeline and ensures
correct operation. Since this is a problem just for the LAR, the
performance impact is minor.
Peritek can supply its software with the read after write operation already
incorporated. When ordering software, be sure to specify the CPU. Known
offenders include 68040 and MIPS R3000 based CPUs.
Summary of Contents for VCD-V
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