Hi all:
In order to run the or1ksim test suite against ORPSoCV2, I wanted to
support NOP codes like NOP_GET_TICKS and NOP_GET_PS in the Verilog test
bench.
The current implementation in or1200_monitor.v already supports a few
NOP codes, look for "small hack to stop simulation (l.nop 1)" in that
file. All of the existing ones read some register value (usually R3) and
do something with them, like writing some trace string in the simulation
log. However, NOP_GET_TICKS and NOP_GET_PS need to write to some
registers in order to pass information back to the software that runs in
the simulation.
CPU registers are read with task get_gpr, which calls task
`OR1200_TOP.`CPU_cpu.`CPU_rf.rf_a.get_gpr. I thought I could write to
CPU registers by calling the set_gpr counterpart. However, it turns out
that the values I write do not stay very long in the CPU registers. For
example, if I write to R3 while executing "l.nop NOP_GET_PS" and the
next instruction is an "l.nop NOP_EXIT", then the value survives, and
the simulation exits with the right value. But if the next instruction
reads from R3, then the new value is lost somewhere in the pipeline, it
looks like R3 goes back to its old value somehow.
I traced set_gpr down to or1200_dpram.v, and the problem might be some
timing issue, as the new value for R3 is not written in a "normal" write
cycle inside the core, but directly to an internal Verilog array by some
foreign code outside that module.
I always wondered why the special l.nop instructions are handled in
or1200_monitor.v , which feels somehow external to the CPU core. I
thought such instructions should be handled inside the core like any
other regular instructions.
Can someone shed some light here? I don't know much about the CPU core
yet. Should I try to find some example code, like the implmentation for
l.add, and implement the special l.nops in the same way, so that the
output registers are written in the standard fashion? Where should I
look for that code? Or is there a quicker hack for simulation purposes
so that writing to registers works?
Many thanks in advance,
Ruben
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