ModelSim SE User's Manual - Electrical and Computer Engineering

Simulating Verilog designs UM-135
ModelSim helps you track down event order dependencies with the following compiler
arguments: -compat, -hazards, and -keep_delta.
See the vlog command (CR-358) for descriptions of -compat and -keep_delta.
Hazard detection
The -hazard argument to vsim (CR-373) detects event order hazards involving simultaneous
reading and writing of the same register in concurrently executing processes. vsim detects
the following kinds of hazards:
WRITE/WRITE:
Two processes writing to the same variable at the same time.
READ/WRITE:
One process reading a variable at the same time it is being written to by another process.
ModelSim calls this a READ/WRITE hazard if it executed the read first.
WRITE/READ:
Same as a READ/WRITE hazard except that ModelSim executed the write first.
vsim issues an error message when it detects a hazard. The message pinpoints the variable
and the two processes involved. You can have the simulator break on the statement where
the hazard is detected by setting the break on assertion level to Error.
To enable hazard detection you must invoke vlog (CR-358) with the -hazards argument
when you compile your source code and you must also invoke vsim with the -hazards
argument when you simulate.
Important: Enabling -hazards implicitly enables the -compat argument. As a result,
using this argument may affect your simulation results.
Limitations of hazard detection
Reads and writes involving bit and part sel ects of vectors are not considered for hazard
detection. The overhead of tracking the overlap between the bit and part selects is too
high.
A WRITE/WRITE hazard is flagged even if the same value is written by both processes.
A WRITE/READ or READ/WRITE hazard is flag ged even if the write does not modify
the variable's value.
Glitches on nets caused by non-guaranteed event ordering are not detected.
ModelSim SE User’s Manual