questasim coverage merge
You can do:
vsim -c -do "coverage save -onexit <TESTSET_CODE_COVER>.ucdb;run -all;exit" -coverage -voptargs="+cover=bcfst" <otherOptions> <TOP>
Here we tell vsim to
1) enable code coverage (-coverage),
2) the types of coverage to collect (via -voptargs=+cover= bcefst") if not using the three-step flow process,
3) and to produce a coverage database file on the exit of simulation (do "coverage save -onexit coverage.ucdb)
Further if you have multiple coverage databases, you can merge them into a single database by using:
vcover merge <ucdbFile1>.ucdb <ucdbFile2>.ucdb ... <ucdbFileN>.ucdb <ucdbFileResult>.ucdb
The finally you can generate a HTML report:
vcover report -html -htmldir <dirToOutput> -verbose -threshL 50 -threshH 90 <ucdbFile>.ucdb