To build the 6502 / BBC Micro simulator for MUSIC on Hercules or Sim390:

Log in to a suitably privileged MUSIC account such as $000.
Create a library to hold the simulator files, $Y65: for example.
If not already done, a UCR record must be created. Give the command:

UCR

and at the ? prompt, enter:

SET,CODE='$Y65',UCR=-1,-1
END

Next, restore / download the source files into the $Y65: library (if
some other location is chosen, appropriate modifications must be made to
files Y6502F.FORTG, Y6502A.S, BBCMOS.FORTG, Y6502 and BBC)

On the MUSIC command line, give the command:

$Y65:Y6502A.S

No warnings or errors should result. Next, give the command:

$Y65:Y6502F.FORTG

A single "IEY038I SIZE WRN." is expected and can be ignored.
Finally, give the command:

$Y65:BBCMOS.FORTG

Two "IEY008I ALLOCATION" errors are expected. I don't know what the
compiler is complaining about and there doesn't seem to be any impact
from these errors.