xcall MIAMEX, MX_SETPROMPT, prompt
MX_SETPROMPT (MIAMEX 6) sets the A-Shell command prompt string. The prompt parameter is a string field of up to 20 characters which should be set to the desired comment level prompt string—i.e. to be used in place of the default "dot" prompt.
You can also set the command prompt with SET.LIT. See MX_GETPROMPT to get the current prompt.