Qbasic AND registers

I have a problem using registers in Qbasic.
If the Interrupts I am using requires numeric data and no registers are combined, then I can simply send the data and receive it no problem. BUT when I try to send a string (for example, for ES:BX, say I make it equal to “TEST”), Qbasic crashes along with the program and Windows shows a system error. something like “This program has performed an illegal operation”. When I clicked on Details, it gives me an address in the form of xxxx:xxxx (fault location) and it shows “interrupts in service: 0”. I was using the EMS interrupt and I had to use VARSEG() and SADD() to get the data in Qbasic to communicate with the registers. Where can I find extensive information on how to sort these problems out?