I've done yet and fails too. It est, compiler gives me an error: "invalid combination of opcode and operands". Also i have put next code:
MOV AX, WORD [OldInt21h+2]
MOV DS, AX
MOV DX, WORD [OldInt21h]
With this, compiler goes well, but executable don't work properly because NTVDM gives an error.
Cheers