int intdos(union REGS *inregs, union REGS *outregs )

	- prototype in dos.h

	- Executes 8086 software interrupt INT 21
	- Copies register values from inregs into the registers
	- if CF is set, an error has occurred
	- unique to DOS
	- see	intdosx()   int86()   int86x()   intr()   REGS