At the time of DOS, x86 didn't have multiple privileges. The system call instruction was typically INT, the software interrupt instruction.
Later on the 386 Intel added virtual 8086 mode which trapped to the kernel privileged instruction exception also for certain instructions that had to be virtualized, among them INT.
Later on the 386 Intel added virtual 8086 mode which trapped to the kernel privileged instruction exception also for certain instructions that had to be virtualized, among them INT.