i386_vm86() will set the process into virtual 8086 mode using the registers and selectors specified by the context pointed to by
vmcp. The processor registers are set from
vmcp->substr.regs, and the emulated processor type from
vmcp->substr.ss_cpu_type.
The kernel keeps a pointer to the context, and uses the tables stored at
vmcp->int_byuser and
vmcp->int21_byuser for fast virtual interrupt handling. If the
n th bit is clear in the first of these arrays, then the kernel may directly emulate the real-mode x86 INT
n instruction handling. If the
n th bit is set, then the process is delivered a signal when an INT instruction is executed.
Since MS-DOS puts many DOS functions onto interrupt 21, it is handled specially: the
k th bit in the
vmcp->int21_byuser array is checked when INT
21 is requested and the
ah register is
k.