JAN91: PORTING UNIX TO THE 386: DESIGNING THE SOFTWARE SPECIFICATION

Other Processor Faults

386 Processor Exceptions
Exception	Description	Pushes an Error Code?
Divide	Division by 0 or division overflow	No
Debug/Trace	Single step or debug hardware condition	No
Breakpoint	Executed an INT3 instruction	No
Overflow	Executed an INTO instruction when OF bit set	No
Bounds Check	Executed an BOUND instruction which failed	No
Illegal Instruction	Executed an unknown instruction	No
NPX DNA	Numeric processor device not available	No
Double Fault	Recursive fault (fault while processing a fault)	Yes
NPX Operand	Numeric processor accessed outside of segment	No
Invalid TSS	Attempted to task switch to incorrect task state	Yes
Segment Not Present	Attempt to access a not present descriptor	Yes
Stack Segment	Problem with current stack descriptor	Yes
General Protection	Protection problem with a segment descriptor	Yes
Page	Page missing or protection problem with address	Yes
NPX Error	Numeric processor signals an error	No

Figure 7 - 386 Processor Exceptions

Along with address space faults, we found we must map 15 other faults (see Figure 7) into the Berkeley UNIX kernel exception-handling mechanisms. The numeric coprocessor presents special fault-handling challenges, for it can be operating when 386BSD switches to another unrelated process. In that case, we can get a trap that should have been passed to a process other than the one currently running.

If 386BSD receives an unexpected fault while running in the kernel, it must immediately force the kernel down (in UNIX vernacular, to "panic") and attempt to save as much state information as possible for diagnostic purposes. Thus, we differentiated user traps from kernel traps. In most other microprocessors, a bit in the processor flags or status word determines if we are running in the kernel, but the 386 offers no such bit. So, 386BSD examines the contents of the CS segment register when a trap occurs (this is saved by the hardware during an exception) to determine if an instruction was executing in user mode.

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