 |
 |
 |
 |
 |
|
|
|
|
|
|
|
The trap is unexpected and with one exception, the reaction is to “panic”. The code executed is the “default” of the “switch” statement:
Print:
the current value of the seventh kernel segment address register (i.e. the address of the current per process data area);
the address of “ps” (which is in the kernel stack); and
the trap type number;
“panic”, with no return.
Floating point operations are only used by programs, and not by the operating system. Since such operations on the PDP11/45 and 11/70 are handled asynchronously, it is possible that when a floating point exception occurs, the processor may have already switched to kernel mode to handle an interrupt.
Thus a kernel mode floating point exception trap can be expected occasionally and is the concern of the current user program.
Call “psignal” (3963) to set a flag to show that a floating point exception has occurred;
Return.
This raises an interesting question: “Why are the kernel mode and user mode floating point exceptions handled slightly di�ferently?”
|
|
|
|
|
|