I'm making a pc emulator, and as such I'm working from cpu specific models(starting with 8086)...though it will eventually support 80386 stuff, I want to be able to set it to "only use 8086" so that people can test to see if their program works on a plain 8086(not some 80486 or whatever)

now my question...
when do something like mov bx,0xFFFF / mov ax,0xFFFF / mov ds,ax / mov [bx],ax
it will write into rom there, will an 8086 just plain reboot, or freeze, or jsut ignore it or what?

now I'm talking about JUST an 8086 not an 80386 in real mode!

and one more thing, the only faults in the 8086 are the divide by zero, overflow(and only by into), and debug single step(int 3 I think) right?

You can't write into ROM, if you try it's just ignored. But the address you gave actually isn't ROM - FFFF:FFFF is physical address 10FFEF, which on the 8086 is truncated to 0FFEF.

oh...yea, your right.. but anyway

8086 real mode exceptions:

fault: int 0, int 6
trap: int 1, int 3, int 4

Stack fault on 8086 should cause shut down.

I'm not sure about int 13 (General Protection Fault) on 8086. It is really implemented since 286?

btw: do not forget pop cs for 8086