Hi, im currently working on a simulator for the Intel 8051. I dont have access to any real microcontroller so all the infomation i put into the simulator comes either from documentation or other simulators. However i have a few questions i hope one of you out there can answer.

According the official documentation for the 8051 the instructions; 'add', 'addc' and 'subb' will set the aux. carry bit if a carry out or borrow on bit 3. In my current implementation i set aux carry if this happens, however i have learned that the 'Jsim' simulator does not set the aux carry bit under certain circumstances. Example: substracting the value 0x08 from 0x10 does not set the aux carry in the above mentioned simulator but it does require a borrow on bit 3.

I am simply confused =) but please help me clear this out.

I have another perhaps simpler question.
What happens if both timers on a 8051 is set to splitmode ??

Thanks in advance,
Peter Sorensen