Thanks Peter. Ok, I'm more awake now than this morning, and after carefully re-reading the document I based my change of opinion on when I said A.Post was right I revert to my previous conviction: the 8051 DOES keep track of if and at what level interrupt it is servicing. To this avail it uses internal logic which cannot be accessed by software.
I advise A.Post, and anyone who's been confused to read the following document, chapter "How interrupts are handled" page 16:
http://www.semiconductors.philips.com/acrobat/v...WARE_1.pdf
The elusive interrupt control logic I mentioned in earlier posts undeniably pops up in the second paragraph on page 18.