Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!rochester!rit!cci632!ccicpg!conexch!rob From: rob@conexch.UUCP (Robert Collins) Newsgroups: comp.sys.ibm.pc Subject: Re: Protected mode and PC/XT cards Message-ID: <30483@conexch.UUCP> Date: 6 Jun 89 01:12:10 GMT References: <158322@neabbs.UUCP> Reply-To: rob@conexch.UUCP (Robert Collins) Organization: The Consultants' Exchange, Orange County, CA. (714) 842-6348 Lines: 34 In article <158322@neabbs.UUCP> alun@neabbs.UUCP (ALUN SAUNDERS) writes: >Can anyone tell me what happens to memory on 8-bit PC/XT add-on cards >installed in a 286 or 386 machine, say video RAM, when that machine is >running in protected mode ? Since the card is only seeing the bottom >20 bits of the 24 address lines, won't the offending lump of memory >appear all over the ........ Suppose you want to address video ram @ B800:0 What happens in 20 bit addresses? The CPU takes the segment, shifts left 4 and adds the offset. Then uses the result to strobe the address lines. Note that since B8000 is below 1M only 20 address lines are needed. Now, do the same thing on a 286 (not in proteced mode). The only difference is that 24 address lines get strobed. What is the value of lines 20-23? 0. Now go to proteced mode. I can only assume you know how addressing works in proteced mode, so I will not give the long winded explanation. You segment register is a pointer to a table that has the actual address in it. If that segment selector points to a table entry that has address B8000 as its address, all 24 address lines get strobed at you write to B8000. What is the value of A20-A23? 0 (still). So your question really becomes quite simple: Does video (or any RAM) below 1M work on a 286? The answer is an obvious yes. It doesn't matter that you only have 20 address lines on your card when A20-A23 are 0 anyways. -- "Worship the Lord your God, and serve him only." Mat. 4:10 Robert Collins UUCP: ucbvax!ucivax!icnvax!conexch!rob HOMENET: (805) 523-3205 UUCP: uunet!ccicpg!turnkey!conexch!rob WORKNET: (805) 378-7901