Path: utzoo!mnetor!uunet!mcvax!unido!tub!tmpmbx!netmbx!hase
From: hase@netmbx.UUCP (Hartmut Semken)
Newsgroups: comp.sys.atari.st
Subject: Re: Expandability
Message-ID: <1822@netmbx.UUCP>
Date: 8 May 88 04:50:42 GMT
References: <880501040119.855327@HIS-PHOENIX-MULTICS.ARPA> <215@obie.UUCP>
Reply-To: hase@netmbx.UUCP (Hartmut Semken)
Organization: netmbx Public Access Unix, Berlin
Lines: 22

In article <215@obie.UUCP> wes@obie.UUCP (Barnacle Wes) writes:
>The addressing limit on the ST without hacking the hardware is 4 meg.
>The MMU chip does the majority of the address decoding, and it only
>deocodes a1..a21.  If you "fall off" the end of the MMU, it will
>(should?) give you a bus error or address error.  This could, of
>course, be changed by replacing the MMU chip.

No. You don't need to do that.
The GLUE chip monitors -AS (adress strobe of the 68000). After a memory
cycle the 68k waits for DTACK or VPA; if neithe comes, no new cycle
starts, so there is no new AS. A simple monoflop, retriggered by AS,
will interrupt the wait by saying "BUSERR" to her majesty.

If you generate your own DTACK on your RAM board, you can easyly expand
memory beyond 4 Meg.
I did it; I added 2 Meg of DRAM (as resetproof Ramdisk) and 256 KByte
battery backed SRAM (for fast booting) by decoding the full adress bus.

hase
-- 
Hartmut Semken, Lupsteiner Weg 67, 1000 Berlin 37 hase@netmbx.UUCP
I think, you may be right in what I think you're thinking. (Douglas Adams)