Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!comp.vuw.ac.nz!dsiramd!actrix!geoff
From: geoff@actrix.co.nz (Geoff McCaughan)
Newsgroups: comp.sys.amiga.tech
Subject: Re: RAM piggy-backing ?? (ooh err!)
Message-ID: <1990Jun8.023356.9590@actrix.co.nz>
Date: 8 Jun 90 02:33:56 GMT
References: <2113.266ea123@cc.nu.oz>
Organization: Actrix Public Access UNIX, Wellington, New Zealand
Lines: 36

In article <2113.266ea123@cc.nu.oz> c8748411@cc.nu.oz writes:
>
>	I have just read (in comp.sys.amiga.tech) a reference made
>to piggy-backing RAM chips.
>
>	What the hell (excuse the french) is RAM chip piggy-backing.

You can piggyback RAM in an A500 or A501. RAM on the A500 M/B is
addressed by DRA8:0, _CASL/_CASU and _RAS0/_RAS1, all except the latter
being in effect in parallel. The _RAS signals select which 512K bank is
being addressed.

You could add 512K to a 512K A500 by piggybacking RAM chips pin for pin,
but leaving pin 4 bent out. Wire all the pin 4's (_RAS) together and
connect to _RAS1. You also need to ground the EXRAM line for this to
work. Still no clock though 8-(.

You can upgrade an A501 to 1M by doing the same thing, but you need a
hardware hack to generate a _RAS2 signal 1st. Doing this to an A501 is a
class A bitch, you need a fine soldering iron, good light and a steady
hand. Getting the shielding back on afterwards is tricky too.

If you try this _PLEASE_ check three times before applying power, make
sure all your chips are the right way around and that you haven't
splattered solder anywhere. The writer takes no responsibility for any
smoked Amigas which may result from this.

Note the info above pertains to the rev. 5 A500 which uses 256x1 chips.
rev. 6 boards have 256x4 chips and room for an extra 512K on the M/B.

Happy hacking!

-------------------------------------------------------------------------------
Geoff McCaughan        Email: geoff@actrix.co.nz  Phone: +64 3 539545 or 852101
Amiga/Hardware/Unix/Quantum Mechanics Hacker      Phax : +64 3 539567
"Anarchy: Think of it as evolution in action."            ^ NEW ZEALAND!