Date: Mon, 24 Oct 2005 03:18:29 +0200 From: =?ISO-8859-1?Q?Antonio_Rodr=EDguez?= <[EMAIL PROTECTED]>
Well, for what I read in Folklore.org (a great site about the history of the Mac's development, written and maintained by Andy Hertzfeld, Macintosh system software lead engineer), the 128's motherboard actually allowed you to raise the RAM to 512 Kb by replacing the 64 kbit chips with 256 kbit ones. I think, also, that the ROM was prepared from the beginning to detect how much RAM was present and use it all (up to the 8 Mb limit imposed by the 24-bit addressing mode, that is). I also think the kind of RAM chips used was the only change between the 128k and the unenhanced 512k.
Unfortunately, the memory map for the 128K, 512K and Plus all have the ROM mapped at $40 000, which is 4 megabytes. Additionally, the Mac Plus (and therefore 512KE) have the SCSI mapped at $58 0000 to $60 0000 (5.5 MB - 6 MB). I think it would be rather challenging to put the RAM from $00 0000 to $40 0000 and then have it take up again at $42 0000.
On the other hand, I wonder how much of a hacking job it would be to remap the ROM to $80 0000 - $82 0000 and the SCSI to $88 0000 to $90 0000. The address space from $80 0000 to $90 0000 appears unused. One would need to change some GLUE chips as well as the ROM code, I suppose, which is probably doable. I mean, they got Mini vMac working, right. But I suspect that one would also have to patch ToolBox mapping tables in the various OSs.
It would be cool to hack 8 MB of RAM into these old machines, but the memory map coded into the hardware and ROM make it not very feasible.
Jeff Walther -- Compact Macs is sponsored by <http://lowendmac.com/>. Support Low End Mac <http://lowendmac.com/lists/support.html> Compact Macs list info: <http://lowendmac.com/lists/compact.shtml> --> AOL users, remove "mailto:"; Send list messages to: <mailto:[email protected]> To unsubscribe, email: <mailto:[EMAIL PROTECTED]> For digest mode, email: <mailto:[EMAIL PROTECTED]> Subscription questions: <mailto:[EMAIL PROTECTED]> Archive:<http://www.mail-archive.com/compact.macs%40mail.maclaunch.com/> --------------------------------------------------------------- iPod Accessories for Less at 1-800-iPOD.COM Fast Delivery, Low Price, Good Deal www.1800ipod.com ---------------------------------------------------------------
