djs@ttrdc.UUCP (Dennis J. Svobod) (07/16/85)
Has anybody out there in net-land successfully expanded their Macs to greater than 512K?. I am attempting a do-it-yourself upgrade and have run into some problems. It seems like it should be easy to piggy- back another 512K and add some multiplexing to address the correct banks. Not having any schematics makes any modification difficult. I have learned that of the commercially availible >512K expansions, there are basically two versions. One that leaves the video memory mapped as it is on a normal 512K Mac. (i.e. at the top of 512K). This version uses the extra memory (above 512) as a ram-disk. The Mac operating system is unable to use the extra memory as it thinks it only has 512K. Apparently the ROMs only differentiate between 128K and 512K Macs. (this MAY change with the new ROMS). Thus, the ROM expects the screen bitmap to be either at the top of 128K or at the top of 512K. The second version of >512K upgrades puts the screen at the top of the new memory. (i.e.at the top of 1Meg for a 1Meg upgrade.) When this is done, the memory is no longer fragmented by the video screen, and can be used by the operating system memory manager. The ROM however must be replaced with a "hacked" version that initializes all the appropriatte global variables correctly so that the Mac knows where the screen has moved to, how much heap space it has etc. I would very much like to here from anyone who has done a >512K upgrade. Either of the 2 versions above would be OK. (it should be trivial to convert one to the other, provided the correct ROMS were availible) Any suggestions from any of you Hardware Hackers would be appreciated. I am willing to try out any reasonable idea on my Mac. It's already pretty well hacked up anyway. Also, if anyone has purchased one of the commercially availible upgrades and has learned how they did it, I would like to here from them as well. If I get enough responces so that I am able to successfully do the upgrade I will post the 'solution' to the net so everyone who wants to can do it as well. The idea of having a 512K Ramdisk on top of 512K Mac seems quite attractive. Frank Owen ..!ihnp4!ttrdc!djs ( a friends account)