Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!wuarchive!rex!uflorida!gatech!purdue!haven!ncifcrf!lhc!adm!smoke!gwyn From: gwyn@smoke.brl.mil (Doug Gwyn) Newsgroups: comp.sys.apple2 Subject: Re: OctoRAM and DMA Message-ID: <15920@smoke.brl.mil> Date: 20 Apr 91 23:27:04 GMT References: <47549@ut-emx.uucp> <7vpgrza@rpi.edu> Organization: U.S. Army Ballistic Research Laboratory, APG, MD. Lines: 15 In article <7vpgrza@rpi.edu> mvk@aix01.aix.rpi.edu (Michael V. Kent) writes: >What's the scoop? 8 256KB SIMMs in an OctoRAM give you 2MB expansion RAM plus the 256KB on the motherboard (1.125MB if ROM03). All of it is usable for any purpose except DMA. Only the first 1MB (4 rows) of the expansion RAM is suitable for DMA. Apple's High-Speed SCSI Card will unfortunately attempt DMA to any address less than 4MB, which would include the non-DMA compatible portion of the 8x256KB OctoRAM. You could disable DMA use by the SCSI Card altogether in such a case. There simply is no way to safely use the upper 1MB of an 8x256KB OctoRAM for DMA transfers. The 8x1MB OctoRAM is a different matter. There, due to a hack in the firmware on the High-Speed SCSI Card, no DMA is attempted (by the SCSI Card) to the DMA-incompatible portion of the OctoRAM.