Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!jarthur!nntp-server.caltech.edu!toddpw From: toddpw@nntp-server.caltech.edu (Todd P. Whitesel) Newsgroups: comp.sys.apple2 Subject: Re: OctoRAM and DMA Message-ID: <1991Apr20.231149.22007@nntp-server.caltech.edu> Date: 20 Apr 91 23:11:49 GMT References: <47549@ut-emx.uucp> <7vpgrza@rpi.edu> Organization: California Institute of Technology, Pasadena Lines: 38 mvk@aix01.aix.rpi.edu (Michael V. Kent) writes: >So, back to my original question. If I plug 8 256K SIMMs into my OctoRAM with >my Apple High Speed (DMA) SCSI card, what happens? Will I get 1 Meg of DMA and >1 Meg of non-DMA; or just 1 Meg of DMA; or an unusable system, or damaged hard- >ware? You should get an unusable system. DMA access to the upper meg will not work correctly and you probably get system crashes moments after you first use the disk. If you flip a switch on the DMA SCSI so it doesn't actually use DMA then everything will work but it will be much slower without an accelerator. >I've been told that with an Apple SCSI card, only the first 4 SIMMs will >function in any way. With a RAMFast SCSI, I've been told, the first 4 SIMMs >will be DMA, and the second 4 SIMMs will function normally. Is this true? Not really. It is the other way around, and with 1 meg SIMMs: Most memory cards can only support DMA for four rows of chips (i.e. 4 SIMMs). If the HD card attempts to DMA into the other rows then bad things will happen and you will get an unusable system. The DMA SCSI does not attempt to use DMA above the 4 Megabyte mark, so if you have an OctoRAM with 1 meg SIMMs it will work with 4 megs DMA, 4 megs normal. If you have 256K SIMMs in it you cannot use DMA at all with the Apple SCSI, and the RAMfast must be told to only DMA to the motherboard (both are DIP switches on the cards). The RAMfast does not to my knowledge put a limit on expansion RAM DMA so you will not be able to DMA to the OctoRAM with it. You can still DMA to the motherboard (you can ALWAYS DMA to the motherboard unless another DMA peripheral is causing a conflict -- this apparently happens if both a RAMfast and DMA SCSI are in the same system, and causes 'creeping crashes' -- trust me, I know, I had both for a week and couldn't use them together for more than five minutes without a crash). Todd Whitesel toddpw @ tybalt.caltech.edu