clye@phoenix.Princeton.EDU (Christopher Lye) (05/28/90)
I just got a Mac SE/30 and have been perusing the manual that came with it. It says in the hardware specs that as denser memory becomes available the SE/30 will be capable of supporting 128MB of RAM. However, I've heard that the virtual memory limit of 15MB also applies to "real" memory. Is this so? If so then why would Apple make the SE/30 capable of supporting 128MB? Is this a software or a hardware limitation? Would the extra memory only be useful i terms of a RAM disk? Thanks for any light anyone can shed on this issue. Chris
marc@Apple.COM (Mark Dawson) (05/28/90)
In article <16786@phoenix.Princeton.EDU> clye@phoenix.Princeton.EDU (Christopher Lye) writes: >I just got a Mac SE/30 and have been perusing the manual that came with >it. It says in the hardware specs that as denser memory becomes >available the SE/30 will be capable of supporting 128MB of RAM. However, >I've heard that the virtual memory limit of 15MB also applies to "real" >memory. > >Is this so? > >If so then why would Apple make the SE/30 capable of supporting 128MB? >Is this a software or a hardware limitation? Would the extra memory only >be useful i terms of a RAM disk? (1) I think it costs Apple next-to-nothing to add support for 16mb SIMMs once they are already supporting 4mb. (2) I believe A/UX can take advantage of the extra RAM. (3) RAM disks are nice things. (4) If Apple decides to modify their OS so that it can address more memory, the existing base of Macintoshes will be able to take advantage of it. -- --------------------------------- Mark Dawson Service Diagnostic Engineering AppleLink: Dawson.M Apple says what it says; I say what I say. We're different ---------------------------------
noah@Apple.COM (Noah Price) (05/28/90)
In article <16786@phoenix.Princeton.EDU> clye@phoenix.Princeton.EDU (Christopher Lye) writes: >[The SE/30 manual] says in the hardware specs that as denser memory becomes >available the SE/30 will be capable of supporting 128MB of RAM. This is true -- it really just means that the SIMM has the address lines wired to it for that much DRAM. [Techno-greek, for those who care] The standard SIMM allows 24 bits of address (12 bit row address, 12 bit column address) for a total of 16 MBytes per SIMM. This means, on an SE/30 or any Mac II family computer, you have four times 16 MBytes, or 64 MBytes maximum per bank. Two banks give you 128 MBytes. For a "normal" SIMM (that is, one with no more than eight parts, or nine for parity), this requires 16 Mbit DRAM which is still a generation of memory away. The 4 Mbit parts that are available now will get you a 4 MByte SIMM, for a total of 16 MBytes per bank on an SE/30 or Mac II family computer, or a maximum of 32 MBytes total for both banks. 1 Mbit parts are still the cheapest $/bit, and conveniently a system full of 1 Mbit SIMMs gives you 8 MBytes -- the most physical (or "real" as you call it below) memory today's Mac OS will take advantage of. So now you're calling me a liar, saying "but Noah, so and so will sell me a (4, 16, 32, whatever) MByte SIMM today!" Yes, I've seen some vendors making "oversize" SIMMs by putting more DRAM chips on each SIMM, often with some additional logic, to get larger memory configurations using today's memory technology. Today, A/UX can take advantage of this, but all you can use it for under Mac OS is a RAM disk. You'll probably see more of it once Mac OS can take advantage of more than 8 MBytes, though if you're doing very disk intensive work you may find you prefer having a RAM disk to having more system memory... >I've heard that the virtual memory limit ... also applies to "real" >memory. > >Is this so? In short, no it isn't. But Macs today won't use more than the first 8 MBytes. There's plenty of discussion on virtual memory going on in comp.sys.mac.misc, so I won't rehash it all here. > ... why would Apple make the SE/30 capable of supporting 128MB? Because that's how many address lines there were on the SIMM. :-) >Is this a software or a hardware limitation? 128 MBytes is a hardware limitation. 8 MBytes (of physical memory) is a software limitation under today's system software. >Would the extra memory only be useful in terms of a RAM disk? Today, more than 8 MBytes of physical memory is only useful under A/UX or as a RAM disk. noah ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ noah@apple.com Mac IIci Hardware Design Team ...!{sun,decwrl}!apple!noah (not the opinions of) Apple Computer, Inc.
philip@Kermit.Stanford.EDU (Philip Machanick) (05/29/90)
In article <41391@apple.Apple.COM>, marc@Apple.COM (Mark Dawson) writes: > (3) RAM disks are nice things. No, they're not. RAM caches are a much better idea - you don't have to preload them, and theey fill up with the stuff you actually use. Would it be a huge extra effort for Apple to redesign the memory allocation for the RAM cache so it could take advantage of such "extra" memory (above the 8M limit the OS currently recognizes)? Maybe this problem will be fixed in System 7 ... Philip Machanick philip@pescadero.stanford.edu