dafa@ihwpt.UUCP (David Fay) (10/08/85)
Until yesterday I intended to upgrade my Mac to 2 Meg with a Levco kit. I could get a kit for about $450 from a local dealer. The memory chips would cost about $100 for a total investment of $550. Now I think I have a better deal. The Max(TM) from MacMemory, Inc. is a 1.5 Meg upgrade (add 1 Meg on a daughter board to your 512K). It has several advantages over the Levco upgrade. First, it draws 1.3 watts instead of Levco's 3 watts. While I know almost nothing about hardware, knowledgeable friends tell me that when it comes to power consumption, less is more. In particular, with this small power draw you don't have to worry about whether your power supply will work (Levco maintains a list of power supplies which won't handle its upgrade). Also you don't need a fan. Second, the upgrade was designed to work with the new megabit chips. It uses 18 pin sockets instead of 16 pin and has additional address decoding. When the megabit chips are available at reasonable prices (say in a year) you will be able to replace the 256K chips with megabit chips and have a 4 Meg Mac (actually 4.5 Meg but the Mac will only address 4 Meg). Third, if you are running a ramdisk in the upper 1 Meg of memory it will not be destroyed by a reset. This means you won't have to reload the ramdisk if you have to reset on a runaway program (assuming the program doesn't write all over the ramdisk). There is one disadvantage also. Unlike Levco upgrades, The Max does not have its own ROM. So screen memory sits at the top of 512K. This means that you can use the extra memory for a 1 Meg RamDisk while programs have 512K to execute in. While this is not as desireable as the Levco solution, which relocates screen memory to the top of physical memory, MacMemory says the new Apple ROMs will do this. So when they come out, all 1.5 Megs will be continuous. MacMemory does provide a ramdisk program that gives you an option of using 512K for applications and 1 Meg for ramdisk or vice versa. So if you want to run Switcher with 6 or 8 programs you can. But here is the clincher. Look at these prices for the kit: Quantity Price 1 $525 2-4 $367.50 5+ $341.25 The Max is more than $200 cheaper than Levco and gives you a path to more memory if you need it. Does anyone have any experience with The Max upgrade? Does anyone see anything wrong with the way they have designed it? Unless I hear about something I missed, I'm taking my Mac to The Max. If anyone else is interested, here's where you can get more information: MacMemory, Inc. 473 Macara Avenue Sunnyvale, CA 94086 (415) 964-4176 David Fay ihnp4!ihexp!dafa -- -------------------- David Fay AT&T Bell Laboratories Naperville, IL 60566 ihnp4!ihexp!dafa
sjl@amdahl.UUCP (Steve Langdon) (10/09/85)
In <501@ihwpt.UUCP> David Fay (ihnp4!ihexp!dafa) writes: > > Until yesterday I intended to upgrade my Mac to 2 Meg with a Levco > kit. I could get a kit for about $450 from a local dealer. The memory > chips would cost about $100 for a total investment of $550. > ... > There is one disadvantage also. Unlike Levco upgrades, The Max > does not have its own ROM. So screen memory sits at the top of 512K. > This means that you can use the extra memory for a 1 Meg RamDisk while > programs have 512K to execute in. While this is not as desireable as > the Levco solution, which relocates screen memory to the top of > physical memory, MacMemory says the new Apple ROMs will do this. > So when they come out, all 1.5 Megs will be continuous. > ... > But here is the clincher. Look at these prices for the kit: > > Quantity Price > 1 $525 > 2-4 $367.50 > 5+ $341.25 > > The Max is more than $200 cheaper than Levco and gives you a path > to more memory if you need it. > ... I have had the Levco 2 Meg upgrade for over two months and I feel that memory > 512K makes a significant improvement to the Mac. I am happy to see alternative sources of this type of upgrade. However, there are a couple of items in the original article that puzzle me. The more important item is the statement that the new Apple ROMs will relocate the screen buffer to the top of the 1.5 Meg address space. As address decode is a hardware, rather than software, function I would want a full and convincing explanation of this claim before I purchased the MacMemory upgrade. The second item that did not seem quite right was the claim of a $200 saving. From the costs quoted this seemed to be true only if you bought more than 1 kit. Unless you own more than one Mac this seems rather impractical. If you have several Macs, you should also check Levco's quantity prices to determine what the real saving is. -- Stephen J. Langdon ...!{ihnp4,cbosgd,hplabs,sun}!amdahl!sjl [ The article above is not an official statement from any organization in the known universe. ]
dafa@ihwpt.UUCP (David Fay) (10/10/85)
> I have had the Levco 2 Meg upgrade for over two months and I feel that > memory > 512K makes a significant improvement to the Mac. I am happy > to see alternative sources of this type of upgrade. However, there are > a couple of items in the original article that puzzle me. > > The more important item is the statement that the new Apple ROMs will > relocate the screen buffer to the top of the 1.5 Meg address space. > As address decode is a hardware, rather than software, function I would > want a full and convincing explanation of this claim before I purchased > the MacMemory upgrade. > > The second item that did not seem quite right was the claim of a $200 > saving. From the costs quoted this seemed to be true only if you bought > more than 1 kit. Unless you own more than one Mac this seems rather > impractical. If you have several Macs, you should also check Levco's > quantity prices to determine what the real saving is. > -- > Stephen J. Langdon ...!{ihnp4,cbosgd,hplabs,sun}!amdahl!sjl It is my understanding that the location of screen memory is determined by a ScreenBase pointer that is placed in low memory. I'm not sure how it gets there, but I would guess that there is a ROM routine that figures out on boot up whether you have a 128K or 512K and then puts the appropriate value in the pointer location. I don't see why the Apple ROM couldn't figure out that you have 1 Meg, 2 Meg or whatever and calculate where screen memory should be. Obviously Levco does something like this with its ROM. The quantity prices apply to a single order. So if you want the lower prices, find someone else that wants to place an order at the same time. -- -------------------- David Fay AT&T Bell Laboratories Naperville, IL 60566 ihnp4!ihexp!dafa
tdn@spice.cs.cmu.edu.ARPA (Thomas Newton) (10/14/85)
> It is my understanding that the location of screen memory is > determined by a ScreenBase pointer that is placed in low memory. > I'm not sure how it gets there, but I would guess that there is a ROM > routine that figures out on boot up whether you have a 128K or 512K and > then puts the appropriate value in the pointer location. I don't see > why the Apple ROM couldn't figure out that you have 1 Meg, 2 Meg > or whatever and calculate where screen memory should be. Obviously Levco > does something like this with its ROM. The ScreenBase pointer does not determine the location of screen memory; instead, the screen locations supported by the hardware determine ScreenBase. It's important that the two correspond. I think that the current Apple ROMs assume that you have either a 128K Mac or a 512K Mac and set ScreenBase for one of these two configurations. In one of the MAUG conferences, Andy Hertzfield said that it was important for people upgrading to more than 512K of memory to get upgrades that placed the screen at the top of memory. Currently, locating screen memory at the top of a >512K Mac requires patching the ROMs to set ScreenBase correctly. The new Apple ROMs should set ScreenBase correctly (= pointing to the top of memory) for Macs with more than 512K of memory. But the new ROMs won't help you (and in fact will hurt you) if you have a >512K upgrade that leaves the screen at the top of the first 512K of memory. I called MassTech about their upgrades, and it sounds like they're doing things right (the extra memory is contiguous with the standard memory, the screen is located at the top of the address space, and the board contains patches to the current ROM to let it know about the extra memory). From the little I've heard, it sounds like Levco is using a similar technique. But I have seen some ads for >512K upgrades that I wouldn't touch with a ten-foot pole. A few questions to ask: (1) Is the extra memory contiguous with the Mac's built-in memory? (the answer to this one should be YES; if the answer is NO, then you'll only be able to use the extra memory as a ramdisk) (2) Is the screen relocated to the top of memory? (the answer to this one should be YES; according to Hertzfield, this is necessary for compatability with the new ROMs) (3) Are the current ROMs patched to know about the extra memory? (the answer to this one should be YES) (4) Does the upgrade require permanent modifications to the Mac? If so, is it possible to buy service on a modified Mac? (One of the places I called modifies the Mac permanently, but will only do service work on their own board. Since regular Apple dealers won't take modified digital boards as trade-ins, you'd be in real trouble if you got one of their upgrades and your Mac's digital board died.) (5) What is done about heat and/or power supply problems? (MassTech installs a fan with all >512K upgrades and installs a modification to the power supply with 2MB upgrades on older Macs; I think that Levco also installs a fan and possibly a power supply mod). -- Thomas Newton Thomas.Newton@spice.cs.cmu.edu
vishniac@wanginst.UUCP (Ephraim Vishniac) (10/15/85)
> > ... However, there are > > a couple of items in the original article that puzzle me. > > > > The more important item is the statement that the new Apple ROMs will > > relocate the screen buffer to the top of the 1.5 Meg address space. > > As address decode is a hardware, rather than software, function I would > > want a full and convincing explanation of this claim before I purchased > > the MacMemory upgrade. > > It is my understanding that the location of screen memory is > determined by a ScreenBase pointer that is placed in low memory. > I'm not sure how it gets there, but I would guess that there is a ROM > routine that figures out on boot up whether you have a 128K or 512K and > then puts the appropriate value in the pointer location. I don't see > why the Apple ROM couldn't figure out that you have 1 Meg, 2 Meg > or whatever and calculate where screen memory should be. Obviously Levco > does something like this with its ROM. I think you're confusing cause and effect. During startup, the ROMs size memory. (The existing roms do it very naively, choosing only between 128K and 512K.) They then *assume* that the hardware has provided for the screen to be a fixed distance below the top of memory. Based on this *assumption*, they set up the ScreenBase pointer. If the hardware doesn't put the screen there, you're up the creek without a paddle... -- Ephraim Vishniac [apollo, bbncca, cadmus, decvax, harvard, linus, masscomp]!wanginst!vishniac vishniac%Wang-Inst@Csnet-Relay
vishniac@wanginst.UUCP (Ephraim Vishniac) (10/16/85)
> In one of the MAUG conferences, Andy Hertzfield said that it was important > for people upgrading to more than 512K of memory to get upgrades that placed > the screen at the top of memory. Currently, locating screen memory at the > top of a >512K Mac requires patching the ROMs to set ScreenBase correctly. > The new Apple ROMs should set ScreenBase correctly (= pointing to the top > of memory) for Macs with more than 512K of memory. But the new ROMs won't > help you (and in fact will hurt you) if you have a >512K upgrade that leaves > the screen at the top of the first 512K of memory. > > I called MassTech about their upgrades, and it sounds like they're doing > things right (the extra memory is contiguous with the standard memory, the > screen is located at the top of the address space, and the board contains > patches to the current ROM to let it know about the extra memory)... > Another point about the MassTech upgrade. The ROM patches are contained in an additional ROM. The board is designed so that with the patch ROM absent, the native ROMs are unaffected. So, when the new Apple ROMs are available, just install the new ROMs, pluck out the patch ROM, and you're in business. -- Ephraim Vishniac [apollo, bbncca, cadmus, decvax, harvard, linus, masscomp]!wanginst!vishniac vishniac%Wang-Inst@Csnet-Relay
dafa@ihwpt.UUCP (David Fay) (10/18/85)
Having spent some time talking to MacMemory and to my hardware friends, I'd like to clarify several issues that were confused in my previous submissions on the MacMemory 1.5 Meg upgrade. First, as several people pointed out, I was most definitely confused about how screen memory location is determined. In fact, screen memory location is determined entirely by hardware address decoding. But in order for programs to know where screen memory is located, a ROM routine figures out where screen memory is and sets a pointer to it in the low memory location ScreenBase. It does this by figuring out how much memory is in the Mac and then assuming that screen memory must be a certain distance below the top of memory either 128K or 512K. Therefore, >512K upgrades must patch the ScreenBase pointer with the correct location if they set -- -------------------- David Fay AT&T Bell Laboratories Naperville, IL 60566 ihnp4!ihexp!dafa
dafa@ihwpt.UUCP (David Fay) (10/18/85)
After talking further with MacMemory and with my hardware friends, I would like to clarify several issues discussed in my previous submissions on the MacMemory 1.5 Megabyte upgrade. As several people pointed out, I was confused about how screen memory location is determined. In fact, screen memory location is determined entirely by hardware address decoding. However, in order that programs can know about the location of screen memory, the ROM puts a pointer to it in the low memory location ScreenBase. This is done by figuring out how much memory the Mac has, and then assuming screen memory starts a certain distance below the top of memory. Currently, the ROM assumes that a Mac can have only 128K or 512K. If you increase memory beyond 512K, the current ROM will set ScreenBase incorrectly, so you must patch it to point to the actual location of the screen memory. This is done by the ROMs that Levco and others provide with their upgrades. The new Apple ROMs will not assume that memory can only be 128K or 512K. Instead, they will figure out how much memory is actually installed and then set ScreenBase appropriately. Megabyte upgrades that move screen memory to the top of physical memory have one disadvantage: they are more expensive. Extra circuitry must be provided for ROMs to override the Apple ROMs. Furthermore, when the Apple ROMs come out in a few months, these non-Apple ROMs will become vestigial. MacMemory has taken a different approach. They have built in the address decoding for locating screen memory to the top of 1.5 Megabytes (AND to the top of 4 Meg with 1 Megabit chips) but have not enabled the relocation in order to avoid the expense of the extra ROMs. When the new ROMs come out, all you have to do is to cut a trace on the board. Screen memory will then be at the top of 1.5 Meg and the Apple ROMs will set ScreenBase correctly. According to the designer of The Max board at MacMemory, who also happens to be Vice-President of Sales and Marketing there, they have already tried out the new Apple ROM with their board and, once the screen memory is relocated, it works fine. Because MacMemory saves the expense of new ROMs and because they have designed the board so as not to require a fan, they can offer significantly cheaper prices. Unfortunately, many of their employees are confused about what the prices actually are. On five separate phone calls, I and friends were given incorrect information about prices, some of which I reported in earlier articles. The correct prices for the KIT, according to the Vice-President, are as follows: List price: $525 Club price (quantity order, I believe): List minus 20% (~$420) Dealer prices: (quantity 2-4) $367 (quantity 5+) $324 So it looks like you can get one for about $420, which is the lowest 1.5 Megabyte upgrade available. The additional advantage, which I mentioned in an earlier article, is that you can upgrade to 4 Megabytes when megabit chips come out by making a further small modification to the board. This is a significant advantage in my view. One further unannounced advantage of MacMemory is that they will provide a free print spooler to all purchasers of the upgrade sometime next month. Finally, they offer a warranty on their board and (for additional money) a service contract on the whole Mac. They are negotiating with Apple right now for a deal like GCC worked out. If they can swing it, a MegaMac upgrade will not void your AppleCare and you will be able to get your Mac (except the daughter board I assume) serviced at an Apple dealer. Now the disadvantages: Until the Apple ROMs come out, you will be limited to 1 Megabyte of continuous memory and 512K of RamDisk (or vice versa). The upgrade is designed to be used with SOLDERED mother boards. So if you have upgraded to 512K yourself and used sockets, as I have, it's not clear The Max will work. We just received a board in the mail and will be reporting in the next few weeks whether the board has enough clearance to fit above a socketed motherboard. MacMemory also has some concerns about the extra inductance(?) added by the sockets. Finally, the 256K chips on the board are soldered, not socketed. This means you will have to desolder them to replace them with 1 Megabit chips. This is exactly analogous to a homebrew upgrade from 128K to 512K and should be no problem for someone handy with a soldering iron. -- -------------------- David Fay AT&T Bell Laboratories Naperville, IL 60566 ihnp4!ihexp!dafa
vishniac@wanginst.UUCP (Ephraim Vishniac) (10/23/85)
> After talking further with MacMemory and with my hardware friends, > I would like to clarify several issues discussed in my > previous submissions on the MacMemory 1.5 Megabyte upgrade. ... > The additional advantage, which I mentioned in an earlier article, > is that you can upgrade to 4 Megabytes when megabit chips come out > by making a further small modification to the board. This is a > significant advantage in my view. > > Finally, they offer a warranty on their board and (for additional > money) a service contract on the whole Mac. They are negotiating > with Apple right now for a deal like GCC worked out. If they can > swing it, a MegaMac upgrade will not void your AppleCare and you will > be able to get your Mac (except the daughter board I assume) > serviced at an Apple dealer. Can they swing it? GCC got the deal they have because the new Hyperdrive is a clip-on. The Apple board is left untouched. I don't believe Apple will approve any upgrade that involves modifying the Mac digital board. > Now the disadvantages: ... > Finally, the 256K chips on the board are soldered, not socketed. > This means you will have to desolder them to replace them with 1 > Megabit chips. This is exactly analogous to a homebrew upgrade from > 128K to 512K and should be no problem for someone handy with a > soldering iron. But again, I can't believe that Apple (or anyone else) will support this after the user has plugged in his soldering iron. The moral: don't depend on what the maker says he's working on. Wait until it's "on the shelf." -- Ephraim Vishniac [apollo, bbncca, cadmus, decvax, harvard, linus, masscomp]!wanginst!vishniac vishniac%Wang-Inst@Csnet-Relay
espen@well.UUCP (Peter Espen) (10/28/85)
In article <523@ihwpt.UUCP>, dafa@ihwpt.UUCP (David Fay) writes: > After talking further with MacMemory and with my hardware friends, > I would like to clarify several issues discussed in my > previous submissions on the MacMemory 1.5 Megabyte upgrade. > > As several people pointed out, I was confused about how screen memory > location is determined. In fact, screen memory location is determined > entirely by hardware address decoding. However, in order that programs > can know about the location of screen memory, the ROM puts a pointer to > it in the low memory location ScreenBase. This is done by figuring out how > much memory the Mac has, and then assuming screen memory starts a > certain distance below the top of memory. Currently, the ROM assumes > that a Mac can have only 128K or 512K. If you increase memory beyond > 512K, the current ROM will set ScreenBase incorrectly, so you must > patch it to point to the actual location of the screen memory. This is > done by the ROMs that Levco and others provide with their upgrades. > If the screen memory location is determined entirely by hardware address decoding, then how can the Apple ROM alter the screen memory location to point to the top of RAM for a 512K Mac? I did the Dr. Dobbs 128 to 512K upgrade on my Mac and I didn't alter any of the hardware address decoding for screen memory at all. The only thing that the upgrade did was add the 256K memory chips and add the address decoding for address lines A17 and A18 to select Memory address line MA8 when needed. My screen memory never-the-less is right up there at 7a700 as it should be. I always assumed that was because the ROM was detecting that I had more than 128K of memory and so was writing the ScreenBase pointer as 7a700. I still don't understand why that ScreenBase pointer can't be made to point to whatever the top of memory is. Perhaps someone out there can clear this up for me. Thanks! Peter Espen
north@apple.UUCP (Donald N. North) (10/29/85)
In article <240@well.UUCP> espen@well.UUCP (Peter Espen) writes: >In article <523@ihwpt.UUCP>, dafa@ihwpt.UUCP (David Fay) writes: >> After talking further with MacMemory and with my hardware friends, >> I would like to clarify several issues discussed in my >> previous submissions on the MacMemory 1.5 Megabyte upgrade. >> >> As several people pointed out, I was confused about how screen memory >> location is determined. In fact, screen memory location is determined >> entirely by hardware address decoding. However, in order that programs >> can know about the location of screen memory, the ROM puts a pointer to >> it in the low memory location ScreenBase. This is done by figuring out how >> much memory the Mac has, and then assuming screen memory starts a >> certain distance below the top of memory. Currently, the ROM assumes >> that a Mac can have only 128K or 512K. If you increase memory beyond >> 512K, the current ROM will set ScreenBase incorrectly, so you must >> patch it to point to the actual location of the screen memory. This is >> done by the ROMs that Levco and others provide with their upgrades. >> > > If the screen memory location is determined entirely by hardware >address decoding, then how can the Apple ROM alter the screen memory >location to point to the top of RAM for a 512K Mac? I did the Dr. Dobbs >128 to 512K upgrade on my Mac and I didn't alter any of the hardware >address decoding for screen memory at all. The only thing that the >upgrade did was add the 256K memory chips and add the address decoding >for address lines A17 and A18 to select Memory address line MA8 when > needed. My screen memory never-the-less is right up there at 7a700 >as it should be. I always assumed that was because the ROM was detecting >that I had more than 128K of memory and so was writing the ScreenBase >pointer as 7a700. I still don't understand why that ScreenBase pointer >can't be made to point to whatever the top of memory is. Perhaps >someone out there can clear this up for me. Thanks! > Peter Espen > > In the 128K/512K MAC, the RAM address decode logic only looks at the upper two bits of the address; A<23:22>='00' selects RAM. The display/sound buffers only need the low 16 word-address lines A<16:01> multiplexed (because they occupy less than 64K total). Hardware pullups keep intermediate address bits high during display refresh access memory cycles, thus the hardware location of the display will 'float' automatically to the top of any RAM memory in the low 4 MByte chunk of address space. Since memory 'wraps' from only decoding A<23:22>, a screen address base of $7A700 will handle physical memory sizes of 128K (screen at $3A700) or 512K (screen at $7A700). If one were to set the ROM-generated screenbase at $3FA700, RAM sizes of 128K - 4096K (the limit imposed by hardware) could be handled transparently to the software.
tk@cvl.UUCP (Tharakesh Siddalingaiah) (10/29/85)
In article <240@well.UUCP> espen@well.UUCP (Peter Espen) writes: >In article <523@ihwpt.UUCP>, dafa@ihwpt.UUCP (David Fay) writes: >> After talking further with MacMemory and with my hardware friends, >> I would like to clarify several issues discussed in my >> previous submissions on the MacMemory 1.5 Megabyte upgrade. >> >> As several people pointed out, I was confused about how screen memory >> location is determined. In fact, screen memory location is determined >> entirely by hardware address decoding. However, in order that programs >> can know about the location of screen memory, the ROM puts a pointer to >> it in the low memory location ScreenBase. This is done by figuring out how >> much memory the Mac has, and then assuming screen memory starts a >> certain distance below the top of memory. Currently, the ROM assumes >> that a Mac can have only 128K or 512K. If you increase memory beyond >> 512K, the current ROM will set ScreenBase incorrectly, so you must >> patch it to point to the actual location of the screen memory. This is >> done by the ROMs that Levco and others provide with their upgrades. >> > > If the screen memory location is determined entirely by hardware >address decoding, then how can the Apple ROM alter the screen memory >location to point to the top of RAM for a 512K Mac? I did the Dr. Dobbs >128 to 512K upgrade on my Mac and I didn't alter any of the hardware >address decoding for screen memory at all. The only thing that the >upgrade did was add the 256K memory chips and add the address decoding >for address lines A17 and A18 to select Memory address line MA8 when > needed. My screen memory never-the-less is right up there at 7a700 >as it should be. I always assumed that was because the ROM was detecting >that I had more than 128K of memory and so was writing the ScreenBase >pointer as 7a700. I still don't understand why that ScreenBase pointer >can't be made to point to whatever the top of memory is. Perhaps >someone out there can clear this up for me. Thanks! > Peter Espen > > The hardware was always look for screen memory starting at location 7A700 but since you orignaly had 128k of memory the 2 MSB wern't being decoded so it seemed that the screen was at 1A700. I belive the ROM did a similar sort of thing i.e. put the address 7A700 in the ScreenBase pointer and the decodeing logic took care of the rest. -tk -- Tharakesh Siddalingaiah --- University of Maryland Computer Vision Lab, (301) 454-4526 ARPA:tk@cvl CSNET:tk@cvl UUCP:{seismo,allegra,brl-bmd}!umcp-cs!cvl!tk
dafa@ihwpt.UUCP (David Fay) (10/29/85)
Thank you Donald North for clarifying how screen memory location is determined. One small correction - screen memory on the 128K is at $1A700 not $3A700. We have now installed our first MacMemory 1.5 Meg upgrade on top of an Apple 512K board. It was not a difficult job, requiring about the same level of skill as a 512K upgrade. Three ICs must be de-soldered on the mother board along with some other minor modifications. Less work is required on a 128K board which has been upgraded to 512K. The MacMemory board appears to be of high quality - at least as good as the Apple board itself. We can not detect any increase in heat coming out of the vents on top. The materials that come with the kit state that the upgrade is to be added to stock Apple boards with soldered (not socketed) RAM. Apparently this is because MacMemory believes socketed memory is not as reliable as soldered. Throwing all caution to the wind, our next upgrade will be on a homebrew 512K upgrade with low profile sockets. We have already determined that there is plenty of clearance for the daughter board even with the socketed memory. -- -------------------- David Fay AT&T Bell Laboratories Naperville, IL 60566 ihnp4!ihexp!dafa
vishniac@wanginst.UUCP (Ephraim Vishniac) (10/29/85)
> If the screen memory location is determined entirely by hardware > address decoding, then how can the Apple ROM alter the screen memory > location to point to the top of RAM for a 512K Mac? I did the Dr. Dobbs > 128 to 512K upgrade on my Mac and I didn't alter any of the hardware > address decoding for screen memory at all. The only thing that the > upgrade did was add the 256K memory chips and add the address decoding > for address lines A17 and A18 to select Memory address line MA8 when > needed. My screen memory never-the-less is right up there at 7a700 > as it should be. I always assumed that was because the ROM was detecting > that I had more than 128K of memory and so was writing the ScreenBase > pointer as 7a700. I still don't understand why that ScreenBase pointer > can't be made to point to whatever the top of memory is. Perhaps > someone out there can clear this up for me. Thanks! > Peter Espen I believe the answer is that the video hardware, even on 128K Macs, addresses the screen at the "Fat Mac" location (7A700). Wraparound does the rest. -- Ephraim Vishniac [apollo, bbncca, cadmus, decvax, harvard, linus, masscomp]!wanginst!vishniac vishniac%Wang-Inst@Csnet-Relay