Re: Zip GS and 14MB RAM??

[rubywand@swbell.net]

In article <33148D94.3BEA@auge.de>,
Martin Kaeser  <martin.kaeser@auge.de> wrote:
>what would a Zip GS or TWGS do with 14MB of RAM in a IIgs?
>a) crash
>b) cache the first 8 MB
>c) cache all 14 MB


     An accelerator card will execute commands, load/store data, etc. 
from whatever system RAM the FPI Controller will permit it to access. A 
good guide to limits upon Expansion RAM is the IIgs Hardware Reference 
Manual. It states that 4MB is the maximum RAM which is fully supported 
for the Expansion Slot.

     The description of FPI functioning and GS schematic indicate that an 
expansion of main memory beyond an addition of 4MB saddles the designer 
with numerous hardware and firmware problems. For a system, with no mods 
to the motherboard, to function well with more than 4MB of Expansion RAM, 
the expansion card, accelerator, hard disk interface, and system software 
would need to be in agreement on how the extra RAM is to be supported and 
accessed.

     There have been claims that some 8MB main memory expansion cards 
(which employ a kludge to obtain an additional address line) work, kind 
of work, sometimes work, etc.. Based upon reports, the extra memory 
"works"; but, there are special limitations on using it due to lack of 
support by some peripherals and system software. A major drawback of the 
8MB expansion is that, for full support, quite a number of changes are 
required; yet, it is a dead end. The user gets another 4MB and that's it.


     For a IIgs accelerator card to have reliable access to 14MB, one 
approach is a mod to the motherboard which allows valid address lines to 
reach the Expansion Slot along with active R/W, Data, and refresh for the 
extra memory. Mainly, you would need to replace the FPI Controller or add 
hardware to undo the restrictions it imposes-- not exactly a beginner's 
hardware project.

     A less dramatic solution is bank-switching. It is too bad that, 
early on, developers of IIgs firmware, software, and add-on hardware did 
not opt to support 'bank switching' of 4MB blocks much as the IIe bank 
switches 64k blocks. A single 16-bit register IC accessed via a soft 
switch would permit access to as much as 262MB of extra memory. 
Everything could go on the mem expansion card. There would be no need to 
modify the motherboard.

     Implementing a hardware bank-switching scheme today would involve a 
number of changes. Even so, it may be the best way to, once and for all, 
crack the IIgs main memory bottleneck.