Solidisk Twomeg and Fourmeg RAM/ROM Expansions

SHARING RAM BETWEEN SHADOW AND SIDEWAYS SYSTEMS
You cannot use the same areas of RAM twice over, but the 256k board can
be shared between the Shadow and Sideways system. The RAM on the
board is physically arranged as eight banks of 32k, each mapped to occupy
a VARIABLE but CONTINUOUS block somewhere between &800 and
&FFFF in the memory map. Each bank is normally used solely as Shadow
RAM or Sideways RAM but not without some compromise; the total
requirement must not exceed 32k.
1. If used as Shadow RAM, the block will normally extend from &3000 to
&B000. In this case the top 12k (between &8000 and &AFFF) is free and
can be used as Sideways RAM.
2. If your program needs a very large Shadow area you can position your
block lower, from &800 to &8800. In this case, the bottom 30k is
available as Shadow RAM and the top 2k can be used as Sideways RAM.
To put the block in this position:
?&FE36=&40 .
3. If your program can be restricted to screen mode 3, the block can be
positioned higher, from &4000 to &C000. In this case the top 16k (
between &8000 and &C000) is available to hold a full 16k ROM image.
To put the block in this position you type in ?&FE36=&20 .
This may be done either from the keyboard or from a program.
4. The block can also be positioned from &8000 to &FFFF. In this case no
Shadow RAM is available, but the Sideways RAM is 32k. To put the block
in this position, &10 should be poked into &FE36—but you must not type
it in from the keyboard or the computer will crash. It must be done in
machine code, as in the Manager ROM. Solidisk will supply the "PATCH"
program to software authors wishing to develop ROMs to occupy this
position.
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