This reduces the potential to flicker, but creates two separate
memory areas in the high memory.
For applications which require a large continuous memory space in
high memory, atarixl-largehimem.cfg is provided. With high IRQ
activity or DL interrupt there might be flicker.
- First chunk checks system (if it's XL and memory is ok).
- Second chunk contains the contents for the shadow memory beneath the
ROM and copies the contents to their destination address.
It also moves the screen memory to low memory in front of the program.
The target 'atarixl' is to be used for Atari XL (and better) machines.
It will disable the OS ROM and enable the Shadow RAM available on
those machine.
Note: This commit is only the inital step towards for this goal that just
replicates the target 'atari' as a starting point!
