A600 8Mb fast expansion

[Matt_H]

@ lordv

From NDK3.1:Examples1/PCMCIA/PCMCIA.General

Because the A600/A1200 PCMCIA implementation is entirely memory mapped, the
maximum common memory address space is limited to four (4) megabytes.  The
upper half of our eight ( megabytes of 24-bit expansion space ($600000
through $9FFFFF) has been used for this purpose.  Another 128K of address
space ($A00000 through $A1FFFF) is used for attribute memory, and the next
128K bytes ($A20000 through $A3FFFF) is used for I/O address space.

This means that our 24-bit CPU systems can use common memory for Execute-In-
Place purposes (SYSTEM RAM expansion, and Execute-In-Place code).  The down
side is that our current implementation does not yet provide a paging register
so that addresses greater than 4MB cannot be generated.  While our system
software cannot currently support XIP and bank switching well, the extended
address space will be useful if we want to support large FLASH-ROM cards.  A
paging register is being considered for this purpose in the future.

The PCMCIA slot does have some limitations.  It is a 16 bit interface, and
most SRAM/PSRAM cards are 200-250ns requiring GAYLE provide wait-state
generation.  A specification for PCMCIA DMA was not finalized for the Release
2.0 specification.  We realize that some of you will prefer to use the common
memory space for 32 bit RAM expansion in the A1200, so a credit-card disable
register is provided in GAYLE.  Software modifications have been added to the
latest V39 release which automatically disables the PCMCIA interface when RAM
is present in card memory space (this does however mean that you lose use of
your PCMCIA slot).