Posted By
gerliczer
on 2014-10-16
13:31:22
|  Re: Basic program address with or without Graphic allocation
In a standard (unmodified) 264 series machine there are two types of memory paging.
One type is the ROM paging that JamesC already mentioned. The machine is capable to map two built-in and two external 16k ROM banks to the $8000-$BFFF memory address space, and another two built-in and two external 16k ROM banks to the $C000-$FFFF memory area (this area also covers the "chipset" I/O address space, so there is a hole in it at $FD00-$FF3F). This works via the registers mentioned earlier.
The other paging type is the ROM-RAM paging, which switches between ROM and RAM access in the $8000-$FFFF memory space (of course excluding the I/O area). This works via TED registers $FF3E (page in ROM) and $FF3F (page in RAM). The act of writing into the register triggers the switching, therefore the written value is indifferent.
There are unofficial but (maybe) plus/4 scene-standard memory expansion solutions, but unfortunately I don't really know them. These expansions are not widely spread, so maybe it is fine to ignore them. Oh, and there are other also unofficial expansions that extend the usable external ROM banks, too.
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