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Are there any areas of the 64k RAM which are permanently unusable by anything, whether by the built-in monitor ROM or user programs?
Looking at $C000-C0FF, this is the "softswitch" area, and as far as I can determine, the 256 bytes of RAM that occupies this space may be completely unusable by anything. Apparently no RAM is required here as these are mapped to I/O control lines for system devices.
Are there any special bank switch tricks to gain access to the RAM that occupies $C000-C0FF ?
Are there any areas of the 64k RAM which are permanently unusable by anything, whether by the built-in monitor ROM or user programs?
No. All is usable. After all, Woz did it. He never wasted a single gate, even less RAM.
Looking at $C000-C0FF, this is the "softswitch" area, and as far as I can determine, the 256 bytes of RAM that occupies this space may be completely unusable by anything. Apparently no RAM is required here as these are mapped to I/O control lines for system devices.
There is not RAM in the $C000..CFFF area and never has been.
The original Apple II had 'only' up to 48 KiB. Later the Language Card added 16 Kib in form of 12 KiB in parallel to the ROM at $D000...FFFF(using /INH to aquire the address space). Since using eight 16 Ki, 4116 type chips, was cheaper than 12 4 Ki ICs,the card ended up with 16 KiB. To not waste the additional 4 KiB (quite a lot back then), a logic was added to switch in the remaining 4 KiB again in parallel as Language Card Bank 2 at $D000..DFFF.
While using 64 Ki RAMs, the Apple //e addressing logic (MMU) uses the same logical scheme.
Are there any special bank switch tricks to gain access to the RAM that occupies $C000-C0FF ?
No, as there is never RAM at that address.
$C08B flips in bank 1.$C083 flips in bank 2.In either case, $E000..FFFF is always bank 1.
$C082 disables the Language Card again.There is also another bunch of combinations for reading ROM while writing the Language Cards RAM and so on, but I guess you get the picture.
Here some viualisation I used for a speech about RAM design.
Yes, all 64kb of RAM is accessible. But you ask an interesting question, if we change one letter. ; - )
Address space and device mapping
You asked about RAM, but how is RAM read? The CPU puts a 16-bit address on the address bus, puts the R/W' line in the desired state, and then expects a value on the 8-bit data bus within a certain time. What sends the data that comes back? Something else connected to these shared buses and configured to respond to the given address. It might be onboard RAM, ROM, a peripheral card, or something else. (I will document a few anomalies below.)
The point being that because the buses are shared, the 16-bit address space is shared, and parts claimed by various devices according to physical (cards etc) and logical (soft-switch) configuration.
RAM
On the original Apple II or II+ there is up to 48kb on the motherboard. The extra 16kb (or more) of the "RAM device" decoding is split between the motherboard and a RAM (e.g. Language) Card.
A 64kb Apple //e is very similar to the II+ with LC, except that the entire 64kb is onboard. In fact in the //e, $DXXX accesses to LC Bank 1 are converted to "physical RAM device" addresses in $CXXX. (For details see Sather's Understanding the Apple //e, page 5-24, third paragraph.) If a //e has 128kb or more then the Auxiliary Slot again divides the RAM "device" decoding.
In most Apple IIs $CXXX only accesses peripheral card RAM (or card ROM etc) and onboard ROM, but the //c+ has has 2k of "secret" RAM accessed through a $20 byte window at $CE00.
ROM
If you replace the RAM in your question with ROM (one letter, as I said) the question becomes more interesting, and you have in fact suggested a good place to look: $C0XX. This address space is always claimed by the softswitch decoding logic, and yet both the Apple //e and //c have ROMs called CD, EF or CF that are large enough to fully occupy $C000 to $FFFF. And in fact they do. You cannot read the $C0XX ROM area from inside the Apple, but if you dump any //c ROM (not //c+) and look at the beginning where $C000 to $C0FF would be, you find this:
Peter Quinn, Rick Rice, Joe Ennis, J MacDougall, Ken Victor, E Beernink, JR Huston, RC Williams, S DesJardin, Randy Bleske, Rob Gemmell, Stan Robbins, Donna Keyes, Doug Farrar, Rich Jordan, Jerry Devlin, John Medica, B Etheredge, Dave Downey, Conrad Rogers
Hidden in this inaccessible $C0XX ROM are the names of the creators of the //c.
Something else
What if no device claims an address read? The Apple II was architected such that the CPU accesses the buses during one of its internal phases, and the video scanner uses the other phase to read video data from RAM to generate video. Bob Bishop discovered in 1982 that reads at unclaimed addresses return the most recently scanned video data. Examples are softswitches that do not return data, and vacant slot memory in $CXXX. A few games used this "floating bus" data to sync their graphics page-flipping with monitor refresh to avoid flicker. The technique is sometimes referred to as "vapor lock".
