Rebuilding SD card access

In my enthusiasm reworking the SD card and F011 code, it turns out I got a bit overzealous, and stripped out the code that provides the memory-mapped sector buffer for SD card access. Clearly not optimal. So I had better fix that up.

Before the rework, there were three separate sector buffers:
1. One for the CPU to read as a memory mapped sector buffer;
2. One for the CPU to write to, and the SD controller to read from; and
3. One for the F011 emulation.

At the moment, only the third one is still there.

This means that SD card access is currently only working for mounted disk images -- but there is no way to mount a disk image, as the Hypervisor can't even read the partition table of the SD card.

We could go back to having three buffers, but that seems a waste of precious BRAMs.  The question is whether we can do everything we need with just the one buffer, or whether we need two.  The answer to that is that we do need a second buffer if we want to have memory mapped access to the sector buffer.  That second buffer would be connected to the MEGA65's FastIO bus for reading, so that the CPU can read from it when it wants to. This buffer would be written to when the SD card controller reads data from the SD card.  To allow the CPU to write to the buffer, we would trap writes to the sector buffer location, and pass them to the SD controller to actually perform the write operation.

The complication is that when the SD card controller is asked to write a sector, it has no way to read this buffer, as only the CPU can cause it to be read.  We can solve this by making the F011 sector buffer 1K instead of 512 bytes (since BRAMs are 4K, this doesn't cost us anything extra), and whenever the CPU writes to the SD card sector buffer, we also write to the second 512 bytes of the F011 buffer.  When the CPU is asked to write a sector to the SD card, it uses this second copy of the data, which it does have read access to, in order to perform the write.

Another wrinkle is virtualised F011 mode, where the hypervisor gets a trap whenever a sector read or write is attempted.  This is used with monitor_load to allow feeding a disk image via the serial monitor interface, instead of using the SD card (handy for some software development tasks, and if your SD card interface is broken, as it is on Falk's r1 PCB).  So I need to preserve that.

Probably the best solution here is to have the two buffers, each with 2 x 512 bytes, with the lower half the F011 buffer, and the upper half the SD card buffer, and have a register bit that allows selecting which one is being accessed.

After a bit of fiddling about, this is all done now and working nicely, and the saved BRAM is also a nice result.

Switching back and forth between the SD card and floppy drive works, but it seems that it is possible for the track numbers to get out of sync, so it is necessary to seek to track 0 after switching from SD card, to make sure that everything matches up.   Ideally we would have some program to allow switching back and forth in a really easy way. Initially modifying the disk chooser menu program is probably the right way to do this, so that there is an option to select "internal drive" as one of the disk choices.

After that, the next step on the floppy drive now is to get writing working, including ideally formatting disks which requires unbuffered writing.