Carte Blanche MMC/SD "disk" update

[Alex Freed <alex_news@mirrow.com>]

I did some more work on the subj. A while ago Stephen A. Edwards of 
Columbia University saw my first Apple on FPGA and wrote his own 
version. One feature was a fully hardware implemented (read only) 
"floppy" interface using an MMC card. A complex state machine 
initialized an MMC card that contained a NIB image of a floppy and read 
a track into a buffer. The rest was the same as in my FPGApple - the 
hardware tracked the "head movement" and fed a sequence of nibbles to 
the CPU. Very cute but not very practical IMHO - only a single NIB image 
on a huge MMC/SD card.

This inspired one improvement I made to the CB disk interface. I have 
added a state machine that initializes the memory card at reset and 
reads a 512K block from MMC into the internal FPGA memory.

The interface works like this. Assuming the card in slot 4.

Write to $C0CC - reset MMC.
Write to $C0C4, C0C5 and C0C6 - low, med and high bytes of a block 
address on the MMC card.
Write to $C0C0 starts a read block operation: 512 bytes read into the 
internal memory that is mapped (after reading from C4xx) to $C800.

So the whole block is directly available in the 6502 memory space. 
Probably makes it the fastest disk access in Apple 2 history :)

There are also addresses for lower level SPI access: set/clear the chip 
select and write/read a byte via SPI which allows any SPI operation by 
an Apple program. For example write multiple blocks.

I may add DMA - read directly into the Apple's main memory if I have 
time but it's a lower priority task.

If anyone want to write a UI and needs more hardware features now is a 
good time to suggest it so it can be included into the default 
configuration.

-Alex.