# Accessing SD cards

SD cards support the SPI protocol. If you have a SPI relay and
a driver for it that implement the SPI protocol (doc/spi),
you're a few steps away from accessing SD cards!

What you need to do is to add the SDC subsystem to your Collapse
OS binary. First, define SDC_DEVID to a mask selecting the
proper device on your SPI relay (this is what is sent to
"(spie)"). For example, a SDC_DEVID or 1, 2, 4, or 8 would
select SPI device 1, 2, 3 or 4.

The subsystem is loaded with "250 258 LOADR".

Once that subsystem is loaded, you need to create aliases that
will plug into the BLK subsystem (doc/blk). Add this to your
xcomp:

ALIAS SDC@ (blk@)
ALIAS SDC! (blk!)

You can now load BLKSUB and end the rest of your xcomp normally.

At runtime, the SD card that was inserted needs to be initial-
ized. You can do it with SDC$. If you have no error, it means
that the system can spek to your card, that sync is fine, etc.
You can read/write right now. SDC$ needs to run every time a new
card is inserted.

Collapse OS' SDC drivers are designed to read from the very
first 512 sector of the card, mapping them to blocks
sequentially, 2 sectors per block.