# Accessing SD cards SD cards support the SPI protocol. If you have a SPI relay (doc/hw/spi.txt) and a driver for it that implement the SPI protocol (doc/protocol.txt), 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 "423 436 LOADR". Initialization of the SDC system is done in multiple steps. First, the BLK system needs to be initialized with "BLK$". Then you can plug SDC@ and SDC! into BLK with "' SDC@ BLK@* !" and "' SDC! BLK! !". That only needs to be done once per boot. Then, the SD card that was inserted needs to be initialized. You can do it with "SDC$". 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.