# 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.