SD Card Command Requests
Drivers use Secure Digital (SD) card command requests to send commands to an SD device. The protocol for SD commands is defined in the Secure Digital Card specification. Drivers can send command requests at any time after the IRP_MN_START_DEVICE IRP that starts the device completes successfully.
This section contains two code examples: a command request that reads a byte of data from a register of an SD card using direct I/O, and a device command request that writes a larger quantity of data to an SD card using extended I/O. The explanation in the second section depends on the first, therefore, readers should study the first section before studying the second: