Most of commands return a response message which has to be clocked out from a card by sending a 0xFF. See details in specifications. What actions must be taken to start working with MMC:

• When card is powered up, send a Reset Card command;

• Then send Initialise Card command which takes some time;

• After Initialization is complete you must retrieve card identification (16 byte). Identification contains manufacturer info, card type, amount of memory in the card.

• Then read write the card information.

Read and write commands uses 32 bit address parameter that allows addressing up to 4GB of memory. Reading can be done from any address and you can read bytes by writing 0XFF bytes to latch out further data. Writing takes more efforts, because memory in the cards is organized in 512 byte groups called sectors. So when writing sectors, the card erases it firstly and then writes data. This is tricky if you want to update a single byte. Usually you would need additional 512 bytes of ram to read whole sector, update ir and then write. To make things easier there is a command which allows to set read and write buffer block size. If block size is set to 512, then read/write command start from 0 or 512, or 1024 …

There are more commands like protection, multi-block write that are documented in specifications.