MultiMediaCard Protocol - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 94

The information that travels on the bus includes commands, command
responses, and tokens that contain data and error indications.
Commands
Each MultiMediaCard command is 48 bits. Table 5-1 shows the format of
the commands.
The command index is a number that identifies the command. Each com-
mand also has a name consisting of “CMD” followed by the index value
(CMD0, CMD1, and so on) and a descriptive abbreviation. For example,
the descriptive abbreviation for CMD17 is READ_SINGLE_BLOCK.
The command argument is a 32-bit value that can provide supplementary
information required to carry out a command. For example, the argument
for READ_SINGLE_BLOCK is the beginning address of the block to read.
For commands that don't require arguments, the host sends stuff bits of
zero.
The CRC value is seven bits that the MultiMediaCard controller can use to
verify that a command arrived without error.
The transmission bit is set to 1. The start bit and end bit mark the begin-
ning and end of the command.
Response Types
On receiving a command and argument, a MultiMediaCard returns a
response. The MultiMediaCard specification defines six response types with
each command having a designated response type. Most commands use
response type R1 or R1b. A few use R2 or R3. Only I/O cards use types R4
and R5.
These are the response types a mass-storage device uses on an SPI bus.
R1
Response type R1 is a single byte. A value of 00h means that the command
completed without error. On error, one or more of the bits are set:
Bit 0: The card is in the idle state and is initializing.
Bit 1: An erase sequence was cleared before executing the command
because an out-of-erase-sequence command was received.