[mogui]

SD cards support a serial interface mode (SPI) that is compatible with MMC cards, though applications can use the faster 4-bit parallel SD mode. If the demands of an application permit, it is logical to use the SPI mode which, though slower, gives MMC card compatibility with only one software stack rather than the two stacks required to support both SD and SPI modes.

Additionally the SD standard is not open to open-source developers, so Linux based devices are not likely to support SD mode. Sony might have its own proprietary drivers under license however.

Compatibility issues exist when older cards are used that adhered to a previous set of standards. Some commands have changed. Some micro-SD cards do not use the SPI interface, so if our Reader uses only SPI (likely) these would not work. SDHC cards do not work. They are a newer standard and a different interface that does not provide backward compatibility to SD/MMC interfaces.

SDHC cards can be addressed up to 32G. It does not appear that the Reader supports SDHC functionality.

The interface to an SD card is via a 4-bit data bus that is usually used in (SPI) one-bit serial mode. Obviously then, the physical addressing of memory is accomplished via registers internal to the SD card, which registers are accessed via a system of software commands. The standard does not permit variations in the physical interfaces of various SD cards, though the physical interface of the MMC card has fewer pins. Wikipedia states that the theoretical memory addressing capacity of an SD card is 128G using 28-bit sector addressing. They don't say whether it is 128G bits or bytes. If bits, that would confer a 16G byte limitation.

The SD Group Technical Committee specifies that SD cards can have a capacity of up to and including 2G. It seems that those producing SD cards in excess of 2G are doing so in violation of the specs. One manufacturers's site projects SD cards of up to 16G capacity.

It is probable then that the limitations to the SD/MMC addressing capability of the Reader are firmware limitations. I found a statement that the SD cards have a greater growth capacity in terms of memory size than the MMC cards. It is possible that the current ceiling of 2G on the Reader's SD capacity reflects an industry-wide tendency to adhere to the capacity limits of the MMC standard for the sake of being compatible with both formats simultaneously using the same firmware. It is also likely that the SD/MMC host controllers available on the market present addressing limitations.