MMCSD HC.LIB
From MCS Wiki AVR
The MMCSD_HC.LIB is an MMC SD-HC card driver library.
See the AVR-DOS topic for an example.
There is an optional constant you can set in your code :
CONST _CS_EXTENDED_PORT=1
You need to set this constant when using a normal AVR chip with the CS pin connected to an extended port. We recommend to use a normal port which allows the CBI/SBI instructions but some times it is required to use an extended port like PORTF on an MEGA2560. Since the extended port needs a register to read-alter-write a bit, the register R23 need to be saved in the lib. When you define the constant and give it a value of 1, the register is preserved.
You can always set this directive, it will only create unneeded code when using normal ports.
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