The Flexible Static Memory Controller (FSMC) is a peripheral which an be configured to control a variety of external memory chips.
Normally, any variables in your program will be allocated space in RAM (notable exceptions are variables marked with const or __FLASH__). Without the FSMC, this space is limited to the amount that comes built-in to the chip’s internal SRAM.
The addition of the FSMC peripheral allows addtional memory to be used. For example, it is used on the Maple Native to interface with the board’s built-in external SRAM chip. However, this extra memory comes at a cost: the FSMC uses a fairly large number of GPIOs.
The FSMC peripheral is currently only available on the Maple Native. On that board, we have broken out a wide variety of the FSMC lines, so that experienced users can add additional external memory chips to suit their own applications’ purposes.