flash memory integration with Nios II in qsys

I'll try to answer to some of your questions,

Parallel flash devices expose address and data ports and you access data directly by writing the target address and wr/rd the data word

Conversely, serial flash are accessed through a serial port interface (usually SPI) with a specific protocol: this allows a minimal pin count (independent from address range and data width) at the expense of a slower access speed and a bulkier sw driver.

CFI stands for Common Flash Interface: this is a standardized specification and it is widely used for parallel flash.

CF stands for Compact Flash and is used for a class of removable memory cards; the interface is parallel; they must not be confused with SD cards which rely on a serial interface.

EPCS is a particular serial flash memory supplied by Altera; its way of operation is very similar to other serial flash devices, like ST M25Pxx series.

Tristate bus allows sharing the bus among more devices: they are connected in parallel and only one of them at a time can drive data, the others being tristated, i.e. not driving the signal lines. The Altera tristate bridge provides a connection between the internal Avalon bus and a generic tristate bus, so it's required to connect to a CFI flash device.