Memo: Several common terms such as SDRAM, NOR Flash, NAND Flash

2015/4/14 Update

SDRAM: No data is saved after power-down, and dynamic refresh is required when running. Read faster than Flash, suitable for start-up program execution, need to initialize before use. It is used primarily for program storage, execution, or computation, similar to the memory of a PC.

NOR Flash: Power-down Save data/program. Instructions can be executed directly, read faster, write is not easy, erase speed is slow. Suitable for small-capacity programs or data storage, similar to a small hard disk, equivalent to the computer's BIOS.
NAND Flash: Power-down saves data/programs. The read speed is slightly faster than nor flash, but it is faster to write and erase. The reliability is slightly lower, need to do loss balance, data check and so on. Suitable for large-capacity data storage, similar to hard disk, storage operating system.

Do not know what your arm processor is model,
In the case of Samsung's ARM7 (s3c44b0x), there must be nor flash and SDRAM, which can be used to run the code in nor flash directly at startup, or to copy nor Flash to the SDRAM.
In the case of Samsung's ARM9 (s3c2440), there can be no nor Flash, with NAND flash and SDRAM, and the code and data in NAND flash must be copied into the SDRAM when booting.
If the arm microcontroller (lpc17xx,stm32, etc.), the built-in flash and SRAM.
Like s3c2440 's ARM9 processor, there are 4 K SRAM used for CPU start-up.

The s3c2440 supports two boot modes: NAND and non-NAND (here is nor Flash).
The exact method depends on the OM0, OM1 two pins. The Mini2440 uses two external 32Mbytes total 64Mbytes SDRAM chips.

Memo: Several common terms such as SDRAM, NOR Flash, NAND Flash