UC-7400-CE User’s Manual
Introduction
1-7
Memory and File Systems
The UC-7400-CE file system allows file storage on the system RAM or on the onboard flash
memory. Files can also be stored on external media such as a USB drive or CompactFlash card.
RAM-based Storage
About 20 MB of the 128 MB SDRAM is available for user applications and data. The operating
system and kernel image occupy the rest of the space. The root directory is stored in RAM, along
with subdirectories such as “Windows”, “Temp”, “My Documents”, “Network”, and “Program
Files”. Files can be temporarily stored in these directories, but they will be deleted when the
system is shut down or restarted. Persistent files and programs should be placed in the
“NORFlash” directory.
Onboard Flash Memory Storage
Onboard flash memory storage is provided through the “NORFlash” directory. Anything that is
saved in this directory will be retained when power is disconnected or lost. 15.5 MB of storage is
available.
The flash memory file system supports TFAT (Transaction-Safe File Allocation Table), which is
safer than FAT. TFAT protects the file system from write corruption during critical events such as
a sudden power loss. After a sudden power loss, the file state is rolled back. TFAT is a superset of
FAT, which means that the embedded computer is able to mount external FAT file systems.
External Media Storage
When external media is installed, such as a USB drive or CompactFlash card, it will be found as
an additional directory. For data that is intended to be shared with a PC, the external media should
be formatted using the PC’s FAT file system instead of the TFAT file system. PCs may not
recognize the TFAT format.
Caution When Storing Data
It is recommend that the onboard NOR flash be used for storing programs only. For log data
generated by your programs, use external storage media such as CompactFlash or a Network File
System. CompactFlash is much easier to replace if it is damaged or full.
NOR flash memory has a life cycle of 100,000 write operations at the block (128 KB) level. It
does not support BBM (Bad Block Management). For this reason, a FAT file system would not
know if a flash block has reached the end of its life cycle. The FAT file system would continue to
scan the block again and again, eventually resulting in an unpredictable state.
In addition, the FAT file system searches for free space sequentially when performing write
operations. As files are deleted, free storage space becomes more and more fragmented, making it
difficult to search. When a file is frequently updated, data is deleted and rewritten to the same
memory blocks over and over again. Eventually, the FAT file system would be unable to read
those blocks, causing the operating system to hang.
Although CompactFlash cards also have a life cycle, most use NAND flash memory with
hardware controllers that implement BBM. This feature allows the FAT file system to flag and
skip any bad blocks. Furthermore, there is much more storage space available on CompactFlash
cards than on the NOR flash memory. This space can be used cautiously to maximize the media’s
life cycle. An effective method is to create a large empty file (around 30 MB) to store log data.
Data is written evenly over the space, and when the end of the space is reached, the write
operations start over from the beginning of the space. This method reduces the number of write
operations performed on each block.
