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UC-7110 Series User’s Manual Software Package
3-3
and uses JFFS2 for the User Directory.
The partition sizes are hard coded into the kernel binary. You must rebuild the kernel to change the
partition sizes. The flash memory map is shown in the following table.
Flash Context Flash Address Size Access control
Boot loader 0 – 0x3ffff 256 K Read ONLY
Kernet &
Root File System
0x40000– 0x3fffff 4 M - 256 K
Read ONLY
ROMFS
User Directory 0x400000 – 0x7fffff 4 M
Read / Write
JFFS2
In addition to the flash file systems, a RAM based file system will be mounted on /var/.
Read-Only File System (ROMFS)
ROMFS is a read-only file system for Linux. This means that it is a non-journaling file system.
Unlike CRAMFS, ROMFS does not support code compression.
A working Linux system requires the kernel and at least some programs, so obviously a file
system is also needed. Most Linux disk file systems are designed to be high performance,
supporting all POSIX features, and sometimes elaborate recovery from crashes (journaling). This
makes the system quite heavy-weighted, and thus often inappropriate for some special purposes.
Additional information about ROMFS is available at:
http://romfs.sourceforge.net/
Journaling Flash File System (JFFS2)
The flash User Directory is formatted by the Journaling Flash File System (JFFS2), which
places a compressed file system on the flash, transparent to the user.
The Journaling Flash File System (JFFS2) was developed by Axis Communications in Sweden.
JFFS2 provides a file system directly on flash, rather than emulating a block device designed for
use on flash-ROM chips, and it recognizes flash-ROM chips' special write requirements, does
wear-leveling to extend flash life, keeps the flash directory structure in RAM at all times, and
implements a log-structured file system that is always consistent—even if the system crashes or
unexpectedly powers down. It does not require fsck on boot up.
JFFS2, the next version of JFFS, provides improved wear-leveling and garbage-collection
performance, improved RAM footprint and response to system-memory pressure, improved
concurrency and support for suspending flash erases, marking of bad sectors with continued use of
the remaining good sectors (thus enhancing the write-life of the devices), native data compression
inside the file system design; and support for hard links.
Key features of JFFS2 are:
y Directly targeted to Flash ROM
y Robust
y Consistent across power failure
y No integrity scan (fsck) is required at boot time after normal or abnormal shutdown
y Explicit wear leveling
y Transparent compression