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DISK EMULATOR OPERATION
GENERAL
The 4C27 has an optional built in nonvolatile disk emulator with a capacity of up to 16M bytes
using NAND flash devices .
The 4C27 disk emulator is viewed as a hard disk by system software. This means that the first
emulated drive will be drive C: , and the next emulated drive will be drive D:.
RELIABILITY
In an embedded system environment where a system that won't boot is basically a failed system, it
is important to understand some characteristics of the DOS operating system that applies to disk
access. When DOS writes a file, it writes to the FAT and directory areas of the drive (emulated or
real).
If there is any chance that a system can be reset or power can fail when writing to this disk, all
information on the disk could become inaccessible, not just the file that was being written.
The reason is that when DOS writes to a directory or FAT area it always writes a full sector, not
just the directory or FAT entry required. If the sector write is not completed, the sector with the
directory or FAT entry that was being written will have an invalid CRC. This can affect any file on the
drive!
In applications that do frequent disk writes, there are two possible solutions to this problem. The
first solution is to disable emulated disk CRC checking. This will make a partially rewritten sector
readable by the operating system. This will only improve the odds of surviving a power off or reset
during a file write, not totally eliminate the problem. Turning off CRC's will also mask possible
hardware problems, so is not generally suggested. The second solution is to configure a two drive
system, with a drive (usually C:) used as the software drive, and the other drive (usually D:) used as
the data drive. Any files writes during normal operation would be done to the D: drive. If any problem
occurs on the D: drive, software on the C: drive can attempt to recover the data, and then re-initialize
the D: drive.
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4C27 CPU USER'S MANUAL