DISK EMULATOR OPERATION
GENERAL
The 4C24 has a built in nonvolatile disk emulator with a capacity of up to 2M bytes using EPROM
or 1 M byte using flash EEPROM.
Two 32 pin sockets on the 4C24 can be used for disk emulator memory chips. These two chips can
be used together as a single drive, or each chip can be configured as an independent drive. When two
drives are configured, the chips can be of different types. For example, a configuration with EPROM
for program storage and flash EEPROM for data logging is a popular arrangement.
When using the built in disk emulator, the disk type configuration jumpers must be first set to
match the type of disk emulator chip(s) used. (See the HARDWARE CONFIGURATION section of
this manual.)
The 4C24 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.
4C24 TOOL CHIP
An EPROM disk chip preloaded with a set of basic 4C24 utilities is available from MESA at a
nominal charge. If you do not have a PC-104 floppy controller, it is suggested that you purchase this
chip to simplify first time 4C24 setup and disk initialization.
Page 22
4C24 CPU USER'S MANUAL
