
W83877AF
Publication Release Date: Dec. 1996
Preliminary Version 0.52
- 114 -
7 6 5 4 3 2 1 0
Floppy Boot Drive 0
Floppy Boot Drive 1
Media ID 0
Media ID 1
SWWP
DISFDDWR
APDTMS2
APDTMS1
APDTMS2 APDTMS1 (Bit 6, 7):
These two bits select the count-down time of the automatic power-down mode counter.
00
4 seconds
01
32 seconds
10
64 seconds
11
4 minutes
DISFDDWR (Bit 5):
This bit enables or disables FDD write data.
0
Enables FDD write
1
Disables FDD write (forces pins WE, WD to stay high)
Once this bit is set high, the FDC operates normally, but because pin WE
is inactive, the FDD will not
write data to diskettes. For example, if a diskette is formatted with DISFDDWR = 1, after the format
command has been executed, messages will be displayed that appear to indicate that the format is
complete. If the diskette is removed from the disk drive and inserted again, however, typing the DIR
command will reveal that the contents of the diskette have not been modified and the diskette was
not actually reformatted.
This is because as the operating system (e.g., DOS) reads the diskette files, it keeps the files in
memory. If there is a write operation, DOS will write data to the diskette and memory simultaneously.
When DOS wants to read the diskette, it will first search the files in memory. If DOS finds the file in
memory, it will not issue a read command to read the diskette. When DISFDDWR = 1, DOS still
writes data to the diskette and memory, but only the data in memory are updated. If a read operation
is performed, data are read from memory first, and not from the diskette. The action of removing the
diskette from the drive and inserting it again forces the DSKCHG pin active. DOS will then read the
contents of the diskette and will show that the contents have not been modified. The same holds true
with write commands.
The disable FDD write function allows users to protect diskettes against computer viruses by ensuring
that no data are written to the diskette.
SWWP (Bit 4):
0
1
Normal, use
WP
to determine whether the FDD is write-protected or not
FDD is always write-protected
Media ID 1 Media ID 0 (Bit 3, 2):