About the Disk Driver
The Disk Driver is a specialized device driver in ROM that you use for storing
and retrieving information on Macintosh floppy and hard drives. That
information is stored in 512K-byte sectors consisting of an address mark that
shows the position of the sector on the disk and a data mark that holds the data
for that sector.
Consecutive sectors are organized into tracks--each of which represents a
ring of constant radius around the disk.
Applications read and write data in sector-sized chunks so it must specify
reads and writes in multiples of 512. The Disk Driver calculates which sector
to access. Your application also specifies where on the disk the data should be
read or written by providing a positioning mode and a positioning offset. You
can read and write data: at the current sector; from a position relative to the
current sector; or from a position relative to the beginning of the first sector
on the disk. The positioning mode constants are defined as:
fsAtMark = 0 current sector
fsFromStart = 1 relative to first sector
fsFromMark = 3 relative to current sector
Remember that relative positions are always given in multiples of 512.
On top of the 512 bytes of address and data information in each sector, there
are also 12 bytes of file tags for floppy drives and 20 bytes of file tags for
hard drives. They are designed to let you rebuild files when their directory
information (or other file-access data) has been destroyed. Whenever the
Disk Driver reads a sector from a disk, it places the sector's file tags in a file
tags buffer. Whenever a sector's file tags are written to the buffer, the
previous tag information is overwritten. Going the other way, whenever the
Disk Driver writes a sector to the disk, it takes the 12 or 20 bytes (depending
on the kind of disk drive involved) in the buffer and writes them to the disk
along with the sector's address and data information.
The Disk Driver disables interrupts during disk access. It uses the time
while interrupts are disabled to store any serial data received from the modem
port. Later it passes the data to the Serial Driver. The result is that you can
receive messages over communications links without generating hardware
overrun errors.
The Disk Driver opens automatically when the system starts up. It obtains
space in the system heap for variables, installs entries in the drive queue for
every drive on the computer and installs a task in the vertical retrace queue.
Writing data to a disk involves calling Device Manager's write procedure and passing the following parameters: floppy drive reference number (either
single- or double-sided) -5; hard drive reference number -2; logical (not
hard-coded) drive number in consecutive, sequential order starting at 1 and
running until all the drives have been numbered; a positioning mode to show
where on the disk the new information goes; a positioning offset in multiples of
512; a buffer with the data you want to write; and the number of bytes to be
written.
In the event of an error, one of the following codes will be returned;
noErr (0) No error
nsDrvErr (-56) No such drive
paramErr (-50) Bad positioning information
wPrErr (-44) Volume is physically locked
firstdskErr (-84) Low-level disk error
through lastDskErr ...lastDskErr is range of disk err codes as follows:
sectNFErr (-81) Can't find sector
seekErr (-80) Drive error
spdAdjErr (-79) Can't correctly adjust disk speed
twoSideErr (-78) Tried to read side two in a single-sided drive
initIWMErr (-77) Can't initialize disk controller chip
tk0BadErr (-76) Can't find track 0
cantStepErr (-75) Drive error
wrUnderrun (-74) Write underrun occurred
badDBtSlp (-73) Bad data mark
badDCksum (-72) Bad data mark
noDtaMkErr (-71) Can't find data mark
badBtSlpErr (-70) Bad address mark
badCksmErr (-69) Bad address mark
dataVerErr (-68) Read-verify failed
noAdrMkErr (-67) Can't find an address mark
noNybErr (-66) Disk is probably blank
offLinErr (-65) No disk in drive
noDrive (-64) Drive not connected
Reading data from a disk involves making a Device Manager read call and passing the same parameters as above, except that you give it the buffer you
want to read into( including the number of bytes it contains) instead of a buffer
to write.
In the event of an error, one of the following codes will be returned:
noErr (0) No error
nsDrvErr (-56) No such drive
paramErr (-50) Bad positioning information
firstdskErr (-84) Low-level disk error
through lastDskErr ...lastDskErr is range of disk err codes as follows:
sectNFErr (-81) Can't find sector
seekErr (-80) Drive error
spdAdjErr (-79) Can't correctly adjust disk speed
twoSideErr (-78) Tried to read side two in a single-sided drive
initIWMErr (-77) Can't initialize disk controller chip
tk0BadErr (-76) Can't find track 0
cantStepErr (-75) Drive error
wrUnderrun (-74) Write underrun occurred
badDBtSlp (-73) Bad data mark
badDCksum (-72) Bad data mark
noDtaMkErr (-71) Can't find data mark
badBtSlpErr (-70) Bad address mark
badCksmErr (-69) Bad address mark
dataVerErr (-68) Read-verify failed
noAdrMkErr (-67) Can't find an address mark
noNybErr (-66) Disk is probably blank
offLinErr (-65) No disk in drive
noDrive (-64) Drive not connected
Make a read call immediately after a write cal to verify that the information
was transferred correctly. A read-verify operation is much the same as a
standard read call except that it contains a constant (rdVerify = 64) in the
positioning mode. Additionally, a dataVerErr (-68) code does not match, will
be returned if an data mismatch occurs.