About the SCSI Manager
About the SCSI Manager Support for third-party drives
Along with three new routines designed to support the message phases of the
SCSI standard, Apple has modified the way the System deals with data transfers
across the SCSI bus and the way external boot drives should be formatted.
mode. The polled mode is slower, but surer, since it checks the DRQ signal on
the SCSI chip before each data transfer and will wait as long as it has to for
each byte. The blind mode is half-again as fast as the polled mode but relies on
timing constraints to get the data back and forth between the Mac and the
external drive. Data has to pass back and forth every two microseconds after
or write data when the drive isn't ready to receive it, and the data will be lost.
If the drive has internal interrupts or some other slow-down that might cause
it to miss the two microsecond deadline, a polled transfer mode is the only one
you can use reliably.
The situation is a little different on the Mac SE and Mac II-family machines.
Here, a hardware handshake was added to make sure the reads and writes stay
synchronized. Even though the transfer routines are still called SCSIRBlind
on the Mac Plus. Timing is a factor again because of the synchronization check.
In the case of the SE, a second byte has to follow its predecessor within 265 to
284 milliseconds. For a Mac II, the timing interval is about 16 microseconds.
In either case, if the interval is exceeded, you'll wind up with a CPU bus error
and you'll need to fall back on the polled transfer mode.
Partition Map
Because some systems may need to support a UNIX operating system as well as
the Mac OS, a new partition map format should be used on external drives that
boot the system through the SCSI bus. While the old format can still be used,
Apple is encouraging developers to use this new one.
When using the new partition map format, bytes 0, 4, 8, C, 10 and 30 are
and they should be defined as indicated above. The rest of the fields may or may
not be used but they've been defined as indicated to give you a standard way to
store driver and operating system information.
Regardless of the format you choose for the partition map, however, there is
no change in the driver de scriptor map.
Error Codes
There is an expanded list of error codes that pertain to bus- specific
conditions:
scArbNBErr (3) Arbitration failure during SCSIGet; bus busy
scMgrBusyErr (7) SCSI Manager already occupied when SCSIGet was called
scSequenceErr (8) Operation out of sequence
scBusTOErr (9) Bus timeout before data ready on blind read or write
scComplPhaseErr (10) Bus not in Status phase; SCSIComplete call failed
In order, scArbNBErr is returned when arbitration fails because the bus is
Calling routines out of order produces a scSequenceErr result code.
is out of the Status phase. What the last error code tells you is that either the
filler bytes were written or bytes were read and lost.
Things to Remember
If you are writing a program to partition a disk you're reminded of the
following:
1) With the sole exception of physical block 0, every block on the disk has to
belong to a partition. Unused blocks have a partition type of their own, called
Apple_Free. The other types of partitions include Apple_MFS for flat file
system; Apple_HFS for hierarchical file system; Apple_Unix_SVR2 for UNIX;
Apple_partition_map for the special partition that holds the partition map;
Apple_Driver for the partition that contains a device driver; Apple_PRODOS
for a partition designated for an Apple IIGS; and Apple_Scratch for partitions
that are empty.
2) There has to be a partition map for every partition, including the
partition that holds the partition map.
3) Device drivers all get their own individual partitions instead of being in
the associated operating system's partition.
4) Repartitioning involves two separate steps. Existing partitions are first
marked as Apple_Free partitions (during which process you set the first eight
blocks to 0 after first copying their contents somewhere else). After that, you
combine the existing partitions with contiguous free partitions to create a
single, larger, area.
5) You can shift existing partitions towards the end of the disk if you need
more room for the partition map as a result of repartitioning. The partition
map is a special case. Its partition is the only one that can be expanded without
damaging or destroying its contents.