Commodore 1581 - Controller Job Queue

Cache Buffer. This buffer is located from $0C00 through $1FFF. All disk
accesses involve an entire physical track at a time. If a single sector is
needed from a particular track, the entire track is read into RAM. Con
sequently, any more requests for sectors from the same track require
only a RAM data transfer, rather than a search of the track on the disk
again. Sector writes are also speeded up considerably, since each con
secutive write to sectors on the same track requires only a RAM to RAM
data transfer. After data is written into the Track Cache Buffer, it is not
written to the disk until one of the following occurs: 1) a request is
made for access of a sector on a different track, 2) a 'dump track buffer'
command is issued, or 3) after 250 milliseconds of no serial bus activity.
Controller Job Queue
The software in the 1581's ROM can be broken down into two
major components—the DOS and the Controller. The DOS (Disk
Operating System) is the software interface between the host comput
er and the 1581 system. The DOS keeps track of the file management
details necessary to create, modify and delete files. It monitors the
amount of free space left on a disk, and keeps track of the file names in
the directory. It remembers where each file starts, and ensures that
none of them overlap. When the host sends commands, the DOS
checks to make sure that the syntax and the parameters are valid. The
DOS is very complex, and its code occupies the majority of the ROM.
The Controller, on the other hand, knows nothing about the
concept of files. The Controller deals only in the physical world of the
disk itself. It is responsible for reading and writing each of the individ
ual sectors on the disk.
The DOS and Controller communicate to each other primarily
through a 'mailbox,' known as the Job Queue. Because the DOS deals
more in the abstract, or logical, world, it does not access the physical
disk itself. If the DOS requires access to the disk, it must ask the
controller to do it. The DOS places a Controller Command Code into
the Job Queue and then waits for the requested operation to be
completed. The Controller polls the Job Queue every 10 milliseconds
(the polling rate is determined by timer B of the 8520) looking for
something to do. If there is a job in the queue, the Controller executes
the job and returns a status byte to the DOS. To speed things up, the
polling of the job queue can be bypassed by a direct jump to JSTROBE
CONTROLLER at $FF54 (refer to the section describing the Vectored
Jump Table). A call to JSTROBECONTROLLER requires the command
in the accumulator and the job queue offset in the x register,
102