ZiLOG Z80-RIO Operating System User's Manual page 135

disk address of the record following the current one in the
sequence. A file is said to be active if these pointers
are valid. OPENing a file consists of locating it in a
directory, reading its descriptor record, and initializing
these pointers.
A
doubly-linked list provides some redundancy in
establishing the sequence of the records. This redundancy
is used when traversing the file to check the file
integrity. For example, the forward pointer of the current
record is used to establish the next record. When the next
record is read, its back pointer is checked to make sure it
indicates the current record.
A
failure of this or a
similar check is what is referred to as a pointer error.
ZOOS is designed to operate with up to 8 floppy disk
drives, each holding approximately 300 Kbytes. The
standard MCZ has two drives, configured as drive
a
(also
referred to as the system drive), and drive 2. The ZOS
also has two drives configured as drive 0 ·(referred
to as the system drive), and drive 1. Wh~n a file
is to be located, and the drive is not specifically
indicated (equivalent to specifying '*'), the drives are
searched in order, starting with drive 1, and continuing
through the highest disk which is attached and ready, and
finally, if still unsuccessful, concluding with drive
o.
Similarly, if a file is to be created without specifying
which disk it is to be on, it will be created on the first
disk which is attached and ready in the same search order.
Under RIO, all I/O calls pass through the operating system
where they are routed to the required device driver
according to the logical unit being requested and the
current routing for that unit. Calling parameters are
passed to the drivers via a 13-byte "parameter vector"
which is pointed to by the IY register. There are two ways
in which I/O calls are handled by the drivers. The driver
may perform the entire operation, then return to the
calling program (referred to as "return on completion"), or
it may perform only the initial setup necessary, then return
to the calling program and let the operation proceed to
completion under interrupt control (referred to as
"immediate return").
A
completion code is provided in the
parameter vector to indicate when the operation is complete
and signal any unusual circumstances of the completion.
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