HP 3000 III Series Manual page 259

I/O System
request is for Unit 2 (uses the next available entry), followed
by a second request for Unit 1. This second request for unit 1
causes the first word of the initial request to point to the next
unused entry, which is then filled with information pertaining to
the second request. Therefore, eventually the 10Q will contain a
queue of requests for Unit 1, a separate queue for Unit 2, and so
on,
plus a linked list of free entries.
Next, an I/O driver is executed to initiate the request. An I/O
program will then be run on a device, using the request parame-
ters given in the IOQ. When the request is completed, the 10Q
entry is returned to the free list. Note that the IOQ only es-
tablishes the priority of requests for each device on a first-in
first-out basis. Questions of priority in executing I/O drivers
are resolved by the Dispatcher. (Refer to the
MPE
General Infor-
mation Manual for a description of the Dispatcher.) Once several
Device Controllers are running I/O programs, priority conflicts
are resolved by hardware service priority.
The DRT (figure 7-4) consists of a number of four-word entries
corresponding to the number of Device Controllers present in the
system. The DRT is located in fixed memory locations beginning
at octal address 14. (Locations 0 through 13 are allocated to
other purposes; refer to table 2-4.) The upper limit for the
table is location 777 which limits the maximum number of four-
word entries to 125 (decimal). Because each DRT entry is always
four words in length, it is convenient for the hardware to map
device numbers to DRT addresses simply by mUltiplying by four.
(Leftshift device number two binary places.) Thus the entry for
device number 3 begins at octal location 14 (i.e., %(3x4)
=
%14).
Because the DRT begins at location 14, device number 3 is the
lowest device number. (Devices 0,1, and 2 do not exist.)
Note
The device number associated with a par-
ticular DRT entry defines a Device Con-
troller, and not necessarily an actual
physical device. Also remember that some
controllers identified by one device num-
ber are capable of driving several physi-
cal devices. Individual identification of
physical devices is made by logical device
numbers. The logical device number is the
value used by the file system in request-
ing I/O, and the I/O system software per-
forms the logical to physical device
number translation.
7-5