HP 3000 III Series Manual page 268

I/O System
1. On completion of the write, the Device Controller generates
another interrupt (12) to TIP. The sequence repeats back to
step h until all characters in the record have been written
out to the terminal.
m. The Dispatcher then returns control (13) to the user process.
7-9. BLOCKED/UNBLOCKED I/O
At the conclusion of all three of the preceding operating se-
quences (general I/O, direct read, and direct write), control is
returned to the user process. While the I/O operation was in
progress, the user process may have been suspended to await I/O
completion (blocked I/O), or may have continued to execute while
periodically checking for I/O completion (unblocked I/O). The
choice of blocked or unblocked I/O is made in the call to
ATTACHIO. (The file system nearly always uses unblocked I/O.)
Paragraphs 7-10 and 7-11 contain a description of the character-
istics of blocked and unblocked I/O.
7-10. Blocked I/O
As shown in figure 7-8, the user process goes into an I/O wait
state as soon as the I/O request is given. The user process re-
mains in the wait sta te while the I/O opera·tions proceed. The
request is entered into the I/O Queue and is ultimately processed
via the hardware I/O system. At the end of the I/O operation,
the resul ts of the transfer are entered into the IOQ. Control is
then returned to the user process. During terminal writes, the
operation is considered completed when the data has been trans-
ferred to the terminal buffers. The user process now continues
to execute from the point following the I/O request.
7-11. Unblocked I/O
During unblocked I/O (figure 7-8), privileged capability is as-
sumed. The process must also specify the action to be taken on
completion' of I/O; either no action or reactivate the process if
in an I/O wait state. This specification (like the blocked/un-
blocked I/O choice) is made in the call to ATTACHIO. After call-
ing ATTACHIO, the process may then continue to execute and may
generate other unblocked I/O requests. It is the responsibility
of the process to synchronize all unblocked requests and to check
for I/O completion. The process also has the capability to put
itself into the I/O wait state and to change the I/O completion
action for any unblocked request at any time. Obviously, how-
ever, the process should not specify no action for all unblocked
requests and then go into the I/O wait state since there is no
way to recover from this situation. At least one request must
reactivate the process.
While the process continues to execute, ATTACHIO enters the re-
quest into the I/O Queue and hardware processing of the request
begins. At the end of the I/O operation, the results of the
transfer are entered into the IOQ. Then the completion action
7-14