HP 3000 III Series Manual page 266

I/O System
One element not previously present is the terminal buffer. The
ter~inal buffer consists of a linked list of buffers, which are
pointed to by an address word in the Device Information Table
(DIT) for a particular terminal. A sufficient number of these
buffers are used to accommodate the line or record length of the
associated device. Data is transferred between the terminal
buffer and the data buffer on a record basis. Thus, the terminal
buffer reads characters fran the device until the complete line
(or record) is read, then transfers the complete line (or record)
to the data buffer. This scheme conserves Main Memory space by
allowing the data buffer to be absent on disc while the compara-
tively slow terminal device is transferring individual char-
acte rs.
7-7. Direct Read
The sequence of operations for direct read, illustrated in figure
7-6, is as follows. (It is assumed that the file request requires
a p hys ical read fr om the te rm inal. )
a. The executing user process generates a file request
(1,
fig-
ure 7-6) to the file system.
b. The file system tests the validity of the request and calls
the At tach I/O proced ure
(2).
c. Attach I/O inserts the request parameters (3) in the I/O
Queue for the requested device. Unlike SIO which uses a
first-in/first-out queue for the requests, terminal requests
are analyzed for relative importance and are then inserted
into an appropriate place in the queue. (For example, factors
such as whether the request is from the system console are
cons ide red.)
d. When all higher priority requests have been completed, the
TERM procedure
(4)
begins execution for this request.
e. The Terminal Monitor issues a CIO instruction
(5)
directly to
the Device Controller, causing TIP to initiate the read
opera tion.
f. The Device Controller enables the device to transmit a char-
acter
(6).
When a key is pressed, the device returns the the
character to the controller.
g. On receipt of the character,
CPU interrupt
(7)
to the
(TI P) •
the Device Controller causes a
interrupt routine for terminals
h. TIP issues an RIO instruction to the Device Controller. This
causes the character (8) to be loaded onto the ICS and also
causes a command to be issued to the device to transmit the
next character. TIP now checks the character on the ICS to
determine if it is a data character or a control character.
7-12