HP 3000 III Series Manual page 279

I/O System
7-30. Typical I/O Program Operation. Figure 7-13 illustrates the
sequence of operations occurring as the result of an SIO instruc-
tion. The sequence is as follows:
a. The SIO instruction (decoded by the CPU) fetches the device
number given at S-K (1, figure 7-l3f in the stack (2), and
puts the TOS into the first word of its DRT entry as the I/O
progr am pointe r.
b. SIO then sends the device number (3) to the lOP Control Reg-
ister and sends an SIO command (4) to the lOP.
c. The lOP issues the SIO command (5) to the Device Controller
and execution by the hardware begins. The CPU is now free to
continue execution elsewhere.
d. On demand from the MUltiplexer Channel, the lOP obtains the
program pointer (6) from the DRT. (The Selector Channel ob-
tains the program pointer directly, not via the lOP.) As il-
lustrated previously in figure 7-4, the address is obtained
by mul tiplying the dev ice number by four. The progr am point-
er is the first word of the four-word DRT entry.
e. The program pointer points to the first doubleword of the I/O
program (7). The pointer is Lpdated to point at each I/O
program double word as the program progresses. (The Selector
Channel, to minimize memory fetches, copies the pointer value
into a register and updates the pointer internally; the Mul-
tiplexer Channel updates the pointer directly in the DRT.)
f. The sample I/O program operates as follows. The first dou-
bleword (7) contains a CONTROL order which enables the hard-
ware I/O subsystem for this device. The second doubleword
contains a READ order which causes the subsystem to read 4096
words (or 8192 bytes) into the data buffer (8) whose starting
location is given in the IOAW word. Since the data chaining
bit is on, the next (third) doubleword is also a READ order
which specifies the remaining count required to fulfill the
I/O request. (Additional READ orders could be given for larg-
er requests.) The IOAW will either specify an additional buf-
fer area contiguous to the first 4096-word buffer if desired
or, in another part of memory.
g. When the transfer is complete, the final doubleword contains
an END order, which obtains the result of the transfer (de-
vice status) and loads it into the IOAW; the END order then
tells the controller to generate an interrupt to inform the
software that the transfer is complete.
h. At the completion of an I/O program, the Selector Channel
returns the current program pointer value to the DRT. The
Multiplexer Channel does not take any special action since it
updates the DRT afte r each order fe tch.
7-25