Device Status Byte (Register R Or Ru1) - Xerox 550 Reference Manual

Table 15. Device Status Byte (Register R or Ru1)
(SIO, no, and HIO only)
Bit
Position
o
Significance
Interrupt Pending. This bit is set to a 1 if
the addressed device has requested an inter-
rupt that has not been acknowledged by the
BP with an AIO instruction. If this bit is
a 1, the current SIO instruction is not ac-
cepted. Condition code bits are set to re-
flect this action and any requested status
information is loaded into the designated
general register(s). SIO instructions will not
be accepted unti I the interrupt pending con-
dition is cleared.
Normally, before a device can request an
interrupt, the following conditions must
prevail:
1. Appropriate flag(s) (IZC, ICE, and/or
IUE; bit positions 33, 35, and 37, re-
spectively) within the I/O command
doubleword must be set to
1.
2. The flagged event (byte count reduced
to zero for the IZC flag, II channel end ll
condition for the ICE flag, or lIunusual
end ll condition for the IUE flag) must
occur.
3. lOP may signal device controller to
raise interrupt without examining inter-
rupt flags, if:
a. A connection address error is
detected.
b, Any error is detected when lOP is
accessing an 10eD.
F or case a, no interrupt status wi
II
be
set in response to an AlO.
For case b, an IUE signal is sent back
in response to an AIO.
An I/o interrupt may also be requested by
certain devices via M modifier bits within
the basic order for that device (see Opera-
tional Command Doublewords).
A BP wi
II
respond to an interrupt request
from a particular I/o subsystem if (1) the
I/O interrupt level (X ' 5C') is armed,
114 Input/Output Instructions
Table 15. Device Status Byte (Register R or Ru1)
(SIO,
no,
and HIO only) (cont.)
Bit
Position
o
(cont. )
1,2
3
Significance
enabled, and not inhibited; and (2) that
there is no higher priority interrupt level
in the active or waiting state.
Device Condition. If bits 1 and 2
are 00
(device IIreadyll), all device conditions re-
quired for proper operation are satisfied.
If bits 1 and 2 are 01 (device IInot opera-
tional ll ), the addressed device has developed
some condition that will not allow it to pro-
ceed; in either case, operator intervention
is usually required. If bits 1 and
2
are 10
(device II unavailable ll ), the device has more
than one channel of communication avail-
able and it is engaged in an operation con-
trolled by a controller other than the one
specified by the I/o address. If bits 1 and 2
are 11 (device II busyll), the device has ac-
cepted a previous SIO instruction and is al-
ready engaged in an I/O operation.
Device Mode. If this bit is 1, the device
is in the lIautomatic ll mode; if this bit is 0,
the device is in the IImanual" mode and
requires operator intervention. This bit can
be used in conjunction with bits 1 and
2
to
determine the type of action required. For
example, assume that a card reader is able
to ·operate, but no cards are in the hopper.
The card reader would be in state 000 (de-
vice II ready
II ,
but manual intervention re-
quired), where the state is indicated by
bits 1,
2,
and 3 of the I/O status response.
If the operator subsequently loads the card
hopper and presses the card reader START
switch; the reader would advance to state 001
(device "ready" and in automatic operation).
If the card reader is in state 000 when an
SIO instruction is executed, the SIO would
be accepted by the reader and the reader
would advance to state 110 (device II busyll,
but operator intervention required). Should
the operator then place cards in the hopper
and press the START switch, the card reader
state would advance to 111 (device
II
busyll
and in "automatic ll mode), and the input
operation would proceed. Should the card
reader subsequently become empty (or the
ope;ator press the STOP switch) and com-
mand chaining is being used to read a num-
ber of cards, the card reader wou Id return to
state 110. If the card reader is in state 001
when an SIO instruction is executed, the
reader advances to state 111, and the input