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2.
After the record has been read, the next instruction is
taken from location X ' 2A ' (provided that no error con-
dition has been detected by the device or input/output
proce ssor).
3.
When the instruction in location X '2A' is executed,
the unit device and device controller selected for
loading are capable of accepting a new SIO instruction.
4.
Further I/O operations from the load unit may be ac-
complished by coding subsequent I/O instructions to
indirectly address location X'25 1 .
LOAD OPERAnON DETAILS
The first executed instruction of the bootstrap program (in
location X 126 1 ) loads general register 0 with the doubleword
address of the first I/O command doubleword. The I/O ad-
dress for the SIO instruction in location X 1271 is the 11
low-order bits of location X I 25 1 (which have been set equal
to the load unit address as a result of pressing the LOAD
switch).
During the SIO instruction, general register 0
points to locations X 1221 and X 123 1 as the first I/O com-
mand doubleword for the selected device. This command
doubleword contains an order that instructs the selected pe-
ripheral device to read 88 (XI
~31)
bytes into consecutive
memory locations starting at word location X '2AI (byte lo-
cation X 'A8 1 ). At the completion of the read operation,
neither data chaining nor command chaining is called
for in the I/O command doubleword. Also, the suppress
incorrect length flag is set to 1 so that an incorrect length
indication wi II not be considered an error. (This means
that no transmission error halt will result if the first record
is either less than or greater than 88 bytes. If the record is
greater than 88 bytes, on Iy the first 88 bytes wi II be stored
in memory.) After the SIO instruction, the computer exe-
cutes a TIO instruction with the same effective address as
the SIO instruction. The TIO instruction is coded to accept
only condition code data. The BCS instruction in location
X I 29 1 will cause a branch back to the TIO instruction as
long as either CC1 or CC2 (or both) is set to 1. In normal
operation, CCl is reset to 0 and CC2 remains set to 1 until
the device can accept another SIO instruction, at which
time the next instruction will be taken from location X'2A'.
If a transmission error or equipment malfunction is detected
by either the device or the lOP, the lOP instructs the de-
vice to halt and initiate an "unusual end" interrupt signal
(as specified by the appropriate flags in the I/O command
doubleword). The "unusual end
II
interrupt wi II be ignored,
however, si nce all interrupt levels have been disarmed by
pressing the SYS RESET/CLEARswitch prior to loading. The
device will not accept another SIO while the device inter-
rupt is pending and, therefore, the BCS instruction in loca-
tion X
l
29 1 will continue to branch to location X128 1 . The
correct operator action at this point is to repeat the load
procedure.
If there is no I/O address recognition of the
load unit, the SIO instruction wi II not cause any I/O action
and CC 1 wi II conti nue to be set to 1 by the
510
and TI
0
instructions; thus causing the BCS instruction to branch.
Loading Operation
99
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