GE 235 System Manual page 68

Checks a r e also made against the conditions which cause an ALERT HALT. This i s indicated by the
turning on of the ALERT HALT light on the tape controller display paneL These conditions are:
A parity e r r o r on instruction words 2 and 3 a s these words a r e transferred from memory to
the tape controller.
Addressing a tape unit that i s rewinding.
Using an address which has not been assigned to any specific tape unit.
Any detectable malfunction of the tape handler.
Specifying a tape handler logical address which has been assigned to two o r more units.
Giving a write command to a tape unit which does not have the write permit ring attached to
the supply reel. Each tape unit i s equipped with a file protection ring which is attached to the
tape supply reel. When this ring i s not attached, the controller does not permit writing on that
tape.
Addressing a tape unit with a read backward instruction (RBB, RBD, o r RBS) when the tape
on the unit i s initially positioned a t i t s beginning.
Parity and other such e r r o r s detected while reading o r writing tape do not cause an ALERT
HALT but do give an indication on the tape controller display panel that a specific e r r o r has
occurred on the last record. These e r r o r s can be detected under program control by means
of the test-and-branch instructions for the Magnetic Tape System.
MAGNETIC T A P E I N S T R U C T I O N S
All instructions executed by the Magnetic Tape Controller consist of 3 memory words, the f i r s t of
which has a s i t s function the selection of the specified controller. This f i r s t word has the operation
code SEL P. Similarly all test instructionspertaining to the status of a controller bear the common
operation code BCS.
The general format of these instructions utilize the following conventions: P
=
controller assign-
ment (channel number); M
=
memory location (starting); T
=
tape handler assignment (number) ;
N
=
length of record (number of words).
Coding under the "Index word" column is recorded in column 20 on the GAP coding sheet.
General Tape Instructions
Mnemonic
Operand Index Time Description
Oper. Code Address Word Microsec.
SE L P 24-36 SELECT the peripheral controller connected to s e -
lector channel number P.
BCS
XXX
P 24-36 BRANCH ON CONTROLLER SELECTOR. The pe-
ripheral controller connected to the controller channel
selector i s tested for the conditionindicated by a mne-
monic placed in the operand address field identified by
xxx.