Singer 40 - Read Operations Procedure; Reading Data from Sectors and Error Handling

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5 READ OPERATIONS
A Read instruction which addresses a disc drive causes the 100 characters
contained in a sector to be read into main memory.
If a Read instruction addresses a disc drive which is not on-line, the instruction
is terminated and Condition Codes 4 and 1 (Fault) in the central processor are
set on.
If the specified sector cannot be successfully read, reading is retried up to
three more times. If, after a total of four attempts, the sector still cannot be
successfully read, the operation is terminated and Condition Code 1 (Error) in
the central proces sor is set on.
If a Read instruction addresses a Model 42 Disc Drive and the requested sector
is one which would be accessed by the middle arm (arm 2), the operation is
terminated and Condition Code 3 (Flag) in the central processor is set on.
If a FILE UNSAFE condition occurs during a read operation, the operation is
terminated and Condition Codes 4 and 1 (Fault) in the central processor are set
on.
Track address verification is performed automatically at the beginning of a
System Ten disc read operation.
If an attempt is made to read a sector which has been marked ''bad'' or if
address verification reveals that the sector has not been initialized, the opera-
tion is terminated and Condition Code 3 (Flag) in the central processor is set
on.
If address verification reveals that the read/write heads have been positioned
at the wrong track, the operation is terminated, Condition Code 1 (Error) in
the central process is set on, and the read/write head mechanism is repositioned
to the outermost track (track 000).
NOTE:
If the Disc Controller is addressed by a Read Control instruction, the
instruction is acted upon as though it were .a Read instruction.
Models 40 and 42 Reference Manual 5-1