Operation Controls - IBM System 360 Operating Manual

OPERATION CONTROLS
Four rotary control switches are provided to allow
any of several modes of system operation. Althougb
these controls are normally used only by the IBM
Customer Engineer, certain of their functions are
useful to the programmer and operator. The Test
light is on if these four switches are not all set to
the Process position.
Rate Switch
The three modes of operation provided by this
control are:
Instruction Step (INSN STE P) Position
When the Rate switch is in this position, one com-
plete instruction (including all pending unmasked
interruptions) is executed each time the Start key
is pressed. After the instruction has been executed,
the A- and B-registers contain the address of the
next instruction. The stop that then occurs is
identical to the stop that occurs when the Stop key
is pressed.
If an input/output instruction is executed, the
I/O instruction and all associated chaining is com-
pleted before the final stop occurs. If the start key
is pressed again during the chaining operation, the
next instruction is processed.
Process Position
This position of the Rate switch is used for normal
program processing.
Single-Cycle Position
When the Rate switch is in this position, the system
advances only one ROS cycle each time the Start key
is pressed. Data overruns may occur
if
this mode
is used for input/output operations.
Check Control Switch
The setting of this switch determines the system
reaction when an error is detected in the CPU or
I/O channels. Several courses of action are allowed
as follows:
10
Diagnostic Position
In this mode, system control in case of an error is
under control of a special microprogram.
Disable Position
When the system is operated in this mode, parity
errors are logged (in associated check circuitry), but
otherwise the failure is ignored. Note, however,
that certain system errors cannot be ignored; that
is, automatic correction of read-only storage
addressing is not provided; and the wrong word will
be accessed. Results obtained while processing a
problem program in this mode may be wrong.
Process Position
This position of the Check Control switch is used
for problem-program processing. When a parity
check occurs, the malfunction trap microprogram
is automatically initiated. This routine stores the
contents of the machine check register and the pro-
gram status word in fixed main-storage locations
and originates a machine check interruption (refer
to the IBM System/360 Principles of Operation,
Form A22-6821).
Stop Position
Detection of a parity error in this mode causes an
unconditional CPU clock stop, regardless of machine-
check masks. I/O data overruns may occur if the
system is operated in this mode.
Restart Position
When an error occurs in this mode, the resultant
system response is dependent upon the setting of
the Address Compare switch as follows:
1. With the Address Compare switch set to SAR
Restart, a System Reset is initiated, the basic
micro-diagnostic and clear-UCW routines are
executed, the instruction counter is set to the
address specified by switches F, G, H, and J,
and an instruction cycle is started.