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Mode Indicator
The mode indicator shows the architectural mode
of operation selected by the last IML operation.
Power Controls
The power controls are used to turn the power on
and off.
The CPU, storage, channels, operator facilities,
and I/O devices may all have their power turned
on and off by common controls, or they may have
separate power controls. When a particular unit
has its power turned on, that unit is reset. The
sequence is performed so that no instructions or
I/O operations are performed until explicitly
specified. The controls may also permit power to
be turned on in stages, but the machine does not
become operational until power-on is complete.
When the power is completely turned on, an
IML operation is performed. A power-on reset is
then initiated (see the section "Resets" in Chapter
!
4, "Control"). It depends on the model whether
the architectural mode of operation can be selected
when the power is turned on, or whether the IML
controls have to be used to change mode after the
power is on.
Rate Control
The setting of the rate control determines the effect
of the start function and the manner in which
instructions are executed.
The rate control has at least two settings. The
normal setting is process. When the rate control is
set to process and the start function is performed,
the CPU starts operating at normal speed. When
the rate control is set to instruction step, one
instruction or, for interruptible instructions, one
unit of operation is executed each time the start
function is performed. For details, see the section
"Stopped, Operating, Load, and Check-Stop
States" in Chapter 4, "Control."
The test indicator is on while the rate control is
not set to process.
If
the setting of the rate control is changed while
the CPU is in the operating or load state, the
results are unpredictable.
Restart Key
Activating the restart key initiates a restart
interruption. (See the section "Restart
Interruption" in Chapter 6, "Interruptions. ")
The restart key is effective when the CPU is in
the operating or stopped state. The key is not
13-4
IBM 4300 Processors Principles of Operation
effective when the CPU is in the check-stop state.
It depends on the model whether the restart key is
effective when the CPU is in the load state.
Save Indicator
The Save indicator is turned on when a
machine-save operation has been successfully
completed. It is turned off when the load-clear,
load-normal, restart, start, system-reset-clear, or
system-reset-normal key is activated. It may also
be turned off when other controls are activated.
The indicator is off after a power-on reset. If an
error is encountered during the machine-save
operation, the indicator remains off.
Start Key
Activating the start key causes the CPU to perform
the start function. (See the section "Stopped,
Operating, Load, and Check-Stop States" in
Chapter 4, "Control. ")
The start key is effective only when the CPU is
in the stopped state. The effect is unpredictable
when the stopped state has been entered by a reset.
Stop Key
Activating the stop key causes the CPU to perform
the stop function. (See the section "Stopped,
Operating, Load, and Check-Stop States" in
Chapter 4, "ControL")
The stop key is effective only when the CPU is
in the operating state.
Operation Note
Activating the stop key has no effect when:
• An unending string of certain program or
external interruptions occurs.
• The CPU is in the load or check-stop state.
Storage-Size Control
The storage-size control is provided when a model
permits more than one size of virtual storage. The
control determines the storage size and, hence, the
value of the page-capacity count. The number of
storage-size settings of the control depends on the
model. (See the section "Storage Size" in Chapter
3, "Storage. ")
A new setting of the storage-size control
becomes effective only as part of the IML
operation performed when turning the power on or
when activating the IML controls.
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