or a channel status word cause a match to be indicat­ ed depends on the model.
The "IC" setting causes address comparison to be
performed when main storage is addressed to fetch
an instruction. The low-order bit of the address set­ ting mayor may not be ignored. The match is indi­ cated only when the first byte of the instruction is
fetched from the selected location. It is not indicated
for the instruction designated by EXECUTE.
Depending on the model and the type of refer­ ence, address comparison may be performed on logi­ cal, real, or absolute addresses, or controls may be
provided to specify the type of address.
The address-com pare-control settings can be
changed without disrupting CPU operations other
than causing the address-comparison stop.
Check-Stop Indicator
The check-stop indicator is on when the CPU is in
the check-stop state. The check-stop indicator may
be a separate indicator, or some models may use a
combination of other indicators to signal the check­ stop state. Performance of CPU reset turns off the
indicator. The manual indicator may also be on in
the check-stop state.
Configuration Controls
Configuration controls provide a means for setting
and controlling the configuration of the system. Some of these controls are associated specifically
with mUltiprocessing. The detailed function provided
depends on the model.
Display-and-Enter Controls
The system console provides controls and proce­ dures to permit the operator to display and enter
information in main storage, the general, floating­ point, and control registers, the PSW, and the keys
in storage. The CPU must first be placed in the
stopped state.
The display-and-enter functions are provided on
some models by means of controls on the operator
section of the system control panel; on other models,
they are provided by the use of the console device.
The CRT -menu-selection method may be provided
to facilitate the selection of storage facilities and
addresses.
Controls may be provided for data formatting,
checking, and error indication by means of interac­ tive procedures used on a keyboard-printer or a
keyboard-CRT device. In some models, enter-and­ display operations cause the manual indicator to be
turned off and may cause the start and restart keys
to b(! inoperative.
Main storage addresses for display-and-enter
operations are real addresses when dynamic address
translation is specified. Some models also include
the capability of specifying a logical or absolute ad­ dress.
Emergency-Pull Switch
Activating the emergency-pull switch turns off all
power beyond the power-entry terminal on every
unit that is part of the system or that can be
switched onto the system.
The switch latches in the out position and can be
restored to its in position only by maintenance per­ sonnel.
When the emergency-pull switch is in the out
position, the power-on key is ineffective.
Enable-System-Clear Key
Activating the enable-system-clear key, in conjunc­ tion with the load or system-reset key, results in the
performance of the system-clear-reset functions
described in detail in "Resets" in the chapter "System Control." In some models, the combination
of the enable-system-clear key with the load key or
the system-reset key is provided as an integrated
selectable function.
IMPL Controls
Controls are provided in some models for initial
microprogram loading (IMPL). These controls are
model-dependent.
Interrupt Key
Activating the interrupt key causes an external­ interruption condition to be generated.
The interruption is taken when the CPU is ena­ bled for the interruption and is in the operating
state. Otherwise, the interruption request remains
pending.
The interrupt key is effective while power is on
the system.
Load Indicator
The load indicator is on during initial program load­ ing; it goes on when the load key is activated and
goes out after the loading of the new PSW is com­ pleted successfully.
Load Key
Activating the load key causes a reset function to be
performed and initial program loading to be started.
Whether the enable-system-clear control is activated
at the same time determines the type of reset func- System Console 245

tion performed and the resultant effect on the sys­
tem. In a multiprocessing system, the effect of the
key is propagated to all CPUs configured, for reset
purposes, to this CPU. See the detailed discussion
under "Initial Program Loading" and "Resets" in
the chapter "System ControL"
Activating the load key may change the configura­
tion, induding the connection with channels, storage
units, and other CPUs. The load key is effective while power is on the
system.
Load-l7nit-Address Controls
The loadl-unit-address controls select three hexadeci­
mal digits, which provide the 12 rightmost I/O ad­
dress bits used for initial program loading. The manual indicator is on when the CPU is in the
stopped state. Some functions and several manual
controls are effective only when the CPU is in the
stopped state.
Power·,Off Key
Activating the power-off key initiates a power-off
sequence when the power-on key is lighted white or
red, that is, when power is on the system.
The contents of nonvolatile main storage (but not
the keys in storage associated with the proteetion
facility) are preserved, provided the CPU is in the
stopped state when power is turned off.
Power·,On Key
Activating the power-on key does the following:
1. Initiates the power-on sequence for the CPU and, depending on the model and configura­
tion, for the main storage, channels, and other
components of the system. The sequence is
performed in such a manner that no instruc­
tions or I/O operations are executed until ex­
plicitly specified.
2. Initiates the initial-microprogram-Ioading (IMPL) sequence for models with volatile con­
trol storage.
3. Initiates a power-on reset. See "Resets" in the
chapter "System Control" for the detailed de­
scription.
The power-on key is effective only when the
emergency-pull switch is in the in position and the
power-on key is not illuminated white.
Associated with the power-on sequence is a series
of power-on key illuminations. Activating the key
turns the lens red, indicating initiation of the power­
on sequence. The lens remains red until the sequence
246 System/370 Principles of Operation is completed without power-check conditions,
whereupon the lens turns white.
Should any condition prevent completion of the
sequence, the lens remains red if partial power is
present. If the power is off (excluding control volt­
ages) and the system is not performing a power-on
sequence, the lens has no illumination.
Should the CPU complex lose power or indicate a
thermal condition after the lens is white, the lens
turns red if partial power remains. After a power­
check condition is cleared, activating the power-on
key results in power being brought up on the system. Operation Note
Should the power-on IMPL sequence fail to be com­
pleted successfully, the manual indicator remains off.
The IMPL sequence should then be reinitiated by
using the IMPL controls.
Rate Control
The setting of the rate control determines the man­
ner in which instructions are executed.
The rate control has two or more settings, de­
pending on the model. The normal setting is labeled "process." When the rate control is in this setting,
the system starts operating at normal speed when the
start key is activated. The second setting is labeled
"instruction step." When the start key is activated
with the rate control in this setting, one instruction
or, for interruptible instructions, one unit of opera­
tion is executed, and all pending, allowed interrup­
tions are subsequently taken. The CPU then returns
to the stopped state.
Any instruction can be executed with the rate
control in the instruction-step setting. The perfor­
mance of input/ output operations is not affected.
When the CPU is in the wait state, no instruction is
executed, but pending, ailowed interruptions, if any,
are taken before the CPU returns to the stopped
state. When the rate control is in the instruction-step
setting, initial program loading is completed with the
loading of the new PSW. The interval timer is not
updated while the rate control is in the instruction­
step setting.
The test indicator is on when the rate control is
not in the process setting.
If the setting of the rate control is changed while
the CPU is in the operating state, the results are
unpredictable.
Restart Key
Activating the restart key initiates the restart inter­
ruption. See "Restart" in the chapter" Interrup­
tions. " The restart key is effective in both the operating