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

and stopped states. The key is not effective when the CPU is in the check-stop state.
Start Key
Activating the start key causes the CPU to enter the
operating state. See "Stopped and Operating States" in the chapter "System Control." The key is effec­
tive only when the CPU is in the stopped state, and
the effect is unpredictable when the stopped state
has been entered by reset.
Stop Key
Activating the stop key causes the CPU to perform
the stop function. See "Stopped and Operating States" in the chapter "System ControL" The stop key is effective only while the CPU is in
the operating state.
Operation Note
Activating the stop key has no effect when a contin­
uous string of interruptions occurs, when the prefix
register contains an invalid address, or when the CPU is unable to complete an instruction because of
a machine malfunction.
Store-Status Key
Activating the store-status key initiates the store­
status function. Although the store-status function is
performed compatibly on all models, the initiation of
the function differs among models. Some models
initiate the function by the use of a special keyboard
mnemonic and some by the use of a pushbutton,
while others may provide a CRT -menu selection. See "Store Status" in the chapter "System Con­ trol" for a description of the function.
The manual control for performing store status is
effective only when the CPU is in the stopped state.
Operation Note
The store-status function would ordinarily be used in
conjunction with a standalone ,dump program for the
analysis of program malfunctions. For such an oper­
ation, the following sequence would be called for: • Activating of the system-reset key. • Initiation of the store-status function. • Normal IPL of a standalone dump program.
System Indicator
The system indicator is on when the CPU cluster
meter or customer-engineer meter is running.
System-Reset Key
Activating the system-reset key causes a reset func­
tion to be performed. Whether the enable-system­
clear key is activated at the same time determines
the type of reset 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 "Resets" in the chapter "System Control." Activating the system-reset key may change the
configuration, including the connection with chan­
nels, storage units, and other CPUs. The key is effective while power is on the system.
Test Indicator
The test indicator is on when a manual control is not
in its normal position or when a maintenance func­
tion is being performed for the CPU, channels, or
storage.
The test indicator is on whenever a control on the
system control panel or on any separate maintenance
panel for the CPU, storage, or channels is in an ab­
normal position that can affect the normal operation
of a program.
The test indicator may be on when one or more
diagnostic functions under control of DIAGNOSE are activated or when certain abnormal circuit­
breaker or thermal conditions occur.
The test indicator does not reflect the state of
marginal voltage controls.
Thermal/CB Power-Check Indicator
The thermal! CB power-check indicator, which is
one or more red lights, goes on when a thermal con­
dition, a circuit-breaker-trip condition, or both con­
ditions are detected in the CPU complex. This indi­
cator is turned off from the customer-engineer
power-control panel. TOD Clock Key
The TaD (time-of-day) clock key provides an inter­
lock in the execution of the instruction SET CLOCK for the purpose of guarding against an unauthorized
or inadvertent change to the time-of-day clock val­
ue.
When the TaD clock key is not activated, that is,
is in the position labeled "secure," the value of the
TaD clock is protected against alteration by the
program, and the execution of SET CLOCK does
not change the value of the clock.
When the TaD clock key is activated, that is, is in
the position labeled "enable set," alteration of the
clock value by means of SET CLOCK is permitted.
System Console 247