The system control panel contains the switches and
lights necessary to operate and control the system.
The system consists of the CPU, storage, channels, on­
line control units, and I/O devices. Off-line control
units and I/O devices, although part of the system
environment, are not considered part of the system
System controls are divided into three sections:
operator control, operator intervention, and customer
engineering control. Customer engineering controls
are also available on some storage, channel, and con­
trol-unit frames.
No provision is made for locking out any section of
the system control panel. The conditions under which
individual controls are active are described for each
case. System Control Functions
The system-reset function resets the CPU, the channels,
panel are the ability to reset the system; to store and
display information in storage, in registers and in the PSW; and to load initial program information.
System Reset
The system-reset function resets the CPU, the channels,
and on-line, nonshared control units and I/O devices.
The CPU is placed in the stopped state and all pend­
ing interruptions are eliminated. The parity of general
and floating-point registers, as well as the parity of the PSW, may be corrected. All error-status indicators are
reset to zero.
In general, the system is placed in such a state that
processing can be initiated without the occurrence of
machine checks, except those caused by subsequent
machine malfunction.
The reset state for a control unit or device is de­
scribed in the appropriate System Reference Library (SRL) publication. Off-line control units are not reset.
A system-reset signal from a CPU resets only the
functions in a shared control unit or device belonging
to that CPU. Any function pertaining to another CPU remains undisturbed.
The system-reset function is performed when the
system-reset key is pressed, when initial program
System Control Panel loading is initiated, or when a power-on sequence is
Programming Notes
Because the system reset may occur in the middle of
an operation, the contents of the psw and of result
registers or storage locations are unpredictable. If the CPU is in the wait state when the system reset is per­
formed, and I/O is not operating this uncertainty is
Following a system reset, incorrect parity may exist
in storage in all models and in the registers in some
models. Since a machine check occurs when informa­
tion with incorrect parity is used, the incorrect in­
formation should be replaced by loading new infor­
Store and Display The store-and-display function permits manual inter­
vention in the progress of a program. The store-and­
display function may be prOVided by a supervisor
program in conjunction with proper I/O equipment
and the interrupt key.
In the absence of an appropriate supervisor pro­
gram, the controls on the operator intervention panel
permit the CPU to be placed in the stopped state and
subsequently to store and display information in main
storage, in general and floating-point registers, and in
the instruction-address part of the psw. The stopped
state is achieved at the end of the current instruction
when the stop key is pressed, when single instruction
execution is specified, or when a preset address is
reached. Once the desired intervention is completed,
the CPU can be started again.
All basic store-and-display functions can be simu­
lated by a supervisor program. The stopping and start­
ing of the CPU in itself does not cause any alteration
in program execution other than the time element in­
volved (the transition from operating to stopped state
is described under "Stopped State" in "Status-Switch­ ing"). Interruption checks occurring during store-and-dis­
play functions do not interrupt or log immediately but
may, in some cases, create a pending interruption. This
interruption request can be removed by a system re­
set. Otherwise, the interruption, when not masked off,
is taken when the CPU is again in the operating state.
System Control Panel 117
Initial Program Loading
Initial program loading (IPL) is provided for the ini­
tiation of processing when the contents of storage or
the psw are not suitable for further processing.
Initial program loading is initiated manually by se­
lecting an input device with the load-unit switches
and subsequently pressing the load key. When the
multisystem feature is installed, initial program load­
ing may be initiated electronically by a signal received
on one of the IPL in-lines.
Depressing the load key causes a system reset, turns
on the load light, turns off the manual light, sets the
prefix trigger (if present), and subsequently initiates
a read operation from the selected input device.
When reading is completed satisfactorily, a new psw
is obtained, the CPU starts operating, and the load
light is turned off.
When a signal is received on one of the IPL in-lines,
the same sequence of events takes place, except that
the read operation is omitted.
System reset suspends all instruction processing, in­
terruptions, and timer updating and also resets all
channels, on-line nonshared control units, and I/O de­
vices. The contents of general and floating-point regis­
ters remain unchanged, except that the reset proce­
dure may introduce correct parity.
The prefix trigger is set after system reset. In man­
ually initiated IPL, the trigger is set according to the
state of the prefix-select key switch. When IPL is initi­
ated by a signal on one of the two IPL in-lines, the
trigger is set according to the identity of each line.
The prefix trigger is part of the multisystem feature.
If IPL is initiated manually, the select input device
starts reading. The first 24 bytes read are placed
in storage locations 0-23. Storage protection, pro­
gram-controlled interruption, and a possible incor­
rect length indication are ignored. The double-word
read into location 8 is used as the channel command
word (ccw) for a subsequent input command. When
chaining is specified in this ccw, the operation pro­
ceeds with the ccw in location 16.
After the input operation is performed, the I/O ad­
dress is stored in bits 21-31 of the first word in storage.
Bits 16-20 are made zero. Bits 0-15 remain unchanged.
The input operation and the storing of the I/O address
are not performed when IPL is initiated by means of
the IPL in-lines.
The CPU subsequently fetches the double word in lo­
cation 0 as a new psw and proceeds under control of
the new psw. The load light is turned off. When the
118 I/O operations and psw loading are not completed
satisfactorily, the CPU stops, and the load light remains
Programming Notes
Initial program loading resembles a START I/O that
specifies the I/O device selected in the loa d -u nit
switches and a zero protection key. The ccw for this START I/O has a read command, zero data address, a
byte count of 24, command-chain flag on, suppress­
length-indication flag on, program-controlled-inter­
ruption flag off, and a virtual command address of
Initial program loading reads new information into
the first six words of storage. Since the remainder of
the IPL program may be placed in any desired section
of storage, it is possible to preserve such areas of stor­
age as the timer and psw locations, which may be
helpful in program debugging.
If the selected input device is a disk, the IPL infor­
mation is read from track O. The selected input device may be the channel-to­
channel adapter involving two cpu's. A system reset
on this adapter causes an attention signal to be sent
to the addressed CPU. That CPU then should issue the
write command necessary to load a program into main
storage of the requesting CPU. When the psw in location 0 has bit 14 set to one,
the CPU is in the wait state after the IPL procedure
(the manual, the system, and the load lights are off, and the wait light is on). Interruptions that become
pending during IPL are taken before instruction exe­
Operator Control Section
This section of the system control panel contains only
the controls required by the operator when the CPU is operating under full supervisor control. Under su­
pervisor control, a minimum of direct manual inter­
vention is required since the supervisor performs
operations like store and display.
The main functions provided by the operator con­
trol section are the control and indication of power,
the indication of system status, operator to machine
communication, and initial program loading.
The operator control section, with the exception of
the emergency pull switch, may be duplicated once as
a remote panel on a console.
The following table lists all operator controls by
the names on the panel or controls and describes
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