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
Emergency Pull Power On Power Off Interrupt
Manual System Test
Load Unit Load Prefix Select\) (I< Multisystem feature
Emergency Pull Switch IMPLEMENTATION Pull switch
Key, backlighted
Three rotary switches
Key switch Pulling this 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. There­
fore, the switch controls the system proper and all
off-line and shared control units and 110 devices.
The switch latches in the out position and can be
restored to its in position by maintenance personnel
When the emergency pull switch is in the out po­
sition, the power-on key is ineffective. Power-On Key
This key is pressed to initiate the power-on sequence
of the system.
As part of the power-on sequence, a system reset is
performed in such a way that the system performs no
instructions or 110 operations until explicitly directed.
the contents of main storage, including its protection
keys, remain preserved.
The power-on key is backlighted to indicate when
the power-on sequence is completed. The key is ef­
fective only when the emergency pull switch is in its
in position. Power-Off Key
The power-off key is pressed to initiate the power-off
sequence of the system.
The contents of main storage and its protection keys
are preserved.
Interrupt Key
The interrupt key is pressed to request an external
The interruption is taken when not masked off and
when the CPU is not stopped. Otherwise, the inter­
ruption request remains pending. Bit 25 in the inter­
ruption-code portion of the current psw is made one
to indicate that the interrupt key is the source of the
external interruption.
Wait Light
The wait light is on when the CPU is in the wait state. Manual Light
The manual light is on when the CPU is in the stopped
state. Several of the manual controls are effective only
when the CPU is stopped, that is, when the manual
light is on. System Light
The system light is on when the CPU cluster meter or
customer-engineering meter is running.
Programming Note
The states indicated by the wait and manual lights
are independent of each other; however, the state of
the system light is not independent of the state of
these two lights because of the definition of the run­
ning condition for the meters. The following table
off off off Not allowed when power is on
off off on Waiting Not operating
off on off Stopped Not operating
off on on Stopped, Not operating
on off off Running Undetermined
on off on Waiting Operating
on on off Stopped Operating
on on on Stopped, Operating
Test Light
The test light is on when a manual control is not in
its normal position or when a maintenance function is
being performed for CPU, channels, or storage.
Any abnormal switch setting on the system control
panel or on any separate maintenance panel for the CPU, storage, or channels that can affect the normal
operation of a program causes the test light to be on.
The test light may be on when one or more diag­
nostic functions under control of DIAGNOSE are acti­
vated or when certain abnormal circuit breaker or
thermal conditions occur.
The test light does not reflect the state of marginal
voltage controls.
Load Light
The load light is on during initial program loading;
it is turned on when the load key is pressed and is
turned off after the loading of the new psw is com­
pleted successfully.
Load-Unit Switches
Three rotary switches provide the II-bit address of
the device to be used for initial program loading. System Control Panel 119
Previous Page Next Page