The leftmost rotary switch has eight positions label­
ed 0-7. The other two are 16-position rotary switches
labeled with the hexadecimal characters 0-9, A-F.
Load Key
The load key is pressed to start initial program load­
ing, and is effective while power is on the system. Prefix-Select Key Switch
The prefix-select key switch provides the choice be­
tween main prefix and alternate prefix during manu­
ally initiated initial program loading.
The setting of the switch determines the state of the
prefix trigger following the system reset after the load
key is pressed.
The switch is part of the multisystem feature.
Operato,' Intervention Section
This section of the system control panel contains the
controls required for the operator to intervene in nor­
mal programming operation. These controls may be
intermixed with the customer engineering controls,
and additional switch positions and nomenclature may
be included, depending on the model. Operator intervention provides the system-reset and
the store-and-display functions. Compatibility in per­
forming these functions is maintained, except that the
word size used for store and display depends on the
physical word size of storage for the model. Switches
for display of the instruction address are absent on
models that continuously display the instruction ad­
dress.
The following table lists all intervention controls by
the names on the panel or controls and describes them.
NAME System Reset Stop Rate Start Storage Select Address
Data Store Display Set IC
Address Compare
Alterna te Prefix Multisystem feature
System-Re!;et Key
IMPLEMENT A TI ON Key
Key
Rotary switch
Key
Rotary or key switch
Rotary or key switches
Rotary or key switches
Key
Key
Key
Rotary or key switches
Light
The system-reset key is pressed to cause a system re­
set; it is effective while power is on the system. The
reset function does not affect any off-line or shared
device. 120 Stop Key
The stop key is pressed to cause the CPU to enter the
stopped state; it is effective while power is on the sys­
tem.
Programming Note Pressing the stop key has no effect when a continuous
string of interruptions is performed or when the CPU is unable to complete an instruction because of ma­
chine malfunction. The effect of pressing the key is
indicated by the turn-on of the manual light as the CPU enters the stopped state.
Rate Switch
This rotary switch indicates the way in which instruc­
tions are to be performed.
The switch has two or more positions, depending
on model. The vertical position is marked PROCESS. In
this position, the system starts operating at normal
speed whcn the start key is pressed. The position
left of vertical is marked INSTRUCTION STEP. When the
start key is pressed with the rate switch in this po­
sition, one complete instruction is performed, and all
pending, not masked interruptions are subsequently
taken. The CPU next returns to the stopped state.
Any instruction can be executed with the rate
switch set to INSTRUCTION STEP. Input/output opera­
tions are completed to the interruption point. When
the CPU is in the wait state, no instruction is per­
formed, but pending interruptions, if any, are taken
before the CPU returns to the stopped state. Initial
program loading is completed with the loading of the
new psw before any instruction is performed. The
timer is not updated while the rate switch is set to INSTRUCTION STEP. The test light is on when the rate switch is not set
to PROCESS. The position of the rate switch should be changed
only while the CPU is in the stopped state. Otherwise unpredictable results occur.
Start Key
The start key is pressed to start instruction execution
in the manner defined by the rate switch. Pressing the start key after a normal stop causes in­
struction processing to continue as if no stop had oc­
curred, provided that the rate switch is in the PROCESS or INSTRUCTION-STEP position. If the key is pressed
after a system reset, the instruction designated by the
instruction address in the psw is the first instruction
executed. In some models, the start key cannot be
pressed after a system reset until a new instruction
address or psw is introduced by pressing the set IC or
load switch.

The key is effective only while the CPU is in the
stopped state. Storage-Select Switch The storage area to be addressed by the address
switches is selected by the storage-select switches.
The switch can select main storage, the general reg­
isters, the floating-point registers and, in some cases,
the instruction-address part of the psw.
When the general or floating-point registers are not
addressed directly but must be addressed by using
another address such as a local-store location, infor­
mation is included on the panel to enable an operator
to compute the required address.
The switch can be manipulated without disrupting CPU operations.
Address Switches
The address switches address a location in a storage
area and can be manipulated without disrupting CPU operation. The address switches, with the storage-se­
lect switch, permit access to any addressable location.
Correct address parity is generated.
Data Switches
The data switches specify the data to be stored in the
location specified by the storage-select switch and ad­
dress switches.
The number of data switches is sufficient to allow
storing of a full physical storage word. Correct data
parity is generated. Some models generate either cor­
rect or incorrect parity under switch control. Store Key
The store key is pressed to store information in the
location specified by the storage-select switch and ad­
dress switches.
The contents of the data switches are placed in the
main storage, general register, or floating-point .regis­
ter location specified. Storage protection is ignored.
When the location designated by the address switches
and storage-select switch is not available, data are not
stored.
The key is effective only while the CPU is in the
stopped state.
Display Key
The display key is pressed to display information in
the location specified by the storage-select switch and
address switches.
The data in the main storage, general register, or
floating-point register location, or in the instruction-
address part of the psw specified by the address
switches and the storage-select switch, are displayed.
When the designated location is not available, the dis­
played information is unpredictable. In some models,
the current instruction address is continuously dis­
played and hence is not explicitly selected.
The key is effective only while the CPU is in the
stopped state. Set IC Key
This key is pressed to enter an address into the in­
struction-address part of the current psw.
The key is effective only while the CPU is in the
stopped state.
The address in the address switches is entered in
bits 40-63 of the current psw. In some models the ad­
dress is obtained from the data switches. Address-Compare Switches
These rotary or key switches provide a means of stop­
ping the CPU on a successful address comparison.
When these switches are set to the stop position,
the address in the address switches is compared
against the value of the instruction address on all
models and against all addresses on some models. An
equal comparison causes the CPU to enter the stopped
state. Comparison includes only the part of the in­
struction address that addresses the physical word size
of storage.
Comparison of the entire halfword instruction ad­
dress is provided in some models, as is the ability to
compare data addresses.
The address-compare switches can be manipulated
without disrupting CPU operation other than by caus­
ing the address-comparison stop. When they are set
to any position but normal, the test light is on.
Programming Note
When an address not used in the program is selected
in the address switches, the CPU runs as if the address­
compare switches were set to normal, except for the
reduction in performance which may be caused by the
address comparison.
Alternate-Prefix Light
The alternate-prefix light is on when the prefix trigger
is in its alternate state. The light is part of the multi­
system feature.
Customer Engineering Section
This section of the system control panel contains con­
trols intended only for customer-engineering use. System Control Panel 121