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SECTION II.
OPERATOR'S CONTROL PANEL
Access to main memory is under control of a me.mory address register (MAR) which con-
~_
tains the address of the memory location being referenced.
In the Type 121 Central Processor,
.....",
the contents of the memory address register are displayed by the ADDRESS push buttons in five
octal groups.
A memory address is displayed as a binary pattern of lights.
If
a button is illuminated,
the corresponding bit is a "I"; if the button is not illuminated, the corresponding bit is a "0".
Buttons for which there are no corresponding bits are never illuminated.
For example, the
high-order button in the fifth octal group is never illuminated unless the system contains more
than 16,384 main memory locations.
Any bit in the memory address register can be changed from a 0 to a 1 by depressing the
corresponding push button; this action sets the bit and illuminates the button.
To change a bit
from a 1 to a 0, however, it is necessary to reset all the memory address register bits to D's
by depressing the ADDRESS CLEAR button (or the INITIALIZE button).
The desired lIs may
then be set by depressing the appropriate push buttons.
The contents of the memory address register can be changed manually only in the STOP
mode, since the ADDRESS buttons are interlocked to be effective only when the STOP button is
illuminated.
The lights in the ADDRESS push buttons are effective at all times, however, and
display the instantaneous contents of the memory address register while the program is running.
CONTENTS Push-Button Indicators
The CONTENTS push-button indicators normally display a character in main memory.
In
the STOP mode, they are not normally illuminated, unless they are manually changed or displayed.
Access to main memory is accomplished via a nine- bit memory local register (MLR)
whose contents are displayed in the CONTENTS indicators as a binary pattern of lights.
(NOTE:
The parity bit is not displayed in the CONTENTS indicators.) The memory local register bits
are set and reset and the push buttons are illuminated according to the same rules discussed
above for the memory address register, except that the CONTENTS CLEAR (or the INITIALIZE)
button resets the register.
2-2
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