Control Data 3100 Reference Manual page 15

Q Register (Arithmetic)
The Q register is an auxiliary register and is generally used in conjunction with the
A register.
The principal functions of Q are:
• Providing temporary storage for the contents of A while A is used for another
arithmetic operation.
• Forming a double-length register, AQ.
• Shifting to the right or left, separately or in conjunction with A.
• Serving as a mask register for 06, 07, and 27 instructions.
Both A and Q may load or be loaded from any of the three index registers without the
use of storage references.
X Register (Arithmetic)
The X register is a transfer register, used only for internal instruction processing.
Contents of this register cannot be displayed by any external indicators.
F Register (Main Control)
The program control register, F, holds an instruction during the time it is being exe-
cuted. During execution, the program may modify the instruction in one of three ways:
• Indexing (Address Modification)-A quantity in one of the index registers
(B
b
)
is added to the lower 15 bits of F for word-addressed instructions, or to
the lower 17 bits of F for character-addressed instructions. The signs of Bb
and F are extended for the addition process.
• Indirect Addressing - The lower 18 bits of F are replaced by new a, b, al1d m
designators from the original address M (modified if necessary, M
=
m
+
B
b
).
• Indirect Addressing (load and store index instructions) - Bits 00-14 and 17 of
F are replaced by new a and m designators from the original address M (no
modification possible).
After executing an instruction, a Normal Exit, Skip Exit or ·Jump Exit is performed.
F is displayed on the console whenever the keyboard is inactive and the computer is
not in the GO mode.
C Register (Main Control)
Quantities to be entered into the A, Q, B or P registers or into storage from the entry
keyboard are temporarily held in the Communication (C) register until the TRANSFER
switch is pushed. If an error is made while entering data into the Communications
register, the KEYBOARD CLEAR switch may be used to clear this register.
The C register holds words read from storage during a Sweep or Read Storage opera-
tion. The contents of C are displayed on the console whenever the keyboard is active.
P Register (Main Control)
The P register is the Program Address Counter. It provides program continuity by
generating in sequence the storage addresses which contain the individual instructions.
During a Normal Exit the count in P is incremented by 1 at the completion of each
instruction to specify the address of the next instruction. These addresses are sent via
the S (address) Bus to the specified storage module where the instruction is read. A
Skip Exit advances the count in P by 2, bypassing the next sequential
inst~uction
and
executing the following one. For a Jump Exit, the execution address portion of the jump
instruction is entered into P, and used to specify the starting address of a new sequence
of instructions.
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