Control Data Cyber 170 State Hardware Reference Manual page 41

Registers
A Registers
The CP contains eight 18-bit A registers (A0 through A 7 ) . The A0 register
serves as an intermediate register for the user's discretion. The A0 register
is used in the compare collate instruction for the collate table address.
Also, the A0 register provides the relative CM starting address in a block copy
operation.
Registers Al through
A7
are essentially
CM
operand address registers associated
one-for-one with the X registers.
Placing a quantity into an address register
( A l through A51 causes a CM read reference to that address and transmits the CM
word to the corresponding X register
(XI
through X5). Similarly, placing a
quantity into the A6 or
A7
register causes the word in the corresponding X6 or
X7 register to be written into that relative address of CM.
8 Registers
The CP contains eight 18-bit B registers (BO through B 7 ) . These registers are
primarily indexing registers to control program execution. Program loop counts
may
also be incremented or decremented in these registers.
Program addresses may be modified on the way to an A register by adding or
subtracting
B register quantities. The B registers also hold shift counts for
the nominal B j
shifts, the resultant exponent for the unpack, the operand
exponent for the pack, and the resultant shift count from a normalize. The BO
register always contains 4, which can be used as an operand.
This register
cannot hold results from instructions.
Support Registers
Eight support registers assist the operating registers during programs
execution. The contents of the support registers are stored in CM, and their
new contents are loaded from CM during a CYBER
170 exchange sequence. With the
exception of the P register, the contents of the support registers cannot be
altered during the execution interval of a CYBER 170 exchange package. When
the execution interval completes, the data in the support registers is sent
back to CM through a CYBER 170 exchange jump.
P Register
The 18-bit program address (PI register loads from CM during the first word of
a CYBER 170 exchange sequence and contains the current program execution
address. The register serves as a program address counter and holds the
relative CM address for each program step.