Control Data Cyber 170 State Hardware Reference Manual page 40

Registers
Operating Registers
The operating registers consist of operand (XI, address (A), and index
( B )
registers. These registers minimize memory references
for
arithmetic operands
and results.
X Registers
The CP contains eight 60-bit X registers (XO through X7).
The XO register is
used in the compare instructions to indicate if two fields of characters are
equal. Also, the XO
register provides the relative
UEM
starting address in
a
block copy operation.
Registers
X 1
through X7 are primarily data-handling registers for computation.
Registers X 1 through X5 are used to input data from CM, and registers X6 and X7
are used to transmit data to CM.
Operands and results transfer between CM and the X registers as a result of
placing CM addresses into corresponding
A
registers.