Page "CDC Cyber" Paragraph 13

As with predecessor systems, the Cyber 170 series had eight 18-bit address registers ( A0 through A7 ), eight 18-bit index registers ( B0 through B7 ), and eight 60-bit operand registers ( X0 through X7 ).

Seven of the A registers were tied to their corresponding X register.

Setting A1 through A5 read that address and fetched it into the corresponding X1 through X5 register.

Likewise, setting register A6 or A7 wrote the corresponding X6 or X7 register to central memory at the address written to the A register.

A0 was effectively a scratch register.