Control Data Cyber 170 State Hardware Reference Manual page 48

Central Memory
CM Ports and Priorities
A priority network resolves access conflicts on a rotating basis, preventing
long-term lockout of any port. In case of simultaneous requests, the CP
has
priority.
The CM also has
a
refresh mechanism that may consume a maximum of 4 percent of
memory time. Refresh requests have priority over port requests. Refer
to
table 2-1 for maximum request lockout time.
Table 2-1. Maximum Request Lockout Time in Bank Cycles
Port Read or Write Requests
Refresh 1
Port 0 4
Port 1
5
Note: One bank cycle equals six clock periods, which
equals
384 ns.