Control Data Cyber 170 State Hardware Reference Manual page 166

PP InputlOutput Instructions
641cm Test and set channel c flag SCF m,c
If the C h a ~ e l c flag is set, this instruction causes a
jump
to m. If the
chamel c flag is clear, it sets this flag and continues with the next
instruction. When m is set to P plus 2, the channel flag is unconditionalLy
set when the program reaches P plus 2 .
If two or more PPs simultaneously issue this instruction for the same channel,
the conflict is resolved as follows:
If one of the competing channels i s channel 17 (maintenance channel), the PP in
the lowest physical level sees the true condition of the flag; the other
conflicting PPs see the flag set (and hence take a jump). I f the competing
chamel is any other channel, software must resolve the conflict. Any five
consecutively numbered PPs (in the same barrel) issue instructions at different
times.
651cm Clear channel c flag CCF m,c
This instruction clears the channel c flag. The m field is required but is not
used.