Control Data Cyber 170 State Hardware Reference Manual page 235

Maintenance Channel Programming
MCH Control Words
Some function words must be followed by two 8-bit control words, which specify
the internal address of the register to be accessed. This is accomplished
by
transmitting two PP words where the rightmost 8 bits in each word are used.
Control words are required.for the following.
Function words to CP with opcodes 415.
Function words to CM and IOU with opcodes 4/5.
Function words to CP, CM, and IOU with opcode 8 (echo).
Refer to tables 5-13 through 5-15 for CP, CM, and IOU internal address
assignments.
MCH Programming for HaWStart (Opcode 0/1)
These operations consist of the output of a function word. A halt opcode halts
the processor without damaging the process being executed,.including the
integrity of the interunit communication of the halted processor such as CDC
CYBER 170 exchange 'request commuaication, central memory communications, and
the process state. If the process is subsequently restarted without performing
any other MCH operations or after performing read/write with certain
precautions, the process continues without damage.
MCH Clear LED (Opcode 3)
This operation clears all LEDs associated with
pak
errors and is intended, but
not required, for use at system initialization. For maintenance reasons, this
operation can also clear LEDs without initializing and master-clearing.