Control Data Cyber 170 State Hardware Reference Manual page 236

Maintenance Channel Programming
MCH Programming
for
Read/Write (Opcode 4/5)
Refer to Programming for PP Data 1nput/output in this chapter for a more
complete procedure. In general terms, proceed as follows:
1. Issue the function with opcode 4/5.
2. Output the first control word.
3 . Verify the error flag is clear.
4.
Output the second control word.
5. Verify the error flag is clear.
6 . Input/output the required number of data words.
7.
Verify the error flag is clear.
Reading a nonexistent register returns all 0's.
Writing to a read-only
register or to a nonexistent register does not alter any register. Most
registers are readlwrite as 64-bit (8-byte) registers, requiring the input/
output of 8 MCH data words.
Most registers that are physically smaller than
8 bytes are right-justified with zero-fill.
Fixceptions are as follows:
a Reading a status summary register repeats the status information in each
byte.
a
The IOU may disconnect the MCH without affecting subsequent MCH operations
in the following cases :
-
After reading
1
to 8 bytes from any maintenance register.
-
After writing 1 byte to a corrected error log register.
-
After writing 1 byte to an uncorrected error log register.
The following MCH operations on CP registers can be performed with the CP
running or halted.
a Read CP status summary register.
Read CP fault status register.
Read CP corrected error log registers.
a
Read CP options installed registers.
Read CP element identifier register.
Readlwrite CP dependent environmental control register.
a
~ead/write test mode control registers.
a Clear errors.
To readlwrite other CP-registers, the CP must be running since these registers
are accessed by microcode.
Refer to the Maintenance Register Codes Booklet
listed in the preface for register bit assignments.
60463560 A