
Progressive Rotate


The CDC6000 series has instructions for testing the highorder (sign)
bit of a register but no other bit addressing. Therefore: 

Test bits by rotating them into the sign position. All rotates are
equally fast so bits can be tested in any order. A register need not be
restored to its original positon between tests so long as the current postion
of the register is known before each test. In complicated test sequences
additional rotates may be added to balance the paths. 

Use the instruction sequence:
LXi ab
PL Xi,label
where a is the bit number currently in the sign postion and b
is the bit number of the bit desired there. Caution, CDC convention numbers
bits from 0 to 59 starting at the least significant bit, while DEC
convention numbers the PDP8's bits from 0 to 11 starting at the most
significant bit. CDC code is written using the CDC convention. 