Rich kids are into COBOL

tony duell ard at
Mon Mar 2 10:24:32 CST 2015

> On the PDP-5/8 and the -7, I am guessing they DON'T incr the
> PC an
> extra time, they set up for increment through the main
> adder, and then
> make a carry-in if they want to double-increment the PC.

FWIW, that's basically how it is done on the HP9800 calculator processor, 
a bit-serial machine. The program counter is incremented by adding 1 to one bit, then
adding 0 to 15 bits (the register shifting after each add). The 9800 processor hardware
allows a microinstruction (like add 0) to be repeated 1-16 times, obvously a useful
feature on a bit-serial machine. For that to work, the carry flag has to be preserved
between microinstructions, of course. 

At the end of the microcode routine for each machine instruction, the microcode jumps
to this increment_P_register routine, which then goes on to fetch and decode the next 
machine instruction. If the carry flag is clear at the entry to increment_P_register then
P (the program counter) is incremented by one. But if the carry flag is set, then it
is incremented by 2, skipping the next machine instruction.


More information about the cctalk mailing list