Chuck wrote: Many moons ago I did some programming on a process control computer made by 3M. The machine was a 24-bit word, bit-serial, transistorized machine with drum as main memory. Each instruction contained both the operand address and the next instruction address (in block/track/sector numbers). Depending on the timing of the instruction, the optimal address for the operand and next instruction were calculated based on offsets given in the instruction set summary. There were no index registers on this machine, only a single accumulator register (implemented as a shift-register). The only way you could do table operations or jumps based on evaluation of an expression was self-modifying code. However, this was tricky, if you wanted the code to run as efficiently as possible (minimizing waiting for rotation of the drum), you had to do optimization calculations as part of the self-modifying code. The other tricky part was the you had to be aware of track and sector numbers and properly deal with overflowing, e.g., max sector number was , IIRC, 30(octal) , so if you added 1 to the address, you'd have to clear the sector to zero, and add one to the track address, also being aware that the track number could overflow. I learned a lot from that old machine. Rick Bensene The Old Calculator Museum http://oldcalculatormuseum.com