12/24-bit CPUs, was Re: Oregon Software Pascal/2

On Oct 13, 2006, at 2:32 PM, woodelf wrote: I unfortunately don't have it (I had a chance, but didn't have a place to put it), but in high school, the school had a 24-bit computer. Quite an interesting machine. It used 8K-word magnetic drum memory. There were two complete redundant CPUs, each with their own 8K drum. There was a "shared register" between the two machines that could be used for inter-CPU communication. The machines also shared a very interesting I/O system that was housed in a large cabinet. The I/O system consisted of a Current Loop interface for an ASR-33 teletype (as well as paper tape reader control), an interface for a wide-carriage IBM output typewriter (individual typebars, not a Selectric-style device), and a large number of digital input and output ports, a number of programmable counters (to be used as interval timers), along with a bunch of A/D and D/A channels. There was also a BCD real-time clock which kept track of the time of day in hours/minutes/seconds, and also toggled a bank of flip flops at various rates which could be polled (no interrupts existed on the machine) to track time in various increments (1/2, 1/4, 1/8 and 1/16-th second intervals, if I remember correctly). There was also a set of thumbwheel numeric input switches which could be read, which, if I recall correctly, was 6 digits long, BCD encoded. The machine was all transistorized. No ICs anywhere. The machine was made by 3M Corp. (Minnesota Mining & Manufacturing). It was a process control machine. Its original use was as a natural gas pipeline pressure monitoring and control system by Northwest Natural Gas Co. in Portland, Oregon. After it was retired, it was donated to our high school. When I started at the high school, one of the drums had failed, so one CPU was inoperative. But, the other worked well. It was a two address machine, with operand address, and next instruction address. Addresses were of the form block/track/sector as the drum was organized. The instruction set was pretty simple, there were five bits in the opcode. There was an accumulator (24 bits), and a "B" register which could be used for temporary storage. The accumulator and B register were discrete registers, not located on the drum. Master timing for the machine was generated by a pre-written clock track on the drum, so all operations of the machine were synchronized with the drum rotation. Some kind of phase locked loop or the like was used to keep the drum rotating at a consistent speed (I think it was 3,600 RPM). The instruction set documentation had a "for optimum programming" section for each instruction, which provided formulas for the operand address and next instruction address to minimize the access time for the instruction. Typically, the operand address was the current address + 3, and the next instruction address was the current address + 6. Using these formulas would mean that the operand would be under the head after the instruction was fetched and decoded, and the next instruction would be under the head when the operation was completed. There were no index registers or base registers or anything like that. Addressing tables and such had to be performed by instruction modification, e.g., load an instruction into the accumulator, increment (or add 3 for optimum programming) to the operand address, store the accumulator back over the top of the instruction, then execute the modified instruction. Made list processing, stacks, and the like rather tedious. Each CPU was in a "drawer" about 8 to 10 rack units high. The drum was located in the center of the chassis, with circuit boards (which, IIRC, were about 7" long, and 5" wide) plugged into a backplane in the shape of a "U" around the drum. The two CPUs mounted in a small rack one atop the other. Controls for the CPU were simple. STOP, START, CLEAR, and LOAD. Clear cleared all of the working registers, and set the program counter to Block 0/Track 0/Sector 0, and halted the machine. Stop halted the computer after the execution of the current instruction was completed. START began program execution. LOAD put the machine into a hardwired loader that would allow address/data pairs to be loaded into the drum expected in octal, with a space separating the address and the data. These controls were momentary pushbuttons, with the START, STOP, and LOAD buttons lit to indicate when the machine was running, halted, or in LOAD mode. Five indicators would show the opcode of the current instruction. I seem to recall that it was possible to single-instruction step the machine by putting the machine into STOP mode, then holding the STOP button down when pressing the RUN button at the same time, but this memory could be incorrect. There was a two-pass assembler called SOAP that could be loaded from tape, then the source run through the paper tape reader twice, with the resulting object code stored on the drum. SOAP was pretty big, though, about 4K resident, which meant that you couldn't write anything much larger than about 4K in size with the assembler. I had spent time at the Oregon Museum of Science and Industry (OMSI) on their "Straight 8" PDP-8 System, and had learned FOCAL. I decided to try to write FOCAL on the 3M machine. I actually got to it to the point where small programs could be entered, edited, and executed. It did integer math only (24 bit), as floating point simply wouldn't have fit. When a line of code was entered, and the RETURN key hit, it would take about 1.5 seconds for the line to be parsed, error checked, and stored. This machine was not very fast. I wrote quite a few programs for this machine, including a blackjack game, tic tac toe, an alarm clock (you'd enter the time on the thumbwheel switches, and when the time came along, the teletype would ring the bell). Never got the IBM output typewriter working. It had mechanical problems (probably needed a thorough cleaning and lubrication). After I graduated from high school, the machine didn't get much attention. Someone tried powering it up at one point, and the drum in the still operating CPU failed..bearings went bad, and (fixed) heads crashed. The school eventually moved to a new building two years after I graduated, and that's when I had the opportunity to take the machine, but I had no place to put it, nor any way to move it. It was rather large (and heavy). The cabinetry was all painted a dark blue color. A white formica table top sat on top of the two CPUs in their small rack, and on top was a console with the real-time clock (using incandescent displays that had a GE-47-style lamp that shined through a piece of film that had the digit on it, which projected the digit on the a satin-finish plastic display screen). The clock was not made by 3M, but by someone else, was about 6 rack units high, and had pushbuttons to manually advance the hours, minutes, and seconds. It was implemented with transistor logic, connected as ring counters for each digit. Also on this panel was the six-digit thumbwheel numeric entry device. The IBM typewriter sat to the right of this panel. The controls for the CPUs were mounted on the front plate of each CPU drawer. To the left of the CPU drawers was the large cabinet that housed all of the I/O circuitry. It was crammed full of transistorized modules that plugged into a massive backplane. Each CPU had its own power supply, as well as the I/O cabinet, and the realtime clock. Each CPU needed a 20A 110V circuit. The I/O cabinet needed a 30A 110V circuit. The clock, IBM typewriter, and Teletype had their own 20A 110V circuit. I've been unable to remember the model number of this machine. I recall that it was 4 digits long. At one time, I had a big binder which contained a bunch of documentation for the machine, including the instruction set information, some information about the I/O subsystem programming, and also complete schematics for the CPU. I've searched and searched for it, and can't find it -- so I think that it must have been lost or accidentally tossed in one of many moves. I've searched high and low on the web looking for any references of any computers made by 3M, and have found nothing. My guess is that they didn't made computers for a very long period of time. My recollection is that the date on the documentation was 1965, but not sure of this. Anyway, probably a whole lot more here than anyone cares about, but it is my hope that perhaps someone out there may recall a machine like this made by 3M. Sure had a lot of fun playing with that machine. Learned a great deal system (initially, HP 2000C, advancing over the years to a 2000C', 2000F, then 2000/Access) which were accessed by dial-up ASR-33 teletypes. A "closed" BASIC environment, which was great, but no machine-level programming. I learned a great deal about "how computers work" from the old 3M machine. Thanks for letting me spew out all these memories. Rick Bensene The Old Calculator Web Museum http://oldcalculatormuseum.com