A CAL TSS debugging tool

One of the artifacts preserved from the CAL Timesharing System project is a deck of 14 80-column binary cards labeled “TSS PM DUMP”. This is a program for a CDC 6000 series peripheral processor unit (PPU) to perform a post mortem dump to magnetic tape of the complete state of a (crashed) system: PPU memories, CPU memory, exchange package, and extended core storage. Another system utility program, TSS PP DUMP-TAPE SCANNER, allows selective display of portions of the dump to either the teletype or one of the system console displays. I believe Howard Sturgis wrote the PPU program and Keith Standiford wrote the CPU program.

I suspected this card deck was a version of a PPU program called DMP, for which a listing exists. Carl Claunch very generously offered to read (digitize) the cards. He produced a binary file tss.hex. I queried the ControlFreaks mailing list re a PPU disassembler, and Daiyu Hurst sent me a program ppdis.c that generates a listing with opcodes resolved and addresses, octal, and textual representations of each 12-bit word. After upgrading it to eliminate some K&R C function definitions and changing its input format to match tss.hex, I ran it, captured the output, and then began annotating it. As expected, it matched the DMP listing very closely, so I used those variable names, labels, and comments to update the output of ppdis.c, and added a few additional comments, including slight differences from the DMP listing.

The first card is a loader that loads subsequent cards up until one with a 6-7-8-9 punch in column one is encountered. The first card is loaded via the deadstart panel. Page 14 of the CAL TSS Operator’s Manual explains:


       Unfortunately, the dump program requires a different deadstart panel from the system dead start program. Reset the deadstart panel to
CAL TSS I, push the deadstart button, read the deck “TSS POST MORTEM” into the card reader, mount a tape on unit 0, and stand back and
watch it go. After the tape unloads, reset the deadstart panel to CAL TSS II and dead start the system as usual. Record the reel on which
the dump was made along with the other information relevant to the crash.

The Operator’s Manual also contains a set of CAL-TSS FAILURE LOG forms recording crashes and attempts to diagnose them.

The program on the cards is very similar to the DMP listing (which doesn’t include the loader card), with slightly different addresses and one or two small changes in the code.

The general structure of the program is to dump PPU 0 (whose memory is partially overlaid by the DMP program), then use this working space to dump PPUs 1-9, the exchange package (CPU registers), the CPU memory, extended core storage, and finally write a trailer record. The console display is used for operator messages: mounting a tape on drive zero, progress messages indicating which phase is taking place, and several error messages.

This doesn’t sound like a terribly difficult task, but it requires about 1000 instructions on the PPU, which has 12-bit words, one register, no multiply or divide, and an instruction time of 1 to 4 microseconds. There are some additional complications:

  1. A PPU can’t access the memory of another PPU. When the overall system is deadstarted, PPU 0 begins running a 12-instruction program loaded from toggle switches on the deadstart panel, and the other PPUs are each suspended on an input instruction on a different I/O channel. Thus PPU 0 sends a short program to each one instructing it to output its own memory on a channel, which PPU 0 inputs and then outputs to the tape drive.
  2. Similarly, a PPU can’t access extended core storage (ECS). So PPU 0 repeatedly writes a short program to the CPU memory that reads the next block from ECS to CPU memory, then does an “exchange jump” to cause the CPU to execute that program. The PPU then reads the block from CPU memory and writes it to tape.

Here is the annotated listing.

Update 2024/01/18: Linked to the copy of the listing (etc.) at https://caltss.computerhistory.org.

CAL Timesharing System: Before computers were personal

In 2023 computers are all around us: our phones, tablets, laptops, and desktops, and lurking inside our television sets, appliances, automobiles, to say nothing of our workplaces and the internet. It wasn’t always that way: I was born in 1949, just as the first stored-program digital computers were going into operation. Those computers were big, filling a room, and difficult to use. Initially a user would sign up for a block of time to test and run a program that had been written and punched into paper tape or 80-column cards.cThe fact that an expensive computer sat idle while the user was thinking or mounting tapes seemed wasteful, so people designed batch operating systems that would run programs one after the other, with a trained operator mounting tapes just before they were needed. The users submitted their card decks and waited in their offices until their programs had run and the listings had been printed. While this was more efficient, there was a demand for computers that operated in “real time”, interacting with people and other equipment. MIT’s Whirlwind, TX-0, and TX-2 and Wes Clark’s LINC are examples.

The ability to interact directly with a computer via a terminal (especially when a display was available) was compelling, and computers were becoming much faster, which led to the idea of timesharing: making the computer divide its attention among a set of users, each with a terminal. Ideally the computer would have enough memory and speed so each user would get good service. Early timesharing projects included CTSS at MIT, DTSS at Dartmouth, and Project Genie at Berkeley. By 1966, Berkeley (that is, the University of California at Berkeley) decided to replace its IBM batch system with a larger computer that would provide interactive (time-shared) service as well as batch computing. None of the large commercial computers came with a timesharing system, so Berkeley decided they would build their own. The story of that project—from conception, through funding, design, implementation, (brief) usage, to termination—is told here:

  • Paul McJones and Dave Redell. History of the CAL Timesharing System. IEEE Annals of the History of Computing, Vol. 45, No. 3 (July-September 2023). IEEE Xplore (Open access)

How did I come to write that paper? In the winter of 1968-1969 I was invited to join the timesharing project. At that time I had about 2 years of programming experience gained in classes and on-the-job experience during high school and college (Berkeley). That wasn’t much, but it included one good-sized project—a Snobol4 implementation with Charles Simonyi—so the team welcomed me to the project. For the next three years I helped build the CAL Timesharing System, performed some maintenance on the Snobol4 system, and finished my bachelor’s degree. In December 1971, CAL TSS development was canceled, and I graduated and moved on to the CRMS APL project elsewhere on campus.

Those three years were hectic but immensely enjoyable. The team was small, with under a dozen people, housed first in an old apartment on Channing Way and then in the brand-new Evans Hall. Lifelong friendships were formed. People often worked into the night, when the computer was available, and then trooped over to a nearby hamburger joint for a late meal. Exciting things were going on around us. There were protests, the Vietnam War, and the first moon landings. Rock music seemed fresh and exciting. I had met my future wife in 1968, and we were married in 1970.

As CAL TSS came to an end, we all agreed the experience could never be equalled. But we didn’t realize people in the  future would be interested in studying our system, so we weren’t careful about preserving the magnetic tapes. However many of us kept manuals, design documents, and listings, plus a few tapes. In 1980 and again in 1991 we had reunions and I offered to store everything until it became clear what to do for the long run. Around 2003 I started scanning the materials and organizing a web site. In 2022  the Computer History Museum agreed to accept the physical artifacts, and this year they agreed to host the web site: