My First Multitasking Operating System (Back to the Disco Era, Yeah; Really Groovy)

This is a slightly refreshed discussion from 2018. I started thinking about MINITS and sleep became impossible.

I have mentioned learning to program on an IBM 1401.  This computer processed class scheduling, attendance records, and grades for the entire Santa Monica Unified School District.  I used my knowledge and access to get myself moved from one section of German to the other because the girl that I was obsessed with was going to be in that section.  Another one of the computer lab rats, Curt, claimed no knowledge of how he had dozens of unexcused absences from P.E. and still received an A.

Anyway, the data processing coordinator for the district somehow rescued a computer pound puppy.  It was an odd dual minicomputer system named MINITS (for Miniature Time Sharing System).  It used two Interdata minicomputers, a Model 3 and Model 4, sharing a common 32K RAM space. A model 4:

The dials on the left and the push buttons at bottom allowed you to enter data into various memory locations, such as when you needed to put the boot code in to load the OS from the drum.  This was a slow process.  One friend, Curt, demonstrated his ability to enter the boot sequence accurately with his toes.

  I do not know if the Model 15 was an Interdata supported product, or something Cerf's team created. The Model 3 was for control of the teletype terminals and modems. The model 4 did the timesharing processing.

This is a model 33 teletype.  It printed and accepted keyboard input at 10 characters per second (we finally managed to find a 30 character per seconds modem and CRT to attach).

Modems were the way you communicated with a remote computer well into the 1980s. (I still remember the thrill of installing a Telebit Trailblazer modem at work and getting 14,400 bits/seconds.) It had an acoustic coupler: two rubber orifices into which the handset fit. The modem (modulator/demodulator) took ASCII electrical bytes, converted them to a variety of interesting sounds and sent them over the phone line. At the far end, another modem converted those sounds back to ASCII bytes. It was, as you might expect, less than perfect. Line noise could garble the characters. This is why teletypes operated in either full duplex or half duplex mode. Full duplex mode: you typed a character. The far end echoed it back so you knew what character it heard. Half duplex: the character you typed appeared immediately. 

An early minicomputer editor was called TECO. It was said that TECO commands were hard to distinguish from line noise.

Yes that is a paper tape punch and reader on the left side, which used this stuff:

There are indeed more than 8 rows.  One is a parity bit, and the other is some other out of band signaling information that I have forgotten.  Slow!  I once did a three pass assemble* of the Interdata Model 4 OS from paper tape as my first summer job after graduating.  (That was 12 hours of profound boredom, fixing paper tape jams and tears.  Why three passes?  No hard disk to store intermediate results, like symbol tables)

Anyway back to MINITS.  This was one of Vint Cerf's brainchildren.  The idea was a relatively cheap timesharing system.  We had scads of listings for the operating system, BASIC interpreter, and some of the utilities and games, printed out on a drum printer that was attached to the Univac 1108, that was supposed to be the mainframe to which this would be attached. 

A drum printer was one of two mainframe printers of the era. Chain printers, like the IBM 1403:

An early line printer that used type slugs linked together in a chain as its printing mechanism. The chain spun horizontally around a set of hammers. When the desired character was in front of the selected print column, the corresponding hammer hit the paper into the ribbon and onto the character in the chain. Chain and train printers gave way to band printers in the early 1980s. 

If this sounds incredibly slow: it was not. An IBM 1403 chain printer:

The original model can print 600 lines of text per minute and can skip blank lines at up to 75 inches per second (190 cm/s), while the model 3 can print at up to 1400 lines per minute. The standard model has 120 print positions. An additional 12 positions are available as an option. A print chain with up to 15 copies of the character set spins horizontally in front of the ribbon and paper. Hammers strike the paper from behind at exactly the right moment to print a character as it goes by. In later models, the print chain is replaced by a print train; print slugs instead of being mounted on a chain are placed in a track.

Drum printers used a series of type slugs mounted on a drum. A print stroke would line up the appropriate slug for each letter (perhaps all the slugs for the entire line to be printed?) and stamp them through a ribbon onto that same distinctive wide fanfold paper used by chain printers:

Because alignment of the type to the line was happening at very high speeds, the letters would often being not exactly centered in the line. You would see characters printed perhaps a few hundredths of an inch off a perfect impression vertically. I suspect this could be adjusted for better output accuracy but the listings we had in binders were all suffering this drunken vertical print alignment.

The MINITS hardware was unique. It had a large magnetic drum that did two things: every few milliseconds as the drive reached its zero track position, it generates an interrupt that told the operating system, "It is time to give another user some time."  (Clock generated interrupts?  Not sure when these appeared on other minicomputers.)  Also when it came time to swap users, the entire user context (the current contents of his BASIC program or whatever else a user was running, which I believe went from X'5800' to X'8000') would be written to the drum.  The OS would then load the next user's context and give him execution time.

How much time?  The OS had a concept of Terminal Control Block, which specified how many milliseconds he was allowed and the priority of his terminal's task.  By adjusting these numbers on the fly, you could annoy the heck out of a remote user.  (Imagine typing a key on the teletype and waiting a second or two for the echo to print on the paper.)  There was one in the math lab several thousand feet away, and another attached by 110 baud modem at Lincoln Junior High, some miles away.  We also borrowed a teletype to take home on weekends, for breaking into ARPAnet; more about that some other day.) When there were problems, either because someone was manipulating the remote user's TCB or because of hardware problems, the long-suffering, but always polite Lincoln math teacher responsible for this learning project, Mr. Winkenhower, would call us to find out why he was not having any luck. 

Anyway, I learned a lot about how operating systems worked in 11th and 12th grades.  The Control-V key would drop you into a debugger where you could by accident do enormous damage requiring a hard reset, including re-entering the boot sequence through the front panel.  The right set of common keystrokes would change RAM in areas that were lethal to the OS.  It was needed, however, to fix some problems on occasion so we could not just disable it.  A fellow student, Curt, and I wrote a utility in Interdata assembly language and then hand compiled it (we had no assembler program) that would do all the  necessary functions but otherwise block access to the debugger.*

I also learned how to redirect the illegal instruction interrupt, which would drop you into the debugger.  At some point I decided to be clever and build an illegal instruction driver that would recognize instructions that were part of the Interdata Model 70 instruction set. These were short relative jumps (branch within 256 bytes so the instruction only took two bytes instead of four) and add, subtract, multiply, and divide instructions (limited to I think 8 bit values; AHM (add halfword memory AHM register, value) and it would simulate these perfectly.  Then I foolishly started adding these instructions into the OS.  It came to a near complete standstill as basic OS functions would keep running through the illegal instruction ISR.

*Computers execute instructions stored in RAM as binary numbers usually expressed in hexadecimal.  X'5A' was the Interdata AHM (Add Halfword to Memory) instruction.  (I remember because they used many of IBM 360 mnemonics and architecture; 16 general registers all of which except 0 could be index registers; and where X'5A', was a 32 bit add of memory to register on the 360, X'5A' was AHM 16-bit add of memory to register on the Interdata),   It not being very easy for most people to remember all the numbers, we generally wrote in assembler language.  AHM 5,VALUE would add the 16 bits at symbol VALUE to register 5.  The program that converted these easy to remember mnemonics into binary was called an assembler.

A Depressing Day: This May Be the End of Electoral Reform

The Babylon Bee a few days ago made fun of Republicans who decided gas prices mattered more than mutilating children. There is doubtless some truth to that. The failure to adequately consider plans for closure of the Strait of Hormuz is perhaps Trump's biggest blunder.

There are signs, however, that the problem is worse than Republican voting.11/3/26 Gateway Pundit:

Pennsylvania resident Jean Franco Urbaez arrived at his polling place in Lower Macungie Township, PA, to vote in person. To his surprise, someone had already voted in person using his identity at 8:11. Urbaez videotaped himself making the discovery until he was told he was not allowed to videotape in the polling place.

This is no surprise. A friend in the Bay Area tells me in 2024, he tracked his mail-in ballot from his mailbox to the election center. To his surprise, he was able to track his wife's mail-in ballot, which was counted the day after the election. She is not a U.S. citizen and has never registered to vote, or voted.

11/3/26 Fox News:

Voting machines in Westchester County, New York, the state's seventh most populous county, are not accepting paper ballots at polling locations throughout the county, the Westchester County Board of Elections said in a statement Tuesday morning.

"The Westchester County Board of Elections (BOE) is aware of an issue involving the paper ballots that is preventing some voting machines from accepting ballots at polling locations throughout the County," the statement read.

The board, in a separate statement issued later in the day, said the issue stemmed from an update to the county's election programming that occurred after initial testing of equipment.

Oh yes, I believe that. As a British friend liked to say, "There are fairies in my garden, too." 

A friend of his moved with his wife to West Point, N.Y, where they voted in the 2024 election. Someone voted in their name in the Bay Area, likely not for the same president.

I was really impressed that Newsom won the recall election after COVID. If anyone should have been vulnerable at that point, it was Newsom. My wife thinks all elections are now so corrupted in Blue States that elections no longer matter.

I suspect until the government starts pushing Jews into boxcars for re-education camps (and the rest of us oppressive white people, shortly thereafter)*, this will continue, with the pretense of elections. 

*This is where it should end. Quite forcefully.

In Spite of Socialist Victory Yesterday, Markets Are Way Up Today

I suspect that in spite of DSA power, the Democrats remain fascist and corporations (especially favorite ones) will still do well.

The Babylon Bee joked a few days ago about Republicans deciding that high fuel prices were more important than child genital mutilation and Irsnian nuclear ambitions. 

Way More Interesting Than Expected

James Thacher's Military Journal 75-6 (1823) recounts his contemporaneous and often eye-witness experience of the war. Some of these stories are not new to me or likely you but there are details that are for me:

October. By some gentlemen from head quarters, near New York, we are amused with an account of a singular machine, invented by a Mr. D. Bushnell of Connecticut, for the purpose of destroying the British shipping by explosion. This novel machine was so ingeniously constructed, that, on examination, Major General Putnam was decidedly of opinion that its operations might be attended with the desired suc- cess ; accordingly he encouraged the inventor, and resolved to be himself a spectator of the experiment on the British shipping in New York harbor. Mr. Bushnell gave to his machine the name of American Turtle or Torpedo. It was constructed on the principles of submarine navigation, and on trial it has been ascertained that it might be rowed horizontally, at any given depth under water, and the adventurer, concealed within, might rise or sink, as occasion re- quires. A magazine of powder was attached to it in such a manner as to be screwed into the bottom of the ship ; and being now disengaged from the ma- chine the operator retires in safety, leaving the in- ternal clock work in motion ; and at the distance of half an hour, or an hour, the striking of a gun lock communicates fire to the powder, and the explosion takes place. It was determined to make the experi ment with this machine in the night, on the ship Eagle, of sixty four guns, on board of which admiral Lord Howe commanded. General Putnam placed himself on the wharf to witness the result. Mr. Bushnell had instructed his brother in the management of the Torpedo with perfect dexterity ; but being taken sick a sergeant of a Connecticut regiment was selected for the business, who, for want of time, could not be properly instructed. He however. succeeded so far as to arrive in safety with his apparatus under the bottom of the ship, when the screw, designed to perforate the copper sheathing, unfortunately struck against an iron plate, near the rudder, which, with the strong current and want of skill in the operator, frustrated the enterprize ; and, as day light had begun to appear, the sergeant abandoned his magazine, and returned in the Torpedo to the shore. In less than half an hour a terrible explosion from the magazine took place, and threw into the air a prodigious column of water, resembling a great water spout, attended with a report like thunder. General Putnam, and others who waited with great anxiety for the result, were exceedingly amused with the astonishment and alarm which this secret explo-sion occasioned on board of the ship. This failure, it is confidently asserted, is not to be attributed to any defect in the principles of this wonderful machine ; as it is allowed to be admirably calculated to execute destruction among the shipping.

The explanation that I have read in the past is that the failure to get an adequate bite into the copper bottom meant that turning the screw just spun the submarine. This makes more sense. 

I Hope This Was Intentionally Unclear

Headline from 11/2/26 BBC News:

Man caught in car with £14k of drugs as a lamb ate chips in the back seat avoids jail

The good news: the lamb was not handcuffed or mug shotted.

It is still a fetching picture of the lamb in the back seat 

Natural Tunnel State Park

Just over the border in Virginia. Railroad runs through a natural tunnel exposed by collapse of sinkhole at both ends. Really big sinkhole, like 300 foot cliffs.

The train still runs daily delivering coal to Eastman Chemical.  The guide said it was for hydrocarbon extraction. 

The trail down to the tunnel is somewhat demanding but there is a chairlift to the bottom. 

Upgrading Broken Focuser

I think I mentioned that a couple telescope parts did not survive the move undamaged. One of them was the refractor's focuser. I bent the damaged parts back into what looked to be the original shapes, but the focuser is rough and it binds. before the move, I had considered replacing the focuser with a dual-speed focuser. These have two knobs; one is the coarse focus typical on telescopes; the other turns at 1/10th speed, allowing very fine focus changes. So I ordered the GSO 2" Linear Bearing Crayford Focuser (made in Taiwan). It is an gorgeous piece of machining:

The only downside is that the flange is 112.5mm OD and the refractor's tube is 125mm ID. So, I need to make an adapter. To quote the opening sequence from The Six-Millionaire Dollar Man:

Gentlemen, we can rebuild him. We have the technology. 

At least, I do. I have a 6" 4-jaw chuck. No, Sherline lathes do not normally go this large. In the past, I have made riser blocks for the headstock and tool holder to allow turning big stuff. 

1. I will start with a 6" x 6" aluminum square.

2. Drill and tap a 3/4"-16 center hole. (This is the Sherline headstock spindle size as well as the size of the rotating table adapter.)

3. Remove 6" chuck.

4. Rotate workpiece onto headstock spindle.

5. Turn workpiece to 124.8mm OD.

6. Make riser block for the tilting table on which rotating table will sit. (I need the clearance to rotate this large of a workpiece.)

7. Tilting table to 90 degrees.

8. Put workpiece on rotating table adapter.

9. Turn workpiece 120 degrees at a time. 

10. Center drill each hole.

11. Repeat for two more holes.

12. Replace center drill with twist drill for a through hole deep enough to get through the 112.8mm layer.

13. Return workpiece to 6" chuck on headstock spindle.

14. Use cutting tool to cut 105mm hole.

15. Use boring bore to enlarge center hole to 112.7mm.