50 cores

I still have a tape. It was used on a PDP-12. The PDP-12 was a machine with 2 instruction sets. I was a PDP-8 and also a machine called a Link. An I/O operation would flip it over to a Link Mode and back.

and 42 tracks.

These were all instrumentation recorders, not computer tapes.

All 1/2 inch computer tapes I have ever seen were 7 or 9 track. These drives had vertical tape columns on both sides of the R/W head.

The vacuum at the bottom of these columns kept the tape firmly at the R/W heads even during high acceleration start/stop operations, in which the heavy tape reel motors could not have followed.

Yeah that was weird. The 8 was a 2's complement machine, and Linc was sign-magnitude!

John

Yes, the Link ALU was complete with the end around carry. IIRC, the Link addressed 2K word pages instead of the 4K "fields" of the PDP-8 part.

The OS such as it was had exactly one error message "no".

In a lot of cases, the recorded data was later processed by a computer. There were companies in Texas that had rooms full of tape drives and spent all of their time reading reformatting and writing tapes.

The DEC tapes were on the order of one inch wide. The reels where all that was driven. They servoed the reel motors to make the speed right.

?0.00 DON'T DO THAT

John

The display was a vector graphics on a scope. The word "no" would appear about mid screen. The only thing that gave a clue about the error was that the size of the word "no" seemed to vary with what the error was. The command line was just the name of the program from the tape to be run and the two arguments for it. You couldn't make a huge number of different errors.

aligned

head

FOCAL printed an error code that was a hash of the program counter at the point of error, like ?4.71 . Every release, all of the error codes changed.

John

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Did it do just one error and then stop? If so, a recursive decent parser sounds like the reason. When in doubt call your self. On error, clear/reload the stack. If they cleared the stack by popping, the popped value would be the return address.

perpendicular

aligned

head

was

tape

Yes.

If so, a recursive decent

The PDP-8 had no stack. It did a JSR by writing the return address at the first word of the subroutine, and then executing the next word. So an error call was a 1-word JSR to the error handler. It fetched its own first word and printed that as the error code. Subs were not inherently reentrant nor recursive.

FOCAL-8 was a combo editor and iterpreter, with floating point and all the usual math functions, that did useful work on a 4K PDP-8. Rick Merrill sythesized stacks and reentrancy and such on top of the barbaric PDP-8 architecture. Its internals influenced the PDP-11 architecture, which in turn affected the 680x and 68K designs. Too bad about Intel.

John

End-around carry? Was it '1's complement or sign-magnitude? End-around carry implies '1's complement.

data

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systems

perpendicular

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there was

tape

recorders.

carrier.

You can still implement the method even one a machine that doesn't naturally do recursive calls. It is a method of design as well as a method of coding.

I would call the PDP-8 minimalist not barbaric. It was designed while counting transistors.

If you want barbaric, look at the CPD1802 or more common

8086. There the goal seems to be to have a large instruction set to boast about without the bother of making the things programmers really need.