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Loading PDP 11 BASIC Papertape


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  Loading PDP 11 BASIC Papertape by Bill Degnan - 09/10/2018 00:16
After replacing the reader (again) I was able to load PDP 11 BASIC using my Teletype ASR 33. The computer system is my recently repaired PDP 11/05 S. Pictured is the pile of tape the makes up the BASIC program. It takes about 20-30 minutes or so to load the program. Click image for larger view.

If everything goes right, immediately after loading the tape the system initiates the BASIC program and displays the initialization prompt, *O. Click image for larger view.

Appendix D - The Bootstrap and Absolute Loaders (step by step how to load BASIC into the PDP 11 using a Teletype).

If you're reading this and need a copy of the absolute loader let me know. These things exist on the WWW.

Don't bother to try to load a papertape until you have run a few CPU, RAM, and serial/TTY echo tests. Make sure your backplane voltages are correct and that you can store instructions in core reliably.

Loading BASIC notes -
1. One can choose to put BASIC into core below the maximum RAM space available. For example you may have a 16K system but you'd like to only use the first 8K for BASIC so you can use the upper 8K for something else. To accomplish this, when you toggle in the bootstrap the "highest available core memory bank" would be 037744 rather than 077744. i.e. enter values starting from 037744.

2. You don't need a Teletype Reader Control 4915D (a.k.a. reader run relay) card in your teletype, but you will then need to engage and disengage the start lever manually on your reader. It's ok if the absolute loader is sitting there mindlessly looping and waiting for you to hit START on the Teletype reader. If the system halts (front panel lights freeze/halt) the reader will keep going if there is no 4915D card so pay attention and disengage the reader. Otherwise the tape will keep going!

3. If you get a checksum error while loading the tape, the lights on the front panel will stop flashing and the system will halt (assuming you're using a front panel PDP 11). Immediately stop the reader. Maybe all is not lost. Re-install the tape in the reader before the point of failure and restart the absolute loader. Next, hit START on the TTY reader and the tape will resume loading. It is possible that the tape got jammed or stuck the first time. Running it through again may work. The way papertape loads one can easily start from any point on the tape as long as the entire tape is read in eventually, it's not a one-shot deal. All that matters is that all of the tape makes it through the reader correctly even if it's loaded a little out of order. The tape will tell the CPU where in memory to store the data.

3. Don't start from the very beginning of an absolute tape, bypass the DIGITAL ASCII labeling, etc. Visual inspection should reveal where the real programming starts.

4. If you have core memory, once the tape is loaded and you've initialized BASIC you can turn off your system and restart the next day - just load memory address 000 and START. Assuming your teletype is attached and in LINE mode it'll restart BASIC with the READY prompt. Old BASIC programs will not be saved if you start from 000 however, you'd have to re-load them. I am not sure as of this writing if there is a start address that saves whatever BASIC programs were in RAM at the time the system was shut off. Something to investigate.

5. You have to be a certain kind of nut to load BASIC from a papertape using a Teletype with so many moving and logical parts that can go wrong. Get a comfortable chair, you may have to repeat the process many times before it works. If your Teletype is not cooperating see if you can get a M9312 Terminator/ROM card and a serial cart (M7800 etc) for your system. With these you can load BASIC from PDPGUI, a modern Windows interface for simulating papertape and TU tape loads.

6. I am still working on how to load load a BASIC program saved to papertape when there is no run reader to control the speed or stop the reader to give the system a chance to keep up. I am thinking that it may be necessary to save the papertapes as memory rather than a script to "retype" the BASIC lines and commands.



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