The story of HPL on the HP 9825


Though this is an RPN-loving crowd, I wanted to let everyone know that the story of how the HP 9825 got and nearly lost HPL is now up on my site. See it here:


Steve Leibson


Great article!

Is HPL documented anywhere?


John Limpert wrote:

> Is HPL documented anywhere?

Try the 9825 Operating and Programming Reference Manual



Interesting article, it seems to imply that there are no known bugs in the 9825A (or B), is this the case?


Barney Oliver found what he thought was a bug (this is in addition the phone call mentioned at the end of the article). When connected to an HP 9862 plotter, an HP 9820 would lift the plotter pen automatically when it executed an "end" statement. The HP 9825 did not. You had to add a "pen lift" command to your program if you wanted the pen raised. Bug or operational difference? You decide.

Other than that, I am the only person that I know of who put a latent bug into the 9825 firmware. I wrote the 98225 Systems Programming ROM about two years after the HP 9825 shipped. It performs two extraneous writes to instruction memory (bad). On a production 9825, these writes are innocuous because they're directed at ROM, which can't be written, so the bug has no effect except to introduce a slight 2-instruction delay. However, the bug was later discovered when HPL was ported to the HP 9826, which loaded its firmware from a floppy disk and stored it in RAM. Someone (not me) had to find out why a couple of words in the firmware were being modified, which was how the bug finally came to light.

Again, it wasn't a bug in the 9825 because it didn't cause improper operation and I didn't catch it because the 9825 hardware simulator I used for debugging my code didn't allow writes to instruction memory (if I recall correctly). However, it was a real bug in the 9826 code, but this was fixed before the code shipped to customers.

The HP 9825 language ROM was never recalled.



Your page is indeed a valuable source of information on the history of computers. Congrats!

About the bug in the System Programming ROM, may I quote your website?

HP initiated a project, code named “Skoal,” to find a way to cram more RAM into the HP 9825A’s already filled-to-capacity memory space. The result was the HP 9825T, which incorporated a special state machine that looked at each memory-access operation and determined if the operation was directed to RAM or ROM. Obviously, store operations were always directed at RAM (with one strange exception caused by a latent bug in the Systems Programming ROM). Memory reads had to be analyzed using knowledge about the operating-system code itself

found at

This made me smile...



Thanks for the note. What did you want to use the quote for?



Hi Steve,

I just wanted to quote the paragraph in this thread (and I have done that right away), since it contained information about the bug in the System ROM and its consequences.

Have a nice day! Klaus

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