HP41 Card Reader 1E


Hi members and guests, I was playing around with the card reader on my HP41cx and came up with a strange display. With the card reader plugged in, I gave the command SIZE 25 not 025 just key in a 2 and a 5. Then with the back arrow key, delete the 5 then delete the 2. You will end up with a display that shows the number 2 on the left side of the display and the word SIZE on the right side of the display. The back arrow will not clear the display but any other key will. Has any one seen this before?


That's a well-known bug in early versions of the card reader ROM. It happens with any function that gives a three-digit prompt, and I think it may happen under some circumstances for two-digit prompts.

I'm not sure whether it is fixed in 1F, but I believe it is fixed in 1G. A more serious bug fixed in 1G is that the card reader can corrupt the contents of an extended memory module in port 2 or port 4.

I spend some time studying the backarrow bug last month, and found that it was due to the card reader ROM overflowing the four-level hardware return stack in the I/O service poll.

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