Peculiar HP 97 card reader quirk

[Dave Britten]

I was experimenting with my 97 a bit, and discovered an odd quirk when reading cards. This was actually inspired by my TI-59, having recently gotten its card reader fully operational. On the 59, you can force a side of a card to be loaded into any of the four 240-byte banks of its 960-byte memory, even if it's not the original bank that the card was recorded from. It's a little bit like the MERGE operation on the 67/97, though you can't specify exactly which program step to load to. This works with both program and data cards.

It dawned on me that this is a handy way to quickly verify a card that you've just written: simply read it into a different (higher) bank that's either empty or partitioned as data registers. If you get no error, the card was recorded properly, and you don't risk blowing away the program you've keyed in if there is a read/write error. Handy! HP 65/67/97 owners are probably familiar with the strategy of recording two copies of a program card on the off chance that one of the copies fails to write cleanly.

So I started experimenting with the MERGE function on my 97 to see if this card verification operation could be replicated. Obviously, you can't force the 97 to load a program card into the data register part of memory, but according to the manual, you can position the program pointer to the last step in program memory that you want to keep, press f MERGE, and feed in the card. So why not GTO .224 and then MERGE, effectively reading the whole card, but not actually loading the program steps anywhere? As long as you don't get a read error, you should be good, in theory.

But that's not actually what happens! At first I was starting to suspect a problem with the cards I was trying to read, but it turns out that GTO .224 MERGE has some weird effects. If you read a two-sided card, the first side will be read and the "Crd" prompt will appear, but when you feed in the second side, the motor runs about a quarter of a second longer than it should and the calculator reports "Error". If you try to merge in a one-sided card, there's no error, but the display register will show strange things like "-0.000000000-0" (yes, only one zero for the exponent). Pressing Enter seems to preserve the strange display, but any other operations normalize it to 0.00.

Could this possibly be some kind of interesting non-normalized number trick with useful side effects? I can't say I've ever seen this mentioned in PPC Journal. Does the 67 behave the same way?