|Re: 2 articles about it|
Message #1 Posted by Robert Blaschke on 23 July 2003, 4:24 a.m.
after browsing some of the wonderful PPC CDs within the last nights I found an appropriate article in Datafile Vol.5, No.2, March, April 1986.
It's from Mark Power and says:
"Screen Scroll Bug
I have recently discovered a bug in the error handling of my HP41CV (serial number 2349S40141). The problem is demonstrated by running the program given below.
01: LBL TST
03: "** HP41-CV *"
05: LBL 01
06: SF 25 (Ignore error)
07: SIN (Short delay)
08: GTO 01 (No LBL error)
09: GTO 01 (Do it again)
It seems that the calculator forgets that there is a message on the screen and treats it like the running symbol and scrolls the screen when the machine performs the GTO which creates he error.
If you stop the program and look at the ALPHA registers you will see that the original message is still intact. This bug is not of great importance but I hope someone will put it to good use.
There follows a remark from the editor:
"(Ed - This is one of the display bugs which all HP41's have. These bugs are described in Appendix C of "Extend Your HP41".)"
Now let's take a look into W.A.C. Mier-Jedrzejowicz' great book:
If you VIEW or AVIEW something and execute an instruction that clears flag 25 then the contents of the display will move around just as if they were the "flying goose". Enter and run the following short program:
LBL "DSPL", PI, SF 25, "?", AVIEW, SF 60, LBL 01, ATAN, GTO 01, END
SF 60 generates an error and clears flag 25. Flag 50, the message flag is cleared at the same time, but the display is not cleared, so the HP-41 acts as if the display had been cleared and the "flying goose" was in it, but actually moves whatever is in the display. The ATAN is just a delaying tactic, without it the display would move too quickly in the LBL 01 loop. You can use this bug to display your favourite message while a program is running, but the message must not be more than 12 display positions long (it can contain extra punctuation though). You can experiment with interesting displays, for example try the string " . , . , . , . , . , . ," instead of the "?" in the program above. Reference: PPCCJ V6N8P24"
That's for the moment. Now we have 3 different examples for creating this nice effect and there is no more need to wonder about flag 99 ;-)
In case I find more interesting articles about this fyling goose replacement I'll let you know.
Best regards and happy programming
Edited: 23 July 2003, 4:28 a.m.