I say quirk since I am not sure it is a bug although I am inclined to say it it.

My Orwell computer is using MS Basic. I took the OSI version and have been modifying and adding to that as I work though my reviews of the 80s Usborne books. I just found some interesting behavior in a listing from "Creepy Computer Games" in a game called Ghost Guzzler.

This game prints a number on screen that moves across to the right. On the right is a : (your ghost guzzler) and another number you control. You make your number match the moving one and when the moving one hits the guzzler it eats it if it's the same. If different you lose a life. The line of code in BASIC of interest is this:

120 PRINT TAB(I);N;TAB(18);":";Y

Where I is the position of the number. N is the number and Y is your changing number.

I starts at 1.

When I run this it it starts printing the number in the third column, not the first one. This seems to be for two reasons.

1. In this version of MS Basic TAB seem indexed from 0. So TAB(0) is the first column, TAB(1) is the second column.
2. When you print a numeric variable it will print it with a space in front of it.

These two things together mean the first character is actually printed in column 3.

Out of interest I found an online Vic 20 emulator and that shows the same behaviour. I also found an online Applesoft emulator and that doesn't. It handles TABs differently and also doesn't print a space before variables.

So I compared the assembly. The relevant bits are below:

MS BASIC (My Orwell BASIC):



Applesoft Basic:



It seems there is a difference in how it checks for how many spaces to print and when it does the decrement of the counter for the number of spaces (which is in x) but I haven't quite got my head around why in the first case it always prints one extra space?

So, would you call this a bug or not? I think it is. Interestingly it means if you enter the listing as it is in the book the program won't work properly on VIC20s!

I am writing up the actual review to the book now and will post the link here when done.

Simon

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My 6502 related blog: http://www.asciimation.co.nz/bb/category/6502-computer

Take a look here: http://www.pagetable.com/?p=46
It describes differences and bugfixes between MS Basic versions. Maybe you can find something there.

-Tor

Yep, that's where I originally got my version from but it doesn't mention this specific issue.

I put up my latest Usborne book review here. Is late here so not checked for typos yet!

http://www.asciimation.co.nz/bb/2014/01/08/review-usborne-creepy-computer-games

Simon

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My 6502 related blog: http://www.asciimation.co.nz/bb/category/6502-computer

I remember stumbling over the leading space in PRINTing a variable for almost every BASIC I used at the time. It is the absence of a negative sign in the number which causes a space to be printed instead of a plus sign. Try printing a negative variable and see, if you still get a space ahead of it!

And no, I don't think it qualifies as a bug. A shift of a tabbed number between rows because of changing sign is not very nice to look at.

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6502 sources on GitHub: https://github.com/Klaus2m5

That's it exactly! I tried it with a negative number and yes you are correct, it prints with no space. Thank you!

I also decided that the TAB indexing isn't a bug either since the TAB in MS Basic is saying move the cursor this many spaces. Saying move 0 spaces, TAB(0), is a valid thing to do.

Thanks again. I knew someone would have seen this.

Simon

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My 6502 related blog: http://www.asciimation.co.nz/bb/category/6502-computer

I've used both flavors of MS-BASIC over the last 30+ years, and I can tell you that it's less frustrating figuring out how to add the leading space character than figuring out how to eliminate it, especially in tight spots.

Mike