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Comment by unsui | Hacker News Reader

My favorite was the "TYPO II" ("Type Your Program Once") application, which was part of every Antic! Magazine program listing:

https://www.atarimagazines.com/v3n9/TYPOII.html https://www.atarimagazines.com/antic/

This was wrapper around the BASIC interpreter that printed out a 2-character checksum of each entered code line.

The magazine printing also had an associated 2-character checksum for each line. Your job: make sure the checksums matched.

As a teenager who only had cassette-based storage (couldn't afford a disk drive) and was addicted to typing in programs from Antic! and ANALOG magazines, this was a lifesaver.

(ANALOG's checksum program wasn't quite as convenient, and, IIRC, required a disk drive?)

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I took a look at the listing[1] and looks like it contains unprintable characters, maybe they were ASCII art of some sort?

The checksum algorithm is fairly simple: line 32150 sums the products of all character positions and character codes, and lines 32160-32180 does a modulus to convert them to printable characters. The multiply-by-position bit is clever because it allows the checksum to flag transposed characters. ISBN-10 uses a similar scheme[2].

[1] https://www.atarimagazines.com/software/displayfile.php?file...

[2] https://en.wikipedia.org/wiki/ISBN#ISBN-10_check_digits

I naturally went down a rabbit hole to see if I could find why those characters weren't printing properly.

https://www.atarimania.com/mags/pdf/Antic_Vol_7_No_4.pdf

On p. 31, you can see the intended characters.

I now remember that Atari actually had their own variant of ASCII, called ATASCII:

https://en.wikipedia.org/wiki/ATASCII

Atari 8-bits were actually really cool computers, in that they let you do things like redefine character sets entirely (to create custom character sets to effectively create tile-based displays), play with display-list interrupts, etc.

Nice find :)

I guess Atari character set has enough overlap with ASCII, so I could get the checksum to match:

    sum = 0
    "32000 REM TYPO II BY ANDY BARTON".codepoints.each_with_index{|c, i| sum += (i + 1) * c }
    print ((sum % 676) / 26 + 65).chr, (sum % 26 + 65).chr, "\n"

(Ruby code, outputs "WB")

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