In the 1980s we downloaded games from the radio

Sometimes the typos were in the magazine itself, and you wouldn't figure out the problem with the code you triple-checked you typed in properly until the errata in next month's issue :)

Also typeset in a non-fixed width font with long lines truncated to fit the copy layout!

The compiler/interpreter couldn’t even tell you what line the error was on!

You’d just get a big error message for the whole program.

After a while, magazines like Commodore Run and Compute started including a short program that would checksum each line as you entered it, so you could check that against a checksum in the magazine. Of course, you had to get that program typed in correctly first before you could use it to enter others.

For the zx81 i think it was usually some encoded binary form, so no compiler/interpreter involved.

I'm curious, can you say more about "as you entered it"?

Do you mean like, "for lines 1-20 the checksum should be 0xDEADBEEF"? This would let you find the error before finishing the program.

Or just at the end, it would checksum the whole thing?

Semi related, I created linesum a few years back https://github.com/moreati/linesum to line by line sha256.

My exact memory. When you did finally get everything correct, the program could take 15 minutes to load from the cassette tape. I remember upgrading my Commodore 64 with a floppy disk and loading programs in 2 minutes (which felt instantaneous by comparison).

I remember borrowing a book from the library, which had a type-in checksum program of this sort. It was done like was common for C=64 things of this kind - there's a BASIC FOR-loop iterating through a memory area, reading in bytes from DATA statements you've typed in and POKEing those bytes into memory, not completely unlike entering a program manually from the front-panel switches of an older computer.

So, after typing that in and probably SYSing (C=64 BASIC command for executing machine code from arbitrary memory location) to some address, it did print out a two-digit (eight-bit) hex checksum after every BASIC line I entered on the C=64 and the program listing in the book had the correct checksums for every line, so spotting errors was more or less instantaneous.

This stuff brings memories.

  FOR I=40960 TO 49152:POKE I,PEEK(I):NEXT I
  POKE 1,54

From top of my head; loop through the BASIC interpreter area, reading byte by byte with PEEK and POKEing those bytes back to the same addresses. Sounds nonsensical? Not so, because the C=64 does have full 64 kB of RAM, but some of it is overlapped by ROMs. What happens here is that you're reading from ROM but writes always go to RAM, so you're copying the BASIC interpreter from ROM to RAM. After that, the POKE statement turns off the ROM overlap and the interpreter is now run from RAM, so you can edit it live - and obviously cause all sorts of interesting crash situations.

It sure did help later with career in IT to have understood this kind of stuff at age of around ten.

A checksum for each line of code. COMPUTE! magazine used one, the article introducing it and explaining how to use it is at https://archive.org/details/1983-10-computegazette/page/n49/....

The code listings had a comment (rem) at the end of each line with a checksum number, when you used the checksum program it would display a checksum at the top of the screen that would match if you entered the line correctly.

An example page of code with checksums is at https://archive.org/details/1983-10-computegazette/page/146/....

A life changing event for those of us entering code from magazine listings in the early '80s.

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?)

I never had a tape deck, and was constantly flustered by “press play on tape” messages.

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")

That's awesome. Thanks!