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Ask HN: What numbers do you associate with old computers?

3 points by stevengoodwin ↗ HN
I'm writing a book on retro computers and the numbers that tie them together. (Full blurb and example prose at https://unbound.com/books/20goto10/)

What are the ultimately geeky, deep knowledge numbers, that aren't as obvious as 64K, 8, or 0's and 1's? (And bear in mind, I might need an explanation!)

Thanks in advance...

16 comments

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AnimalMuppet
36. It's the number of bits you need to represent 10 (decimal) digits fixed point, or 8 digits floating point. It's enough to calculate the differences of atomic masses. A number of early mainframes had 36 bit words for this reason.

80. The number of columns in a standard punch card.

132 (?). The number of columns in an extended punch card.

16407. On a TRS80 Model 1, the address of... something. Video controller register, maybe? Poking values into this register would result in weird (and sometimes entertaining) behavior on the screen. It often took a reboot to recover from, though.

jonjacky
I've never seen any 'extended punch card', or any machine that could punch a 132-column card.

132 was the line length of IBM line printers, imitated by many other popular brands, including Printronix and DEC. They printed on that wide green-stripe, fan-fold paper, which came in a standard size that fit 132-column printers.

pinewurst
28 is the I/O block size for Univac 1100s and their descendants, originating from FASTRAND horizontal drum devices.
logotype
4000, 2000, 1200, 1000, 600, 500, 128, 64 - Commodore/Amiga
philipswood
18.2 timer ticks per second on the old IBM PC timer interrupt.
logotype
0x4489 - the MFM sync mark for Amiga Double Density (720k) floppy disks.
jonjacky
12, the number of bits in the word of the PDP-8, the famous and influential minicomputer of the 1960s-70s.

4096, the number of words that can be addressed by 12 bits -- so, the size (in 12-bit words) of the PDP-8 memory. There was a whole culture of writing useful programs that fit in 4096 words -- not as a stunt or tour-de-force, that was just the order of the day.

rachelbythebay
I see someone linked my post earlier (thanks!) but hey, I thought of a few more based on your request.

3583: BASIC bytes free on an unexpanded VIC-20. 38911: same idea for a C-64.

Those were both prominent numbers since they showed up on the screen right when you turned it on and sat there until you got cranking on something else.

eesmith
4.77MHz = frequency of the original IBM PC.

0xB8000 is the base address for the CGA video buffer on a 1980s era PC. 0xB0000 was for MDA (eg, a Hercules card).

MDA's memory range didn't overlap with CGA, EGA, or VGA, so dual-head systems were possible, eg, program display on the color display, debugger in the monochrome display.

My go-to book for the internals, with all sorts of numbers like the above, was "The Peter Norton Programmer's guide to the IBM PC", available at https://archive.org/details/peternortonprogr00nort .

For example, philipswood at https://news.ycombinator.com/item?id=28053235 pointed out the IBM PC timer interrupt was 18.2 ticks per second. Norton explains that at https://archive.org/details/peternortonprogr00nort/page/148/... - the main clock, the 8284A, oscillates at 1,193,180 times per second. The clock interrupt is every 65536 oscillations = 18.20648... ticks per second. And 1,193,180 * 4 = 4.77 MHz.

Decimal 27 = hex 1B is a number I associate both with writing ANSI escape codes (still useful today), and with writing printer commands to my Epson MX-80, like setting to quad density mode for higher quality output. With 9-pins it could make very nice graphical printouts.

300, 1200, and 2400 baud were important modem speeds in my era. The earlier Bell 101 era used 110 baud.