iiinova — where your digital self keeps evolving

Jeff Jetton @jeffjetton.bsky.social · 27/09/2026

I know I'm supposed to be noticing that KIM-1, but ERMAGERD! A Heathkit H8 with pre-hex-"upgrade" octal keys? That gorgeous Sol? And sweet mother of Ada Lovelace, there's a Kenbak-1 sneaking into that first pic. A pretty rare/expensive unit to put on display like that. (Is it a reproduction?)

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Jeff Jetton @jeffjetton.bsky.social · 22/09/2026

Hey! That's the same comic book my profile pic is from! www.ep.tc/intunewithfu...

Jeff Jetton @jeffjetton.bsky.social · 14/09/2026

Well that one didn't have to travel far to get to its first home. :-)

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Jeff Jetton @jeffjetton.bsky.social · 29/08/2026

Well DEC did design the PDP-11 to be single-stepped.

Jeff Jetton @jeffjetton.bsky.social · 17/08/2026

Yeah, a lot of old minis and mainframes (and the Kenbak-1) lacked a hardware stack, so that was typically how you had to pull it off. It's not too bad once you get used to it. Of course, that prevents recursion. I'm on the fence about whether that's a minus or a plus. :-P

Jeff Jetton @jeffjetton.bsky.social · 15/08/2026

If you don't care if fractional results round up or down, you can even skip the CLC. For example, the true mean of 1 and 100 is 50.5. If carry is clear, the result will be 50 (rounded down). If it happens to be set, you'll get 51 (rounded up). Non-fractional results will be correct either way.

Jeff Jetton @jeffjetton.bsky.social · 13/08/2026

I say this *all the time* when finding my seats at a concert/theater/whatever (even though my wife has no idea what I'm talking about).

Jeff Jetton @jeffjetton.bsky.social · 12/08/2026

Yes, the difference between even a high-percentage totality and actual, full-blown, 100% totality is huge. (Like night and day, one might even say.)

xkcd cartoon showing a graph of solar eclipse "coolness" vs. how close you are to the path of totality. There is an enormous spike in coolness at the path of totality, while the area on either side is low and flat.

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7/7 The same basic idea lets you do any operation to your memory variable you want (not just INC), and you can make either nibble, or for that matter any random bit positions, be the part you preserve, based on what you AND with.

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6/7 Finally, we use EOR as a "bit flipper", using the difference bitmap now in A. This flips back any high nibble bits that were affected by the INC to what they were before the INC.

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5/7 In bogax's trick, we preserve the original value in A, then INC what's in COUNTER. We then use EOR as a "difference detector" to get a bitmap of positions that were affected by the INC. The AND zeros out any 1 bits in the low nibble, since we only care about changes to the high nibble.

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4/7 The previous example of 1011 EOR 1001 = 0010 is still true, but now we're thinking about it in a totally different way: Flipping the bits on the end. Interestingly, direction doesn't matter. Flipping the bits in the second byte based on the 1s in the first byte gives the same result!

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3/7 You can also think of it flipping certain bits in a byte to their opposite, with the bit positions specified by the 1s in the other byte. (This is why EORing with 11111111 is the same as a NOT operation, by the way.)

2/7 The result of EORing two values is a bitmap with a 1 marking the positions where the bits are different. 1011 EOR 1001 = 0010, for example, since the second-from-right bit (bit 1) is the only difference.

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1/7 This is a very clever trick, and I thought I'd unpack it a little in case anyone's interested: The key is that EOR (exclusive OR, more commonly abbreviated XOR) is a versatile tool that can work as both a "difference detector" and a "bit flipper", depending on how you think about it.

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Say what you want about DEC, but they by-golly knew how to make a front panel.

Jeff Jetton @jeffjetton.bsky.social · 03/08/2026

Hmmm... I don't know. I've listened to "We Are Never Ever Getting Back Together" and she seems pretty concerned about convergence to me.

Jeff Jetton @jeffjetton.bsky.social · 02/08/2026

SEI and CLI only govern maskable interrupts, triggered by an IRQ (pin 4 low). Non-maskable interrupts (NMI, pin 6 low) can't be disabled in software. The STOP key on the KIM-1 triggers an NMI and doesn't care about a missing CLI. A frozen KIM is more likely due to not setting the NMI vector.

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Jeff Jetton @jeffjetton.bsky.social · 30/07/2026

(I don't own "real" ones, which are quite expensive, so I'm going the replica/emulator route.)

Jeff Jetton @jeffjetton.bsky.social · 30/07/2026

The PDP-8 was a minicomputer, introduced in 1965. You can write all sorts of stuff for it. The Kenbak-1 was a very early "training" computer from 1971 that is somewhat useless, but still fun! Since they predate the microprocessor, they both feature CPUs made of discrete components.

Jeff Jetton @jeffjetton.bsky.social · 30/07/2026

I find a lot of the old-school CPUs to be a total joy to write for. 6502 for sure, but I've also been digging PDP-8 and Kenbak-1 assembly lately too.

Jeff Jetton @jeffjetton.bsky.social · 27/07/2026

We've been running a pi-hole for almost five years now. Total game-changer, and the perfect use for that old Pi 2 or 3 that you're not using for anything else.

Jeff Jetton @jeffjetton.bsky.social · 26/07/2026

Well, I guess technically you would "use" indexing because the 6502 doesn't have a non-indexed indirect addressing mode (apart from JMP). But you'd keep X or Y set to zero or some other constant.

Jeff Jetton @jeffjetton.bsky.social · 26/07/2026

I don't think the AI that I assume is writing these got this one quite right. First, 8 bits can index 256 bytes: offset 0-255. Second, for larger arrays you likely wouldn't use indexing at all, but rather set up a 2-byte pointer and just increment or decrement that, which is fairly straightforward.

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Jeff Jetton @jeffjetton.bsky.social · 20/07/2026

And if you want to add a constant, but you know the carry flag is already set by prior operations, you can still skip an explicit CLC by just adding (constant - 1) instead.

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Jeff Jetton @jeffjetton.bsky.social · 20/07/2026

But a lot of times you can know the state of the carry flag based on what had happened before. For example, if you got to the ADC via a BCC, you can skip the CLC and save a byte (and two cycles).

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Jeff Jetton @jeffjetton.bsky.social · 18/07/2026

Interesting! What would've been the reason back then (or even now) to spend the extra money to run MS BASIC over the free/built-in Atari BASIC?

Jeff Jetton @jeffjetton.bsky.social · 11/07/2026

I had the board game and the lunchbox! :-)

Photo of a 1970s-era metal lunchbox and thermos, with painted images of the TV show "Emergency" on them.

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Jeff Jetton @jeffjetton.bsky.social · 15/06/2026

Love that Wired-esque, neon-cyber-grunge graphic design.

Jeff Jetton @jeffjetton.bsky.social · 09/06/2026

Huh. I always thought it was "You can't figure out the bad guy then." TIL

Jeff Jetton @jeffjetton.bsky.social · 29/05/2026

I love the lighting. You never see the TV, but you know it's there.

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Jeff Jetton @jeffjetton.bsky.social · 23/05/2026

Definitely brilliant design. You were essentially pre-tokenizing the keywords, eliminating the time/ROM/RAM required for the interpreter to scan them from raw text. It would've been a great idea even if the keyboard wasn't pants.

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Jeff Jetton @jeffjetton.bsky.social · 09/05/2026

You might be Gen-X, but are you "played with Lego people back when they were made of Lego bricks" Gen-X?

Photo of five little Lego people. They predate the "minifigs" that are more well-known these days. They consist of a head sitting atop a 2x2 brick, and this brick has flexible (and detachable) arms coming out of the sides. The rest of the bodies are various combinations of compatible Lego bricks, giving them a vaguely human shape. The plastic hair on their heads is detachable and interchangeable too, which is fun.

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Jeff Jetton @jeffjetton.bsky.social · 06/05/2026

Exactly. If people want to open/close stores and schools an hour earlier in the summer, move TV shows and movies up an hour, and get up / go to bed an hour earlier... go for it. But lying about what time it is to make it seem like you're NOT really doing everything an hour earlier is just goofy.

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Jeff Jetton @jeffjetton.bsky.social · 02/04/2026

I like small instruction sets and I cannot lie

Distracted boyfriend meme where the man pictured (labeled "ME") is checking out a woman walking by (labeled "PDP-8") while his presumed girlfriend/wife (labeled "6502") gives him an annoyed look.

Jeff Jetton @jeffjetton.bsky.social · 30/03/2026

And IIRC, the underlying mechanism is that a SBC is just an ADC of the negative. Only instead of doing a full two's complement (NOT and add one), it just does the NOT part. The normal add of the carry bit becomes the "add one" part. Thus, the borrow automatically happens when carry is clear.

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Jeff Jetton @jeffjetton.bsky.social · 30/03/2026

It's a "reverse borrow" flag. I think of the SEC as putting a borrow bit on the shelf in case it's needed. If it's still there when you're done, there wasn't a borrow.

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Jeff Jetton @jeffjetton.bsky.social · 28/03/2026

The dad of some of my neighborhood friends built one. Every time I'd go visit, it would be a little bit further along. Eventually it was finished and we all played around with it. Wait... you didn't grow up in southern Indiana, did you? :-)

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Jeff Jetton @jeffjetton.bsky.social · 24/03/2026

Fun fact: If you celebrated this today at 7AM Chicago time, you were actually an hour early! March 24th, 1984 would've still been under Standard Time, since the switch to Daylight Saving Time wasn't until April back then.

Still of the high school from the "The Breakfast Club", a movie that takes place on a single day of detention. There's a subtitle that shows that the date is Saturday, March 24, 1984. A heading further specifies that detention started at 7AM.

Jeff Jetton @jeffjetton.bsky.social · 11/03/2026

(Which of course made recursion impossible.)