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A brief tour of the PDP-11, the most influential minicomputer of all time (2022)

117 points by jensgk

by kens

5 subcomments

I'd suggest the Datapoint 2200 as the most influential minicomputer of all time since half of you are using an instruction set based on it and it is largely responsible for the creation of the microprocessor.
Now mostly forgotten, the Datapoint 2200 was a programmable desktop computer introduced in 1970. It had a processor built from TTL chips, along with shift-register memory from Intel. Datapoint discussed with Intel and Texas Instruments the possibility of building a single-chip processor to replace the board of TTL chips. TI was first with the TMX 1795 processor, followed by Intel's 8008, both copying the Datapoint 2200 instruction set.
Datapoint decided that these chips didn't have enough performance and fatefully gave up rights to them. TI tried to sell the TMX 1795 to Ford, but got nowhere and abandoned the chip. Intel decided to sell the 8008 as a standalong microprocessor, which was used in early personal computers like the Mark-8. Intel improved the 8008 to form the 8080, then made a somewhat compatible 16-bit version, the 8086, which started the x86 architecture. (Because the Datapoint 2200 was little-endian (to use shift-register memory), x86 is little-endian.)
To summarize its influence, without the Datapoint 2200, the microcomputer industry would have been greatly delayed (since the 4004 wasn't suitable for a personal computer) and x86 wouldn't exist.

by rswail

0 subcomment

My first ever computing experience was in 1977 in year 9 at high school where we were lucky enough to have a PDP-11/10 with 16K(!) and 3 ASR-33s and a VT-52.
Learned how to program in octal on the front panel. I've still got an old front panel (the rest is a rotting collection of wirewrap boards in my garage).
It had a multi-user basic that left out string functions if you went multi-user. You loaded the bootstrap via the front panel, which read the "absolute loader" from paper tape on one of the TTYs, which then read the BASIC interpreter from the same source.
I still have the small reference card with the instruction set and some old paper tapes around somewhere.
The whole structure of the registers with R7 as the PC and R6 as the SP and the various addressing modes was just elegant.
We used to make jokes that your programmed a PDP-11 with 3 fingers (octal) and a VAX with 4 (hex).

by allenrb

2 subcomments

I’m not quite old enough to have grown up with the PDP-11. They were gone from university (or at least the parts I had access to) by the time I arrived. But I did see exactly one in the wild.
In 1997-1998, I was working for a small company in Atlanta who did tape backup systems. At a client in maybe Knoxville (?), a hospital had a PDP-11/70 live in their machine room. Amusingly, right next to racks of then super-fancy Cisco gigabit fiber networking.
I was told that the PDP handled payroll. Guess that was important. Wonder how long it lasted there?

by JKCalhoun

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Obsolescence Guaranteed makes a PDP-11 replica [1]. At the heart is a Raspberry Pi running emulation software. All switches, LEDs are physical—all wrapped up in a clever case that looks like the original.
I have bought a couple of their kits and can vouch for the quality of them.
[1] https://obsolescence.dev/pdp11.html

by ggm

2 subcomments

I'd argue the pdp8 opened the floodgates. That's when the cost of digital computing dropped to the point a research grant or even just discretionary spending in a university department could pay for one, and you didn't need a special room and power supply.
the 11 was when it became more useful. But the 8 was how people realised you could move beyond a calculator to a computer.

by not2b

3 subcomments

It was the preferred lab computer in the mid to late 1970s and into the 80s. I got my first job because I knew PDP-11 assembly language, and worked with both DEC's operating systems for them (RT-11 and RSX-11) and later Unix (the lab I worked with had some machines running Version 6, though Version 7 was the first that I used seriously. It had a very clean and symmetric instruction set that used the program counter as if it were another general purpose register. I had an LSI-11 board (the single-board version of the machine) with 4K 16-bit words of core memory and a paper tape punch with a tiny loader in ROM to read in the tape and peek and poke memory, and I'd sometimes initialize the core memory to a known state by running the one-instruction program
mov -(pc), -(pc) or 014747 in octal. It would fill all of memory with 014747.

by EvanAnderson

1 subcomments

There's a parallel worldline out there where the PDP-11 made the transition into a desktop PC[0] and the IBM PC didn't take over the world. In that worldline our servers are from the PDP-11 lineage, and not the IBM PC.
[0] https://en.wikipedia.org/wiki/DEC_Professional_(computer)

by wduquette

0 subcomment

Heathkit sold an LSI-11 as a kit; my father built one in the mid-70's. I learned BASIC, a bit of Fortran, a bit of PDP-11 assembler, and Pascal on that machine. We started with paper type, and eventually moved up to dual 8" floppies. We started with 16K of memory, IIRC, and ended with 28K.
Been a programmer ever since, me.

by WalterBright

1 subcomments

I had a PDP-11, in the form of the Heathkit H-11.
I loved that computer. Like a fool, I sold it for $25. There's a picture of it on my X profile.
The -11 had an instruction set that fit on one page.

by budman1

0 subcomment

+1 to pdp11. The workhorse of the 1970's. They were everywhere. City Library. Auto parts stores. And reliable; Maytag ain't got nothing on a pdp 11.
Downside, programs are pretty simple that run in 64k. And extended addressing in any form, sucks.

by jakzurr

1 subcomments

Used several of these at school in the 90's. IIRC intro classes used VAX/VMS (was the machine called VAX-1170?). But all the higher undergrad classes used Unix on PDP-11/70's, like in the picture. Of course, I hardly ever saw one up close. :p

by TomMasz

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Back in my running days in the 80s, the people who did the timing and race results had a van with a PDP-11 in it that I assume ran off a generator. I wish I had asked them more about it but I was too busy worrying about running.

by TradingPlaces

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My high school had one of them in the early 80s. Me and a few others were the only ones who ever turned it on. Then a friend got an Apple ][ and we never touched the PDP-11 again.

by gerdesj

1 subcomments

"PDP-11 with UNIX opened the floodgates for inexpensive interactive computing, which then led to an explosion of office productivity. "
Well before we get too misty eyed: "inexpensive" needs looking at "for inexpensive interactive computing".
I'm not old (55) enough to have really got to grips with a PDP11. I do still own (yes: present tense) a C64 from 1986. The C64 was bought by my dad via the NAAFI in West Germany so I have no idea what it costed. Let's wind forward a bit:
I had a 80286 based PC in 1987ish with 1MB of RAM, 20MB RLL hard disc. The graphics card (ISA) had a whopping 512 bytes of RAM. That thing costed about £1200. I added a 80287 later at about £120 so I could run a pirated copy of AutoCAD.
In 1990ish I had a 80486 with 4Mb RAM and 40MB HD - that costed something like £1600.
Nowadays £1600 will buy quite a decent laptop and 35 years of inflation.

by nickcw

1 subcomments

I'm not old enough to have used a PDP-11 so I read through the assembly code description with interest.
Wow, it seems so modern. I've used a lot of assembly languages over the years but I'd feel immediately at home here. Nice sensible orthogonal instruction set with enough registers and a stack pointer. It reminded me immediately of ARM assembly - what a breath of fresh air that was when it came it.
I never realised quite how much influence the PDP-11 had on computer architecture. I knew about it's software legacy but I suspect that was enabled by it's ground breaking architecture.

0 subcomment

by PopePompus

1 subcomments

What happened if you used auto-decrement addressing with the PC? Did that hang the computer?

by ubermonkey

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There's a concept in history whose name escapes me, but which was evoked often when Harrison Tyler died in 2025. Tyler was a private citizen, but he was of note because he was the grandson of the US's 10th president John Tyler -- who died in 1862.
The gist is how surprising bridges to the past are closer than you realize -- as is the past itself.
At my first corporate job in 1994, we had a machine room. Those weren't uncommon back then. What WAS uncommon was that, over in a corner, sandwiched between racks of shiny new DEC Alphas, was a PDP-11 that was still running production code.
My employer then was TeleCheck, which did point of sale risk analysis for checks. The business had originally been run as independent state-by-state franchises, and back then someone had the bright idea to create an IT company that provided services to these franchisees -- and, occasionally, to other companies, too. By the time I was hired, the franchises AND the IT company had all been brought under one ownership, and all the IT company's external clients had gone elsewhere EXCEPT ONE.
That holdout was perfectly happy with what they got from that ancient computer.
I assume it eventually died, but TeleCheck had a DEEP bench of DEC talent, so it could've kept running a long, long time.