[not2b]

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.

[amirhirsch]

014747 makes me smile <3

I built the PDP-11/70 emulator that controls the nuclear reactors in Ontario. That was 20 years ago and I'm probably still the youngest person who can read PDP-11 assembly (and the raw octal)

[ErroneousBosh]

There's a decent chance you're younger than me, but I can still read it. Coincidentally about 20 years ago I got asked to fix a bug in some PDP11 code that ran on real hardware because it was tripping up an emulated system. Nothing wrong with the emulator as such, it was just a logic bug in the program that they no longer wanted to work around.

I made the fateful mistake of saying "Sure, sounds fun, how hard could it possibly be...?"

[le-mark]

Not pdp-11 but once I was tasked with fixing a c program the offshore team had failed to fix for over a year. I basically took a month off (at my desk) after I fixed it the first day. They were happy, I was a legend at that company!

[amirhirsch]

Love your handle

The FPGA emulator we made had to pass the xxdp test suite which was provided to me on punched tape and microfiche. The emulator had a specific test for FDIV overflow which even tested the accuracy of the partial result. None of the software emulators I tested did this. I emailed Gordon Bell who introduced me to Bob Supnkk, and we found the original flowcharts for it so I could replicate the divider logic precisely. Imagine a nuclear reactor dependent on this lol.

[mrandish]

> It had a very clean and symmetric instruction set

Indeed. Motorola's 68000 CPU took so much inspiration from the PDP-11's ISA, it was almost a spiritual successor. The 68000's 8/16-bit little brother, the 6809, widely considered the most powerful 8-bit CPU ever - was also heavily inspired by the PDP-11.

[tralarpa]

Somes lines in PDP-11 assembly from 2.11BSD:

  mov r1,-(sp)
  mov $1024.,-(sp)
  mov $outbuf,-(sp)
  mov fout,-(sp)
  jsr pc,_write
  add $6,sp
  mov (sp)+,r1
  tst r0
  bpl 2f
  jmp wrterr

[getpost]

Ha! The instruction that copies itself. I posted also posted it, 6 years ago. https://news.ycombinator.com/item?id=24820583