Retrocomputing Stack Exchange
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Questions tagged [pdp-11]

For questions regarding the DEC PDP-11 minicomputer.

The PDP-11 minicomputer from the Digital Equipment Corporation (DEC) was a 16-bit machine introduced in 1970 and produced, in various generations, until the 1990s.

Please use this tag specifically for the PDP-11. Other PDP machines from DEC have their own tags.

67 questions
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May the PDP-11's stack grow in either direction?

According to this answer, the stack grows downward. It's in §4.4 of the cited document: As another example, the DEC PDP11 range has a hardware stack which grows with decreasing store addresses. Now, but the way this works is by the…
Omar and Lorraine
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Does the in-code argument passing conventions used on PDP-11's have a name?

Some subroutine calls on PDP-11 machines would be coded with the arguments appearing immediately after JSR or EMT instruction, something like this (apologies for not knowing the exact MACRO syntax): jsr r5,WRITE_STRING .byte 'This is some…
John Källén
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PDP-11/34 bit 8 in MMR0: maintenance mode - what does it do?

While attempting to write a PDP-11/34 emulator, I noticed that there's a bit called "maintenance" in the MMR0 (SSR0) register (the 8th). What does it do? In the manual I read: Maintenance/Destination Mode, Bit 8 - Bit 8 specifies that only…
Folkert van Heusden
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Was it really possible for a PDP-11 to address more than 32K words memory without an MMU?

In an answer by user RichF, it is claimed that the previous-address-space moves were able to access 'all of memory' without memory management being enabled. The instructions MFPI (move from previous instruction space), MFPD (… previous data space),…
dave
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Were any PDP-11 systems with Corita Kent side-panels actually sold?

Somewhere around 1980, I heard about PDP-11 systems that had side-panels designed by Corita Kent (a nun and pop artist, who amongst other things designed a stamp for the US post office, and gas storage tanks along the Boston southeast expressway).…
dave
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10
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How to view register contents on the PDP-11 console?

I have a PiDP-11 kit, which emulates a PDP-11/70 console. I've been using it as a launch point to gradual learning about the original machines and so far have been able to read/write memory and launch simple machine code routines from the…
natevw
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What was the clock speed and ips for the original PDP-11?

What was the clock speed of the first generation of the PDP-11 with the KA11 cpu? This sounds like something that should be easily googlable but so far I have found nothing. I'm also interested in what the instructions per clock (or clocks per…
JanKanis
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8
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simh skipping parts of the v5 unix boot process?

I tried to run v5root.gz on simh V3.8 (unix research version 5). This works insofar as I get a login prompt. If I though look at the source code of main.c (line 55) of the kernel in that image, then I see the following: printf("mem = %l\n",…
Folkert van Heusden
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Why is the start address in the PDP-11/23(+) micro-ODT a 18-bit address?

The KDF11 processors (as used in the PDP-11/23 and PDP-11/23+) contain an 18-bit implementation of ODT, called Micro-ODT. Micronote 058 states that the standard start address for a large (18- or 22-bit) memory system is 773000. That that is the…
JosF
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Does anybody have the PDP-11 CAPS-11 source code?

CAPS-11 is the Casette Programming System for the PDP-11. The manual is available at Bitsavers, binaries can be found at SimH. The manual contains instructions how to assemble the system from source, the source code should be available somewhere. I…
pm100
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4
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1 answer

How MOVB, TSTB and all byte instructions works in odd address read in PDP11?

I'm writing a PDP11 emulator. When CPU read odd address on Unibus odd address exception is launched. But the code below INC increments PRTPTR by 1 which make TSTB @PRTPTR generate a exception when it try access odd address. PRTAST: TSTB…
Fabio
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PDP-11 CIS MOVTC instruction... does what?

I'm looking over the PDP-11 CIS instruction set. Here is the listed description of the MOVTC (and MOVTCI) instruction: MOVTC(I): Move translated character A longer description can be found in the related text: The MOVTC(I) instructions move a…
Maury Markowitz
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PDP-11 ASH instruction: who is right?

I'm trying to figure out why emulation of the ASH instruction by my emulator is marked as faulty. I use the XXDP+ "EKBBF0" test to verify its cpu emulation. If I look at the source-code of EKBBF0, then I see: 3468 006500 072100 ASH R0,R1 ;…
Folkert van Heusden
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PDP 11/70: byte addressing of I/O space

The highest memory page in the PDP-11 series is the I/O-page. On some addresses devices can be accessed. Usually via words and they are also on even addresses. The cpu-registers are also 'visible' in that space and can be accessed there. Strange…
Folkert van Heusden
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pdp11 grace stack, when does grace time end

some models (45 for example) have a stack grace area for use after a stack overflow Stack Limit Violations. Quote from handbook When instructions cause a stack address to exceed (go lower than) a limit set by the programmable Stack Limit Register,…
pm100
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