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Questions tagged [pdp-11]

For questions regarding the DEC PDP-11 minicomputer.

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6 answers
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How did the `long` and `short` integer types originate in C?

I was wondering why C has the long and short integer type in addition to the plain int, and came across the following quote in the C99 Rationale (thanks to this answer here): [...] In the 1970s, 16-...
user51462's user avatar
  • 511
3 votes
0 answers
147 views

Hierarchical/navigational database engine available on RSX-11M

Around 1988-1990 I worked for a company that was using an old RSX-11M system to host an ERP application. The application used a third-party database product of which I am trying to recall the name. ...
Anthony X's user avatar
  • 441
6 votes
1 answer
522 views

In the RL02 disk system, why does the “read” command take CHS coordinates, when there is also a separate “seek” command?

Why is it that the RL02 disk system of the PDP11 has a seek command yet the read command also knows where to read from as the cylinder/head/sector are given via the 'DAR' register? http://www....
Folkert van Heusden's user avatar
3 votes
1 answer
582 views

Why does this code trigger a TRAP on a PDP11/70?

On the PDP11/70, in the XXDP BKTCB0.BIC program I saw the following code: 012716 177777 mov #-1,(KSP) 006627 MTPI (PC)+ According to the comments in the source-code, this triggers a trap....
Folkert van Heusden's user avatar
3 votes
0 answers
231 views

DEC (probably PDP-11) operating system with a guest mode allowing to run BASIC programs

In the mid 1970's I used an OS on DEC equipment (I believe PDP-11) that had a guest mode. If you weren't an authorised user you could login as guest mode where you supplied a name and if it didn't ...
hptsb's user avatar
  • 115
1 vote
1 answer
154 views

When was the PDP-11/50 released?

I've seen varying dates for the PDP-11/50 release, ranging from "sometime in 1975" to "June 1976, at the same time as the PDP-11/55." What was the actual date of release?
cjs's user avatar
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11 votes
2 answers
1k views

Why did the VAX compatibility mode not implement the MARK instruction?

This question was inspired by an older question about the MARK instruction which recently bubbled up to the top of the active pile. The VAX-11/780 and several follow-on VAXen implemented a PDP-11 ...
dave's user avatar
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5 votes
1 answer
336 views

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-...
JosF's user avatar
  • 153
1 vote
1 answer
175 views

Interrupt latency on the PDP-11/23

I'm trying to understand the interrupt latency on the PDP-11/23 (more specifically the KDF11-A). Diagnostic JKDBD0 test 375 (Test that a TTY interrupt causes an overflow trap) contains the following ...
JosF's user avatar
  • 153
14 votes
1 answer
1k views

PDP-11 Program to Execute Only in the Registers

The contents of processor registers of a PDP-11/05, etc were available at UNIBUS memory addresses and it was possible to execute a small test program just in the processor registers without accessing ...
PDP11's user avatar
  • 710
2 votes
0 answers
95 views

What encoding is this BK-0010-01 FOCAL file in?

I want to keep copies of the programs I write in BK FOCAL on my modern laptop, and in the end to be able to transfer text file programs to the BK. My current system for transferring files between the ...
harlandski's user avatar
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11 votes
4 answers
1k views

What is the difference (if any) between these two expressions in FOCAL?

I am trying to understand the 1969 FOCAL program Hamurabi in detail. The program was originally written for the PDP-8 version of FOCAL, but I am running it on the BK 0010-01 which presumably is a ...
harlandski's user avatar
  • 2,953
9 votes
1 answer
521 views

Was the design of the PDP-11 Floating Point Processor responsible for C's willingness to do arithmetic on floats at double precision?

The Wikipedia page on PDP-11 architecture has a very interesting bullet point in the section on the Floating Point Processor extension to the basic architecture: full floating point operations on ...
John Dallman's user avatar
  • 13.4k
2 votes
0 answers
183 views

How to install RSX-11S?

I'm trying to install an RSX-11S system on my LSI-11 (PDP-11/03) simulator. I was under the impression that an RSX-11S system could be generated (via 11SGEN.CMD) from an RSX-11M or RSX-11M PLUS system....
JosF's user avatar
  • 153
3 votes
2 answers
602 views

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 ...
Maury Markowitz's user avatar
2 votes
1 answer
146 views

DEC RLV12 Disk Controller Register Access

The RLV12 Technical Description (EK-RLV12-TD-001) describes the RLV12 registers as "word-addressable". I presume that means these registers can only be accessed as words and byte access ...
JosF's user avatar
  • 153
13 votes
1 answer
372 views

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/...
Folkert van Heusden's user avatar
3 votes
1 answer
229 views

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 ...
Folkert van Heusden's user avatar
31 votes
4 answers
4k views

What motivated the weird boolean instruction repertoire of the PDP-11?

The PDP-11 has seven dyadic instructions (I'm not counting the byte/word varieties separately), which take a full six bits for each of its operands. That's twelve bits to specify the operands, and ...
Omar and Lorraine's user avatar
8 votes
2 answers
759 views

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 ...
Folkert van Heusden's user avatar
12 votes
1 answer
1k views

Can PDP-11 general registers be referenced by memory addresses?

From time to time it is mentioned in RC that the PDP-11 general registers R0-R7 are accessible at addresses 177700 to 177717 (considered as 16 bit program addresses). The mapping is as follows, again ...
dave's user avatar
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7 votes
1 answer
507 views

PDP 11/70: how does MTPI work?

I'm trying to understand the MTPI (and its MFPI counterpart) instruction on the PDP/11. As far as I understood, it: it retrieves a value from the "current" memory space with the current ...
Folkert van Heusden's user avatar
3 votes
1 answer
240 views

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 ...
Folkert van Heusden's user avatar
0 votes
1 answer
313 views

getting PDP-11 DL11 output via telnet

I'm trying to run the (a slightly adapted version but the original gives the same problem) example program from How to implement a 'write' function in pdp-11 / MACRO-11 / ODT using simh? in ...
Folkert van Heusden's user avatar
6 votes
1 answer
330 views

'Trap stack push abort' on the PDP-11 with SIMH

I'm trying to run a bare-metal example program from the DL11 manual that echoes user input. When I try to run it via SIMH however I get a 'Trap stack push abort' error when it tries to access address ...
DivergentClouds's user avatar
13 votes
1 answer
389 views

"MOS" - Mini Operating System for PDP-11?

Around 1977 or so, I worked with MOS, a "mini" operating system for the PDP-11. I think it was originally developed at SRI, to support the BBN Packet Radio networking research. It was ...
John R. Strohm's user avatar
4 votes
1 answer
386 views

When did an overlay linker first appear in a PDP-11 UNIX OS?

On a 16-bit system with at most 64K of RAM available for a user program, one would think of having an executable overlay mechanism as an indispensable tool to maximize the amount of memory available ...
Leo B.'s user avatar
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6 votes
0 answers
354 views

What was a DEC PDP-11/85 intended to be, and why might it be able to use DECsystem-10 software?

In this DEC PDP-11 Strategy Memo from Sept 1974, there is this curious question. HOW MUCH OF THE DEC SYSTEM 10 SOFTWARE CAN THE 11/85 USE, OR DOES THE 11/85 HAVE TO START FROM SCRATCH? CONFLICTING ...
dave's user avatar
  • 35.9k
6 votes
2 answers
1k views

What was the most popular operating system on the PDP-11 computers?

What was the most popular operating system on the PDP-11 computers? And is it known what percentage of PDP-11 computers used this operating system?
user20556's user avatar
0 votes
3 answers
353 views

Should C be regarded as an Intermediate Language? [closed]

Should C be regarded as an intermediate language on a virtual machine named PDP-11 Architecture (which have a plain memory space and stacks), like opcode to jvm, msil to dotnet, asm.js to v8, which ...
Schezuk's user avatar
  • 3,754
15 votes
2 answers
1k views

Trying to understand some assembly syntax in the Unix v7 write system call

so here is the code: (which comes from here, I've also verified this source is in my unix v7 distribution). For reference, this is running on a PDP-11 simulated with the simh program (so please keep ...
LainIwakura's user avatar
3 votes
2 answers
239 views

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 ...
pm100's user avatar
  • 269
3 votes
2 answers
556 views

How to implement a 'write' function in pdp-11 / MACRO-11 / ODT using simh?

So, I'm working on a project where I'm aiming to get the first winner of the IOCCC (obfuscated c code contest) dissected down to the last detail... The program is mullender.c which you can find by ...
LainIwakura's user avatar
5 votes
5 answers
1k views

What’s the most advanced os I can run on a non memory management pdp11 (ie 11/20)

Building my own emulator. I only have the basic , no memory management, model going (11/20?). I started with caps11. What’s my best choice for a disk based os. EDIT: and of course if anybody has the ...
pm100's user avatar
  • 269
4 votes
1 answer
465 views

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, ...
pm100's user avatar
  • 269
1 vote
0 answers
66 views

Understanding PDP 11 ' JSR PC,(RR)' instruction [duplicate]

I am new (1 week to PDP 11, writing my own emulator for fun) using CAPS11 as my starting point. I am confused by this from the very first bit of bootup code 000110: 012703 000130 mov #130,r3 ; ...
pm100's user avatar
  • 269
6 votes
1 answer
523 views

What was the DEC CR11 card reader 'compressed Hollerith code' for?

In this manual for the DEC CR11 card reader controller, the controller can deliver the card content to the PDP11 in either of two forms: Column binary: the 12 rows of hole/no-hole are delivered in ...
dave's user avatar
  • 35.9k
3 votes
1 answer
344 views

Could the PDP-11 console be accessed "raw" by software?

My rough understanding of the lights-and-switches console that many PDP-11 models had is that they were not hardwired into the CPU itself or anything, but implemented as just another Unibus device. ...
natevw's user avatar
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21 votes
8 answers
3k views

Was the MC6800 based on the PDP-11 or PDP-8?

According to Gordon Bell, the 6800 was based on the PDP-11. According to Chuck Peddle, it was a PDP-8. Can anyone with knowledge of the PDP's pass judgement?
Maury Markowitz's user avatar
4 votes
1 answer
186 views

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 ...
Fabio's user avatar
  • 41
11 votes
2 answers
1k views

What causes the monitor to reduce to four lines on BK 0010 Focal?

When working with the Focal cartridge inserted on my BK 0010, the area displayed on the monitor suddenly reduces to just four lines , instead of the usual 20 Each time this has happened when I've ...
harlandski's user avatar
  • 2,953
10 votes
2 answers
879 views

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 ...
natevw's user avatar
  • 2,947
19 votes
4 answers
4k views

Why was Logo created?

The Logo programming language was created by researchers at MIT's AI Lab. Its post-1980s use has been predominately in the educational field, but is this what the language was originally designed for? ...
wizzwizz4's user avatar
  • 18.7k
11 votes
3 answers
992 views

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 ...
dave's user avatar
  • 35.9k
3 votes
1 answer
354 views

Why was the DEC Q-Bus called the Q-bus?

The bus used for DEC's LSI-11 microcomputer implementation of the PDP-11 was originally and prosaically referred to as the LSI-11 bus, but thereafter it was universally referred to as the Q-bus. Does ...
dave's user avatar
  • 35.9k
13 votes
4 answers
2k views

How did 2-chip CPUs work?

The 1970s saw a big transition from CPUs built from thousands of discrete components, to CPUs implemented on a single chip, with the occasional use of bit-slice components along the way. There were, ...
rwallace's user avatar
  • 62.8k
13 votes
3 answers
1k views

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): ...
John Källén's user avatar
3 votes
3 answers
1k views

PDP-11 JMP and JSR - how was the target specified?

As I understand it, the PDP-11 has a 16-bit address space with a fully loaded system having 28k words (56k bytes) after reserving some of the address space for I/O, which means if instructions must be ...
rwallace's user avatar
  • 62.8k
10 votes
1 answer
324 views

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 ...
dave's user avatar
  • 35.9k
15 votes
1 answer
926 views

Why does AT&T syntax use * and $?

In a comment to an answer about AT&T assembly syntax, another-dave asked the following: DEC used #foo for an immediate operand in -11 assemblers; the Unix guys apparently preferred $foo, which ...
ninjalj's user avatar
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