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Questions tagged [pdp-8]

For questions regarding the DEC PDP-8 minicomputer.

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5 votes
1 answer
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Does the PDP-8/E service an interrupt after any instruction, or after instructions not referencing memory only?

While researching this answer about incrementing the program counter, I found something I thought was a little odd in the FETCH state flow diagram, which is figure 3-8 of the PDP-8/E & PDP-8/M &...
Omar and Lorraine's user avatar
9 votes
1 answer
545 views

PDP-8 conditional skip and 2-word instructions

PDP-8 heavily relied on conditional skips: instructions that under some circumstances would skip the following instruction. Among them are, for example: ISZ SZA/SNA/etc. group of instructions many IO ...
lvd's user avatar
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10 votes
1 answer
1k views

PDP-8 floating point numbers

I'm trying to find a single clear statement of the limits to the numbers represented in the PDP-8's three-word format. I found the original documents on this, but they are, ahem, not exactly ...
Maury Markowitz's user avatar
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
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11 votes
1 answer
934 views

How did FOCAL convert strings to a number?

Reading over the manuals for FOCAL-69 and -71, as well as U/W-FOCAL, I realize there is one bit of the workings that I can't find an explanation for. FOCAL did not have string variables, so to allow &...
Maury Markowitz's user avatar
14 votes
2 answers
3k views

Did the PDP-8 designers consider a stack?

The PDP-8 was a remarkable exercise in minimalist computer design; some of the aspects of its design are discussed in detail at PDP-8 transistor count One feature it did not have was a stack. Instead, ...
rwallace's user avatar
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2 votes
2 answers
305 views

SAPCOL Japanese digital typesetting machines

In a comment to this question, Japanese SAPCOL digital typesetting machines are mentioned, which apparently used a PDP-8, and contained fonts for lots of Kanji. What were those SAPCOL machines? What ...
dirkt'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
3 votes
0 answers
100 views

What happens on the Straight-8 when changing to a non-existant data field?

The extended memory controller on the PDP-8 (the original one, sometimes called the Straight-8) apparently had difficulty when selecting a datafield that didn't exist: On machines with 8K or more, ...
Omar and Lorraine's user avatar
3 votes
3 answers
340 views

Was 1.10 a valid line number in FOCAL?

I'm looking over examples of FOCAL code, and I see that group numbers in the examples start with a 0 if need be. That is, it's 01.10, not 1.10. Was a leading zero optional?
Maury Markowitz's user avatar
5 votes
3 answers
514 views

In what way does the Straight-8 expand on the PDP-5?

A quote from the Wikipedia: The PDP-5's instruction set was later expanded in its successor, the PDP-8, to handle more bit rotations and to increase the maximum memory size from 4K words to 32K ...
Omar and Lorraine's user avatar
8 votes
1 answer
326 views

Are there any PDP-8 assemblers that create linkable object files?

I am currently writing a compiler for the PDP-8. The compiler is going to translate source code into PDP-8 assembly code that calls some runtime routines to perform high-level functions such as ...
fuz's user avatar
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5 votes
2 answers
363 views

Does the Intersil 6100 do interrupts differently from other PDP-8s?

As I recall, the original PDP-8 actually does a jsr 0000 when an interrupt happens, so that the interrupt service routine started at address 0001. I think that saves a few gates, since the instruction ...
Omar and Lorraine's user avatar
20 votes
5 answers
3k views

Size of the PDP-8

What is the physical size of a PDP-8? The photographs I found seem to be evenly divided between showing it as a compact unit that looks maybe one foot high, and as a tall cabinet maybe five or six ...
rwallace's user avatar
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16 votes
4 answers
3k views

PDP-8 transistor count

https://en.wikipedia.org/wiki/Transistor_count provides some transistor count figures for early microprocessors, e.g. 8008 and 6502 both listed at around 3500. The PDP-8 was a 12-bit computer with a ...
rwallace's user avatar
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14 votes
3 answers
2k views

How did the PDP-8 handle strings?

The PDP-8 was a popular 12-bit minicomputer of the late sixties. It used word addressing, so that it always dealt in 12-bit words, of which it could address 4096 with a full bank of core memory. In ...
rwallace's user avatar
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8 votes
3 answers
1k views

Arithmetic operations on multiple words on the PDP-8

For the PDP-8 with its strange ISA, I've always wondered what the "standard" (i.e. fastest) way to do arithmetic operations (add, subtract, maybe multiply) on integers consisting of multiple words was....
dirkt's user avatar
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15 votes
3 answers
1k views

Conditional skip instructions of the PDP-8

I am interested in the SKIP instructions on the PDP-8. These test the accumulator and Link bit, and they skip the next instruction if the condition holds. The instructions are: OR GROUP SMA - Skip ...
Omar and Lorraine's user avatar
27 votes
4 answers
4k views

Use of undocumented opcodes

I figure this is fitting for Retrocomputing.SE because CPUs like the 6502 and Z80 and the PDP-8, and probably others, have undocumented instructions and will happily execute them, in contrast with ...
Omar and Lorraine's user avatar