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33 votes
Accepted

Use of undocumented opcodes

In some cases undocumented instructions may be useful as shortcuts to accomplish tasks that are "difficult" to achieve with the official instructions. Going with the Z80, it is common practice ...
Konamiman's user avatar
  • 1,105
27 votes

Use of undocumented opcodes

In many cases, undocumented opcodes were not deliberately created, but are merely the result of designers including the minimal circuitry necessary to create a specified set of opcodes. On the 6502, ...
supercat's user avatar
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27 votes

Use of undocumented opcodes

More “serious and modern” CPUs than the 6502, Z80 and PDP-8 have undocumented instructions too, and they don’t necessarily cause the CPU to stop. The Intel x86 line has quite a few instructions which ...
Stephen Kitt's user avatar
26 votes
Accepted

PDP-8 transistor count

The PDP-8 was a 12-bit computer with a deliberately simple instruction set. That's part of your answer right there: the 6502 was in many ways more complex than the PDP-8: the 6502 has 56 machine ...
Warren Young's user avatar
26 votes
Accepted

Did the PDP-8 designers consider a stack?

Without detailed documentation on the PDP-8 design process, we cannot say for sure. I suspect that while they may have briefly considered it, it was never a serious prospect. The PDP-8 is just the ...
RETRAC's user avatar
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23 votes

Size of the PDP-8

There were different models of the PDP-8, but most models (say, PDP-8/E) consisted of a number of rackmounted boxes, one of which you could call the CPU (or, compared to a modern PC, maybe the ...
dirkt's user avatar
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19 votes
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How did the PDP-8 handle strings?

Technically, the three common methods for storing text in a PDP-8 memory were: two 6-bit chars per word, using the 64 glyph TTY character set padded to 8-bit bytes, packed three bytes per two words ...
A. I. Breveleri's user avatar
18 votes

Size of the PDP-8

The photographs I found seem to be evenly divided between showing it as a compact unit that looks maybe one foot high It is most likely, based on sales numbers, that you're describing a PDP-8/e or ...
Warren Young's user avatar
17 votes

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

In spirit it's both, thus eventually neither. Features of the 6800 can be put in line with many CPUs of that time - from PDP-8 and -11 all the way to TI's 990 or even IBM's /360 - but none will put ...
Raffzahn's user avatar
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17 votes

PDP-8 floating point numbers

The format is pretty basic, so much so that it's easy to miss the description. The exponent is simply a signed 12-bit integer. There is no bias mentioned, so it's capable of representing very large ...
John Dallman's user avatar
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16 votes
Accepted

Conditional skip instructions of the PDP-8

If you compare the octal opcodes for the skip instructions, a bcd | ||| 7500 SMA = 111 101 000 000 7440 SZA = 111 100 100 000 7420 SNL = 111 100 010 000 ...
dirkt's user avatar
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15 votes
Accepted

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

Most sources say it was based on PDP-11. Here are citations from the book "Early Home Computers", summarizing the similarities and the differences: Unlike the PDP-11, 6502 and 8080, the ...
Martin Maly's user avatar
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14 votes

Size of the PDP-8

The original (straight) 8 is a fancy shaped box of about 2 ft wide, 3 ft height and 2 ft deep. Later models where mostly standard 19" rackmount boxes, roughly 2x1x3 ft3 until the last ones made ...
Raffzahn's user avatar
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14 votes

PDP-8 transistor count

The classic PDP-8 made extensive use of what were called DCD gates. These diode-capacitor-diode gates performed the logical AND function between two inputs, one a pulse and one a level signal. So, ...
Douglas W. Jones's user avatar
13 votes
Accepted

Was 1.10 a valid line number in FOCAL?

Yes. This is FOCAL 69 running under TSS/8, on a PiDP-8. *1.1 TYPE "HELLO WORLD",!! *GO HELLO WORLD *WRITE ALL C-FOCAL,1969 01.10 TYPE "HELLO WORLD",!! Observe I gave the line number as 1.1; the ...
dave's user avatar
  • 35.9k
13 votes

PDP-8 transistor count

The PDP-8 processor was implemented in diode-transistor logic, whereas most early microprocessors were implemented in some variety of MOS logic, most commonly NMOS. This accounts for some of the ...
Jules's user avatar
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13 votes
Accepted

How did FOCAL convert strings to a number?

While the PDP-8 indeed frequently used 6-bit characters packed into a word, the way FOCAL handled "alphanumeric numbers" seemes to be different. You can easily run a PDP8 FOCAL in simh, and ...
dirkt's user avatar
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13 votes
Accepted

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

Not really an answer, but posting as an answer since I need the space to work in. Working through both cases... FITR(10 x FABS(FRAN())) FRAN gives (-0.9999, 0.9999) FABS gives (0, 0.9999) x 10 gives (...
dave's user avatar
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12 votes
Accepted

PDP-8 conditional skip and 2-word instructions

The various PDP-8s have a few subtle differences, which could potentially include whether you can skip double-word instructions like your question implies, but I doubt that. There is no mention of ...
Omar and Lorraine's user avatar
9 votes

How did the PDP-8 handle strings?

When I used the PDP-8, 7-bit ascii was in use. Strings would start on an word boundary, and be packed 1.5 8-bit bytes per word, and the last nibble might be unused. You'd use a subroutine to output ...
Erik Eidt's user avatar
  • 3,357
9 votes

How did the PDP-8 handle strings?

In practice, a typical minicomputer would spend a great deal of its time dealing with text one way or another. Not really. PDPs where mainly used to automate machinery and related number crunching (...
Raffzahn's user avatar
  • 226k
8 votes

PDP-8 transistor count

Why did the PDP-8 not distinguish between data and address widths like the microprocessors? You have your question the wrong way around. Up until the advent of microprocessors, it was quite normal ...
JeremyP's user avatar
  • 11.7k
8 votes

Use of undocumented opcodes

Not sure that this really counts as using undocumented op-codes, but might be sufficiently related to be of interest... On the 8086, there are often two ways of encoding the same instruction (...
TripeHound's user avatar
  • 1,091
8 votes

Arithmetic operations on multiple words on the PDP-8

Doug Jones' PDP-8 Programmer's Reference Manual is very helpful here. There's pretty much just one way to do multi-word precision add and subtract. Memory addressing complicates things so let's just ...
George Phillips's user avatar
8 votes

Conditional skip instructions of the PDP-8

I'd like to add a few words on conditional branching. This is quite different from the conditional branching on the PDP-11 and the 6502 computers. It simply doesn't fit into the PDP/8 ...
Ralf Kleberhoff's user avatar
8 votes
Accepted

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

The PiDP-8 project has a wonderful Field Guide to PDP-8 Assemblers, which reviews most (all?) of the PDP-8 assemblers available. SMAL, SABR and RALF have linkable objects that may fit your needs. A ...
RETRAC's user avatar
  • 13.8k
8 votes

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

I was taking the EE Computer Organization class at UT Austin a few years after the MC6800 came out. We had a guest speaker one day, a local consultant who had been a Motorola employee and who had ...
John R. Strohm's user avatar
7 votes

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

I wonder if there isn't a minor mistake in the CPU being discussed in one of those. It's very easy to see the 68K as nearly a direct descendant of the PDP-11. The 68K has separate data and address ...
Jerry Coffin's user avatar
  • 5,173
7 votes

Did the PDP-8 designers consider a stack?

I'm going to say no. The PDP-8 was chiefly designed for compatibility with the PDP-5, and this machine also had no hardware stack. There is not enough room in the instruction space to add push and ...
Omar and Lorraine's user avatar
6 votes

Size of the PDP-8

Before the PDP-8/e, two other models of the PDP-8 had their electronics all fitting entirely beind the front panel. The PDP-8/S, which arrived shortly after the original PDP-8, was in this category; ...
John Savard's user avatar

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