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0

I say the 6800 is not like a PDP-11 in any useful way. It does not have general registers and it does not have addressing modes as a separate construction from the registers used by the mode. For example, the PDP-11 'indexed' mode is X(Rn), for n = 0 to 7; the case of n = 7 (the PC) gets you PC-relative addressing. By contrast the 6800 has only 1 register ...


1

In overall feel, the MC6800 is clearly more like a PDP-11 than a PDP-8. Word vs. Byte Architecture Like many machines of the 1950s and '60s, from the IBM 701 to DEC's PDP-10, the PDP-8 was a word-oriented machine. Addresses pointed only to full words, loads and stores were always of a full word¹, and every instruction was exactly one word in size, with any ...


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I have something to add. It's not exactly an answer, but it's too long for a comment. You are linking to 11Logo (which I put on GitHub, courtesy of CSAIL), but this wasn't the first version of Logo. It was first implemented on PDP-1 at BBN, and later updated for a PDP-10. The PDP-10 version was moved to MIT (the files still have a BBN copyright notice), ...


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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 registers, but programming it is mostly quite similar to programming a PDP-11. I'd say the 6800 is (much) closer to a PDP-8. If memory serves, the 6800 has two ...


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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 it decisively into being based on either. In fact, many of the arguments that can be used to put the 6800 into PDP-8/-11 heritage can as well be applied to the ...


13

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 6800 was big-endian, as was the IBM 360 (...) Unlike the PDP-11 and 6502, but like the 8080, the 6800 used borrow carry (...) Unlike the 8080 and especially ...


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As far as the Unibus is concerned, all reads are word-sized (and from an even address), and the CPU simply ignores the portion of the word it wasn't interested in. Thus, to read an odd-address byte, the CPU reads the even-address word containing that byte, and uses only the top 8 bits of the result. Unibus writes are allowed to be byte-sized (there are 2 ...


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