Thread: 68k details
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Old 13 August 2018, 10:03   #4
meynaf
son of 68k
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Join Date: Nov 2007
Location: Lyon / France
Age: 46
Posts: 3,541
Ok, this time it's about the x86 page. I can not tell for the PDP-11.


Overall the tone says it clearly : in 68k page it was "bad" cpu and here it's "good" cpu. *Cough*. But let's go to the details (there is less to say here).

The 8086 isn't "one of the best processors made in the 70's". Actually, one of the worse. It would have disappeared long ago if it weren't used by IBM on the PC (for economic reasons and nothing else).

While the architecture indeed doesn't adhere to abstract theories, it's nothing about balance nor steadiness either (and not even further development).
It has shortcuts for relative rare operations, some common things are just missing, and so on. Yukk.

And, oh, yes. The segmentation stuff is far from "brilliant". Actually one of the worse things ever invented.
While it may look good in the paper, it's the typical thing that only works in theory - in practice, segments were too short and it led to the near/far pointer horror.

I can just laugh when reading :
"It's hard to say that in the 8086 command system, something is clearly missing. Quite the contrary."
Well, the most problematic are already mentioned in the above post. But it's true that the main issues are with addressing modes, and restricted use of registers.

CMPXCHG and CMPXCHG8B aren't specific to x86, 68020 has CAS/CAS2 that do the same thing. Of course XADD is the perfect example of a totally useless instruction.

Talking about "ease of programming" in case of x86 looks like the author never attempted to write any code.

It's true that many instructions have several opcodes. You may also mention the "de-facto standard" of TEST instruction duplicated at F6-F7 /1.


If you still think x86 is a good cpu overall, i suggest you try to write a disassembler in asm for it (I did that for 68k long ago). There you will more clearly see all the quirks, the first being that it's impossible to know about all the extensions that have been added in "modern" iterations. Say, for example what's encoded at 0F 38 xx exactly, i can't tell.


Yeah, i've maybe been hard with you litwr, but, hey, you nearly asked for it
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