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A comment on A Memo on the Secret Features of the Hitachi 6309 processor (1992). Parent comment

Re:A Memo on the Secret Features of the Hitachi 6309 process... by tempguy9999 on Sunday November 10, 2019 @05:57AM (#21497481)
One reason is the extra instructions are an artefact of the decode process - undefined behaviour that just happens to be useful - but woe to you if you rely on them (although I have heard sometimes they are so useful they get publicly supported, I think something like that happened with the z80 and something to do with addressing/address space).

But here, it's weird, especially something like division which presumably isn't cheap to implement and is useful to end users. I don't get it at all.

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Re:A Memo on the Secret Features of the Hitachi 6309 process... by mysterydip on Sunday November 10, 2019 @06:49AM (#21497610)
Not memory addressing, but there is this "special reset" undocumented by Zilog but discovered from a patent http://www.primrosebank.net/computers/z80/z80_special_reset....
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Re:A Memo on the Secret Features of the Hitachi 6309 process... by GoForthAssemble on Sunday November 10, 2019 @09:58AM (#21498151)
I remember this 'scene' was very exciting. Pre-Internet you had to be in the in-crowd (of nerds) on bulletin boards, or at a meet up to ever find out. It felt like a free upgrade to your processor! here's another that's quite good: http://z80.info/zip/z80-documented.pdf
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