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Path: utzoo!attcan!uunet!seismo!sundc!pitstop!sun!amdcad!ames!mailrus!tut.cis.ohio-state.edu!bloom-beacon!apple!darin
From: darin@Apple.COM (Darin Adler)
Newsgroups: comp.sys.mac,comp.sys.mac.programmer
Subject: Re: Bus and Address Error insights (was Re: Listing of error...)
Keywords: bus error address error macII interrupts
Message-ID: <18022@apple.Apple.COM>
Date: 30 Sep 88 22:00:35 GMT
References: <975@psu-cs.UUCP> <630@ethz.UUCP>
Reply-To: darin@apple.com.UUCP (Darin Adler)
Organization: Apple
Lines: 21

In article <630@ethz.UUCP> macman@ethz.UUCP (Danny Schwendener) writes:

[a discussion of bus and address errors here]

> While we're on the topic, here's another reason for the Bus Error
> waves: The 68020 allows you to read data on an odd address. The
> 68000 doesn't: a Mac Plus will crash with an Address Error if you
> try to access an odd address (by a dangling pointer, or some
> chaotic program code). The Mac II will probably crash too, sooner
> or later, when the program goes completely nuts and tries to access/
> run/write/execute code on an address that doesn't physically exist.
> Or when it tries to WRITE on an odd address boundary. (reading
> is allowed, writing not. Another mysterious feature of the 68020).

Both reading and writing of non-aligned (odd) addresses is allowed.
However, if you jump to an odd address, you will get a bus error. In
other words, code to be executed must be word-aligned.
--
Darin Adler					       AppleLink: Adler4
UUCP: {sun,voder,nsc,mtxinu,dual}!apple!darin	  CSNET: darin@Apple.com