[Coco] "Reading" non-readable bytes with PEEK vs ZBUG
robert.gault at worldnet.att.net
Wed Jan 23 23:11:00 EST 2008
Darren A. wrote:
>>Does it make any difference in these tests if the CPU is in double-speed mode?
> No Roger. I have tested it on a 128K CoCo3 68B09 at both speeds. There is no difference in behavior. I believe Robert G. has confirmed that the same results are seen on a 6309 in emulation mode (don't know about native mode).
> One thing to consider about your LOADM trick is that the behavior is dependant on the sequence of instructions executed. Chances are fairly good that all third-party versions of Disk Basic have simply copied the LOADM code used in Tandy's version, but that's just a guess. As someone stated in a previous post, creating a process that relies on some undefined behavior makes the entire process undefined.
The one thing clear from this discussion is not whether the CPU or GIME
are misbehaving or if clock speed is important. What is clear is we
should not try to read non-readable bits! Any attempt to get info from
the MMU registers should mask off non-active bits. It is silly to insist
that it is safe to take the entire byte as meaningful.
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