68300

Hi Jeff,
the 68K does signed extensions of 8 and 16 bit values to full 32 bit 
addresses, so you are able to access the lower 32K and the upper 32K with
16 bit absolut addresses.
The Adress lines A24-A31 are ignored by the cpu and the address logic
of the 68331 so Adresses $FFxxxxxx and $00xxxxxx are the same.

greetings Leopold

-----Ursprüngliche Nachricht-----
Von: Jeff Andle [mailto:andle@...]
Gesendet: Freitag, 20. September 2002 17:12
An: 68300@yahoogroups.com
Betreff: [68300] 683xx address space and modes...


I am porting some CPU16 code in which the addressing was performed using
ZK:ZX = 0 and the SCIM control registers were accessed at negative 16-bit
displacements from the index register.

I like this addressing mode, which would be (d16,An) in CPU32.  I am trying
to determine if address calculations are full 32 bit in the 683xx or if they
are still 24 bit addresses with roll-over.

IE, if the scim registers are at $FFF800-$FFFFFF, are they only at $00FFF800
or at $xxFFF800?

I was off doing microwaves and RF the entire period of time between learning
6800 assembly in 1983 and self-teaching the extension to CPU16 a couple
years ago.  I missed the entire 68000 family until a month or so ago and I
am slowly learning it...  I have P&E Micro assembler, so using C and letting
the compiler deal with it isn't an option...

Does the scim map to $FFFF F800?
Is (d16,An) really going to save as many clock cycles and bytes over direct
32 bit addressing as it appears?




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