Re: A86: 24-bit addressing


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Re: A86: 24-bit addressing



On Fri, 5 Sep 1997, Matthew Johnson wrote:

> How does 24-bit addressing work? Odviously it would require three registers
> since 8 * 3 = 24, I guess two registers
> is not enough, from which two registers could only address 65,535 bytes.. I
> think.

Yes, it requires three registers.  The ROM seems to rely mostly on AHL and
BDE.  Here are routines that manipulate 24-bit pseudoregisters.

462F _TRANS_AHL_TO_PAGED_AND_SET_PAGE
4633 _TRANS_AHL_TO_PAGED
4637 _INC_AHL
463B _DEC_AHL
463F _INC_BDE
4643 _DEC_BDE

Those first two calls translate a 24-bit virtual address into a 16-bit
physical address.  4633 puts the RAM page in A.  462F goes a step further
and sets the page for you.

--------
Dan Eble (mailto:eble@cis.ohio-state.edu)
         (http://www.cis.ohio-state.edu/~eble)


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