[openMSX-devel] [ openmsx-Feature Requests-3218248 ] io-base configuration for the MemoryMapper

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Feature Requests item #3218248, was opened at 2011-03-17 03:41
Message generated for change (Comment added) made by guyver800
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Category: None
Group: None
Status: Open
Priority: 5
Private: No
Submitted By: FRS (sd-snatcher)
Assigned to: Nobody/Anonymous (nobody)
Summary: io-base configuration for the MemoryMapper

Initial Comment:

This feature request is a subset of the RFE #3209252: Acid2Test

Currently, the memory-mapper I/O address is hardcoded and cannot be changed. It simply ignores the <io base> tag on the hardwareconfig.xml file.  It would be a needed for the Acid2Test that the MemoryMapper I/O address could be configured, like this example shows:

        <MemoryMapper id="Main RAM">
          <size>1024</size>
          <mem base="0x0000" size="0x10000"/>
          <io base="0xFC" num="4" type="O"/>
          <io base="0xFC" num="4" type="I"/>
        </MemoryMapper>


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Comment By: Patriek Lesparre (guyver800)
Date: 2011-03-20 17:41

Message:
Memory Mapper I/O ports are specified by the MSX standard. Having them in
another location is a violation of the MSX standard. BACK TO SCHOOL, FRS!

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Comment By: Maarten ter Huurne (mthuurne)
Date: 2011-03-20 17:29

Message:
The hardcoded I/O addresses are in MSXMotherBoardImpl::createMapperIO().
The purpose of that method is to have only one MapperIO instance per
machine, to make sure that if multiple mappers are present, they all switch
on one I/O port write.

Inside MapperIO itself we assume that the mappers are aligned on a
multiple-of-four port number. I think that is an acceptable restriction. In
any case, many other devices also have such restrictions.

One way to allow different I/O addresses would be to explicitly add
support for that in MSXMotherBoard, by keeping a MapperIO object per
4-ports block (NULL for most entries).

Another approach would be to get rid of MapperIO and use a generic I/O
(de)multiplexer when multiple devices share the same port. We must have
something like that already to support for example an FM-PAC in a machine
that has MSX-MUSIC internal, but I don't know the details. One complication
is that some MSX engine models (like the one in the turbo R) only support
reading 5 mapper bits instead of 8. That is also handled in MapperIO at the
moment, so it would have to be implemented differently if we want to drop
MapperIO.


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