Re: [B2-devel] About {VIA, SCC} patches

[Christian B. <cb...@st...>]

Hi!

On Tue, Jan 30, 2001 at 04:42:28PM +0100, Gwenole Beauchesne wrote:
> Therefore, I traced down changes to those LowMem globals (0x1d4, 0x1d8,
> and 0x1dc) and the most "frequent" ones are those initialized by some
> code located at ROMBase + 0x92a. The code there is a little bit
> obfuscated

Let's see...

SetupHWBases
A00910: MOVE.L   ($DD0),D0              ;AddrMapFlags (basesValid)
A00914: MOVEA.L  ($DD8),A0              ;UniversalInfoPtr
A00918: ADDA.L   (A0),A0		;decoderInfoPtr
A0091A: LEA      ($A0094A,PC),A2
A0091E: MOVE.W   (A2)+,D3		;Offset in DecoderInfo (/4)
A00920: BMI      $A00930
A00922: MOVEA.W  (A2)+,A3		;Address of LM global
A00924: BTST     D3,D0			;Base valid?
A00926: BEQ      $A0091E
A00928: LSL.W    #2,D3			;Yes, set LM global
A0092A: MOVE.L   (0,A0,D3.W),(A3)  
A0092E: BRA      $A0091E
...
A0094A: 0002 01D4       ;VIA 1 Base    
A0094E: 0003 01D8       ;SCC Read Base 
A00952: 0004 01DC       ;SCC Write Base
A00956: 0011 01D8       ;SCC IOP Read 
A0095A: 0011 01DC       ;SCC IOP Write
A0095E: 0005 01E0       ;IWM Base
A00962: 0010 01E0       ;SWIM IOP
A00966: 0006 0B0A       ;PWMBuf1
A0096A: 0006 0312       ;PWMBuf2
A0096E: 0007 0266       ;SoundBase (RAMSndBuf)
A00972: 0008 0C00       ;SCSI Base 
A00976: 0009 0C04       ;SCSI DMA  
A0097A: 000A 0C08       ;SCSI Hsk  
A0097E: 000B 0CEC       ;VIA 2 Base
A00982: 000C 0CC0       ;ASC Base  
A00986: 000D 0CEC       ;RBV Base  
A0098A: 000F 0C00       ;SCSI Base 
A0098E: 000F 0C04       ;SCSI DMA
A00992: 000F 0C08	;SCSI Hsk
A00996: 0012 0CEC	;OSS Base
A0099A: FFFF		;End

The UniversalInfo contains as its first element (decoderInfoPtr) a pointer
to a list of hardware base addresses. The AddrMapFlags (which also comes from
the UniversalInfo) contains one bit for each of these addresses, signifying
its validity.

So you can replace hardware addresses by patching the DecoderInfo of the
UniversalInfo used in the ROM (for example at the beginning of rom_patches.cpp/
patch_rom_32()). IIRC, ShapeShifter on the Amiga does this in exactly this
way (all addresses except for the ASC base are redirected to ScratchMem).
I chose not to do this in Basilisk II because it was more helpful during
development to have Mac hardware accesses cause a segfault (so it drops
into a debugger and you can see what went wrong). For a production version
of B2 it seems better to silenty ignore illegal accesses (and have a "debug"
option to turn on strict checking).

Bye,
Christian

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