[B2-devel] Patches for Real Addressing

[<gb...@di...>]

Hi,

So far, I got only two crashes with B2 in real addressing mode under
Linux/i386:
- the one I told you yesterday
- another one related with AppleShare DRVR 41

1) The "Lo3Bytes" bug

I determined that the unfriendly code starts at ROMBase + 0xdb9c. Bit 0
of the byte located at 0x1efc (what is it ?) is always tested. Then two
branches are possible: one with the address stripped down to 24-bits and
the other keeping all the bits of the address. In normal operation, the
latter branch is always taken but, for some reason, at the end of the
Speedometer Graphics test, the branch with the stripping by AND'ing with
Lo3Bytes is taken!

My fix that has to be improved:
Changing the BNE to a BRA...
A little barbarous, isn't it ? ;-)

2) The AppleShare bug

This morning I tried to boot with extensions on, it failed at the
AppleShare extension from MacOS 8.1 (vers 3.7.4) because it tried to
read some data from 0x3fff.

I finally found that the unfriendly code is located in resource DRVR 41
(Driver: .AFPTranslator) at address 2372.

The problem:

A word at 0x28e (ROM85) is sign-extended to an address register. Then, a
word at that address is fetched. But as that address turns out to be
0x3fff on my system, so B2 crashes.

My fix:

Replacing six of its instructions with:
movea.w         ROM85,%a0
adda.l          #RAMBaseMac,%a0
movea.l         %a0,%a2
nop

In the original code:

0x48e7, 0x1c20, // movem.l      d3-d5/a2,-(a7)
0x382e, 0x0008, // move.w       $0008(a6), d4
0x3a2e, 0x000a, // move.w       $000A(a6),d5
0x554f,         // subq         #$2,a7
0x3eb8, 0x028e, // move.w       ROM85,(a7)
0x301f,         // move.w       (a7)+,d0
0x48c0,         // ext.l        d0
0x2440,         // movea.l      d0,a2
0x2040,         // movea.l      d0,a0
0x3010,         // move.w       (a0),d0
0x0c40, 0x3fff, // cmpi.w       #$3fff,d0
0x6316          // bls.s        #$00000016

Stack seems to be used just as a temporary since a push is immediately
followed by a pop. I am therefore assuming the value stored in is not
used afterwards and directly move the address in the required address
registers.

Do you have other ideas ?

I will probably commit the changes after I tried real addressing under
Solaris.

Bye.
-- 
Gwenolé Beauchesne