Am 18.09.2010 15:27, schrieb gdgqler:
If you add a Thing to the list of Things by using the software described in the
manual for the Trap #1 routine sms.lthg you will find that the linkage block
you have provided is partially completed (which is correct) but that, in
addition, the long word 12 bytes before the start of the linkage block is set
with the linkage block's address.
Does anyone know the reason for this bizarre and potentially dangerous
operation?
The source code for this is:
LEA TH_FRZAP+IOD_IOLK-IOD_CLOS(A1),A0
MOVE.L A0,CHP_DRLK-CHP_END(A1)
A1 points to the linkage block. Now, given that:
TH_FRZAP = $0C
IOD_IOLK = $18
IOD_CLOS = $24
CHP_DRLK = $04
CHP_END = $10
the two instructions become:
LEA (A1),A0 [a slightly unusual way of putting A1
into A0?]
MOVE.L A0,-12(A1)
I can't refrain from commenting that these two instructions could have been
written more shortly as:
MOVE.L A1,-12(A1)
which perhaps shows more clearly the peculiar nature of this part of HOT_REXT.
George
George,
you seem to be taking this from HOT_REXT. Where did you get the sources
for this?
When looking into SMSQ/E sources, there is nothing comparable to what
you see there (sms.lthg is in util_thg_lthg_asm. Nothing in there
touches memory on negative offsets to the linkage block (and, instead of
linking in "manually", the mem.llst vector is used), at least in my
version of SMSQ/E.
And, you're right, the code you cite boils down to the two instructions
you mention. The documentation mentions that the linkage block needs to
be memory allocated through sms.alchp, so -12(a1) actually seems to be
pointing into the heap header and mangles something in there.
Cheers,
Tobias
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