From:	russell DOT thamm AT dsto DOT defence DOT gov DOT au
Newsgroups:	comp.os.msdos.djgpp
Subject:	Accessing Reflective Memory using Pointers
Date:	Tue, 12 Jan 1999 03:33:53 GMT
Organization:	Deja News - The Leader in Internet Discussion
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To:	djgpp AT delorie DOT com
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Reply-To:	djgpp AT delorie DOT com

I am trying to use reflective memory from DJGPP. I have no problems
accessing the reflective using _farpeek/_farpoke. However, the
main attraction of using reflective memory is that it appears to
be normal memory. Reflective memory access should be transparent
to the program.

Therefore, I wish to access this memory using pointers eg rfm->field1.

The FAQ says that this can't be done (under DJGPP 2) but offers no
explanation as to why.

I am running under CWSDPMI with virtual memory disabled. It occurred to
me that I should be able to create a pointer that points to the start
of the reflective memory by finding the base address of the segment
whose selector is in the DS register and subtracting it from the
physical address of the Reflective Memory.

However, when I examine the entry in the LDT, I noticed two problems:

Firstly, the base address is 0x1000000 which is incomprehensible to me.
I am assuming that a linear address is the same as the physical address
because virtual memory is disabled.

Secondly, the limit for this segment is too small. This limit appears
to increase as memory is allocated but does not appear to decrease as
memory is freed. Can I modify the limit to include the entire address
space?

Following is a printout of the LDT:

80 Code-cRA DPL 3 00017EB0:0FFFF Flg 0
88 Data-eWA DPL 3 00017EB0:0FFFF Flg 4
90 Data-eWA DPL 3 00017DB0:0FFFF Flg 0
98 Data-eWA DPL 3 00017B70:0FFFF Flg 0
A0 Code-cRA DPL 3 10000000:5FFFF Flg 4        CS
A8 Data-eWA DPL 3 10000000:5FFFF Flg 4        DS,SS,ES
B0 Data-eWA DPL 3 10000000:5FFFF Flg 4
B8 Data-eWA DPL 3 00000000:FFFFF Flg C        GS
C0 Data-eWA DPL 3 00020DC0:007FF Flg 4
C8 Data-eWA DPL 3 000219C0:00087 Flg 4
D0 Data-eWA DPL 3 000215C0:003FF Flg 4        FS

I'd be grateful for any assistance,
Russell Thamm



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