🚀 go-pugleaf

 * Original <23 May 91 00:16:27> was from Rhoda Neimand to All 

 > RN: From: rhoda@mercury.sybase.com (Rhoda Neimand)
 > RN: 
 > RN: A unix person has just asked me a curious question.  I'm kind
 > RN: of puzzled

Yes, unix people tend to do that to us. :-)

 > RN: because I've never thought of doing this before.  To me, a
 > RN: far pointer
 > RN: is a far pointer and a near pointer is a near pointer, period.

Good enough for me.

 > RN: 
 > RN: His questions:
 > RN: 
 > RN: Can you cast a far pointer to a near pointer?  My answer is
 > RN: that if you did,
 > RN: you would lose its "farness", therefore it would no longer
 > RN: point to anything
 > RN: unless you were sure that its segment address was currently
 > RN: in DS.  Is this
 > RN: correct?

Yup. You loose the segment address and are left only with the offset. This can 
be very dangerous indeed. Especially when doing lost of nasty segment work.

 > RN: 
 > RN: Can you cast a near pointer to a far pointer? 

No problem. This is safe.

 >RN: Not sure here.  If you did,
 > RN: would the segment address be 0?

I don't think so, in the assembly I've looked through,  DS is generally pushed.

 >RN: Does having a segment address of 0 mean
 > RN: something special?

Yes, this is where the 80x8x series of processors keep their interrupt vectors. 
The IBM PC BIOS uses segment 40h ( or 0:0400h ) as the BIOS data area. In any 
mode pointing to anywhere ( that is not owned by yourself ) is a very dangerous 
thing to do. Hence, under DOS bad code can be hard to find. In a protected mode 
environment, such as OS/2 or Windows in 286/386 modes, the memory protection 
will trap this and nail your program - before it does any damage.

 > RN:  MAKEINTRESOURCE does something like this.
 > RN:  I am actually confused about the purpose of MAKEINTRESOURCE.

Have a look in Petzolds Book:

First edition ( the grey one ), page 335-336
Second edition ( the white one ), page 313-314

It explains the special case of having a segment value of zero.

 > RN: 
 > RN: I feel that either casting is not meaningful.  Is this correct?

Yes they are. See above.

 > RN: 
 > RN: Thanks

No problems. Hope this helps.

 > RN: ----------------------------------------------------------------------
 > RN: Rhoda Neimand			                  [I speak for myself]
 > RN: {sun,lll-tis,pyramid,pacbell}!sybase!rhoda -or- rhoda@sybase.com
 > RN: "I didn't want to, Jim"		  Mr. Spock in _This Side Of Paradise_
 > RN: ----------------------------------------------------------------------

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