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HarryTuttle



Joined: 26 Sep 2003
Posts: 211
Location: Poland
HarryTuttle
what's a different in calls to far or near addresses?

is it any difference in protected mode?

what about calls and stack ?

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Post 14 Feb 2005, 10:21
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MCD



Joined: 21 Aug 2004
Posts: 604
Location: Germany
MCD
That's a quiet nebie question, but I will try to answer it anyway.

Near addresses have only an offset value, whereas far addresses have a (always) 16bit segment value/selector too.

Thus, there are 6 basic possibilites for the call instruction with direct values:
Code:
in RM:
call 16bit_offset    1+2 bytes
call 16bit_segment_value:16bit_offset    1+2+2 bytes

in 16bit PM:
call 16bit_offset    1+2 bytes
call 16bit_segment_selector:16bit_offset    1+2+2 bytes

in 32bit PM:
call 32bit_offset    1+4 bytes
call 16bit_segment_selector:32bit_offset    1+2+4 bytes
    

Note that you can (try to) code calls with 32bit offsets in both RM and 16bit PM, but they won't usually work (CPU limited code <1Mb). You can also try to code calls with 16bit offsets in 32bit PM. I both of these cases, where you call an offset with another size, the machine instruction needs an additional prefix, e.g. the addr. size 67h.

So, there are far calls in PM, but you usually don't use them in PM, since most 32bit PM OSes user code uses a rather flat memory approach and thus you don't need segment selectors. It's usually reserved for the Kernel and some drivers.
Post 14 Feb 2005, 10:46
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Madis731



Joined: 25 Sep 2003
Posts: 2141
Location: Estonia
Madis731
I think you haven't done your homework Smile
but the general rule is when the address that is being jumped to is ±127 bytes, then you can use near jump, otherwise you need more bytes to describe how far you'd like to jump.

Totally different is absolute jump that doesn't add to(sub from) but replaces the address of the destination with (E)IP.

calls and stack? What about them?
Post 14 Feb 2005, 11:05
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HarryTuttle



Joined: 26 Sep 2003
Posts: 211
Location: Poland
HarryTuttle
is it true that if I calls far procedure then CS is pushed on the stack after offset?

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Post 14 Feb 2005, 14:42
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MazeGen



Joined: 06 Oct 2003
Posts: 975
Location: Czechoslovakia
MazeGen
That's not true, CS is always pushed before EIP. Note that in 32-bit protected mode is pushed CS padded with 16 high-order bits.
Post 17 Feb 2005, 16:58
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