For example, say I have something like:
0x33:0x10
and I want to get an address in the form:
0x????????
from it. What is the easiest way to go about doing this? This is on Linux x86.
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On 32 bits x86 or 64 bits x86-64 Linux systems there is only a flat address space! (no segmentation) – Basile Starynkevitch Aug 03 '12 at 17:02
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The why do I see in assembly code like gs:0x10? – Bhubhu Hbuhdbus Aug 03 '12 at 17:03
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The gs segment register is, IIRC, for thread local storage (but I could be wrong) on x86/32 bits, and it is not really a segment anymore, just a way of indexing... – Basile Starynkevitch Aug 03 '12 at 17:09
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There are also others such as ds:OFFSET, es:OFFSET, etc. – Bhubhu Hbuhdbus Aug 03 '12 at 17:16
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It would be useful to have a quick way of converting these to regular addresses. – Bhubhu Hbuhdbus Aug 03 '12 at 17:16
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Why are you asking? Most compilers rarely generate such code. – Basile Starynkevitch Aug 03 '12 at 17:24
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Really? I was looking @ PHPs source code in assembly using GDB and it was filled with such code. – Bhubhu Hbuhdbus Aug 03 '12 at 17:26
1 Answers
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The selector in the segment register is decoded to give a descriptor table specifier and a descriptor table index. The base given in the table and index is added to the offset in order to give a final address within process space. In order to convert process memory to physical memory, the page tables must be read and decoded.
Ignacio Vazquez-Abrams
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