Wednesday, December 1, 2010

What is an address space

When a program is built(compilation, assembler invocation, and linking)
it is stored in a file with a special format. One such format is
ELF(Executable and Linking Format)in Unix like systems. It has the
necessary information ( in the ELF header and program header) to help
the program loader to load and create the process image in memory at
runtime, and also do run time linking (for shared libraries linked
At the time program is built, storage for all uninitialised data (static or
global) are not allocated, but only the total size is noted in program
header table of the ELF file. Later when program is being loaded, loader
read the metadata from the ELF file and allocate one large chunk (page
aligned) of memory to hose all such variables. This section is called
BSS(Block Static Storage or Block started by symbol).
Data( storage for all initialized static and global variables) and STACK
section should be familiar to you. HEAP is created by dynamic invocation
by memory-alloc routines of your program. In addition to this there will
be shared memory mappings as well. If you process a file using mmap()
system call interface, those regions are also mapped to your program.
Now the sum total of all such memory is called the address space of your

Now what’s an address? Is that a virtual address, physical address
or logical address?
The value you see when you apply an & operator to a C variable
is actually the program-relative logical address. It has to be
processed by segmentation unit(coupled with the value in the CS
register) to create a linear address and then processed by the
paging unit to create the physical address in your RAM. So these
details are not at the control of C, but is at the control of the
memory management subsystems of the operating system.