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CS253 Memory Segments

Segments

• There are several “segments”, or areas of memory, where parts of your program live, in a typical computer architecture:
  • Text: executable code (machine instructions)
  • Data: initialized static data
  • BSS: uninitialized static data
  • Stack: local variables
  • Heap: allocated via malloc() or new

Text: executable code

• The text segment contains executable code (cpu instructions). Don’t obsess over the name—it’s not text strings.
• If the hardware architecture permits, text is marked as read-only and executable, where other segments are not.
  • This allows sharing. When several users are running bash, we only need one copy of bash’s executable code in RAM. This saves memory and and increases the chance of instructions being in cache.
• The text segment is stored in the a.out file.
  • That’s the executable machine language program on the disk, called a.out (assembler output) by default by g++.

Data: initialized static data

• The data segment contains initialized static data.
• It is read-write at run-time—programs may alter their variables.
• The data segment is stored in the a.out file.

BSS: uninitialized static data

• The BSS segment contains “uninitialized” static data.
• It is zero-initialized at initial load of the program.
• It is read-write at run-time—programs may alter their variables.
• BSS has no associated data, so the a.out file only has to know how big the BSS segment is, lay out that much space at run-time, and clear it. Almost no space required in a.out.
• Clever compilers put zero-initialized static variables in BSS.
• The origin of the name is unclear—perhaps the assembler pseudo-op Begin Section started by Symbol.

Stack: local variables

• The stack segment contains local variables (declared at function scope).
  • If no initial value is given, the value is unknown.
    • It contains whatever value the memory had before that.
    • It might be initially zero due to the OS clearing memory.
  • If an initial value is given, it is assigned at run-time.
• It is read-write at run-time—programs may alter their local variables.
• It is not stored in the a.out at all, except implicitly, in subroutine preamble code.

Heap: allocated via new

• The heap segment is contains uninitialized dynamic variables.
• It is read-write at run-time—programs may alter their local variables.
• It is not stored in the a.out at all.
• Traditionally, free/allocated dynamic memory is kept track of via a “heap” data structure, hence the name.

Example

int b1[3], d1=45;
const int t1 = 299'792'458;

int main() {
   static double b2 = 0.0, d2 = 2.718281828;
   int s1 = 10, *s2 = new int[s1];
   vector<int> s3 = {123,456,789};
   map<const void *, string> stuff = {
      {&s1, "s1"}, {&t1, "t1"},
      {&s2, "s2"}, {&b1, "b1"}, {&d1, "d1"},
      {&s3, "s3"}, {&b2, "b2"}, {&d2, "d2"},
      {&s2[0], "s2[0]"}, {&s3[0], "s3[0]"},
      {&cout, "cout"}, {(void *) exit, "exit"},
   };
   for (auto p : stuff)
       cout << setw(14) << p.first
            << ' ' << p.second << '\n';
   delete[] s2;
}

      0x401b50 exit
      0x4050f8 t1
      0x608118 d1
      0x608120 d2
      0x608140 cout
      0x608258 b1
      0x608268 b2
     0x25e32b0 s2[0]
     0x25e32e0 s3[0]
0x7fff058afdc0 s3
0x7fff058afde0 s2
0x7fff058afde8 s1

Summary

Text	Data	BSS	Stack	Heap
instruction/const	initialized global	uninitialized global	local var	dynamic memory
static data			run-time data
read-only	read-write
occupies space in a.out		no space in a.out