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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
- 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.
- It is typically read-only at run-time.
- It can be shared—if several users are running
bash, then 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.
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.
- Since bss has no data associated with it, the
a.out file
just 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
the a.out file.
- The origin of the name is unclear. It may have been an old assembly
pseudo-op for Begin Section started by Symbol.
Stack: local variables
- The stack segment is contains uninitialized local variables.
- 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 a, b=45;
const int c = 299'792'458;
int main() {
static double d, e = 2.7182818284590452353;
long *f = new long[10];
vector<int> g = {123,456,789};
cout << "&exit: " << (void *) &exit << "\n"
"&c: " << &c << "\n\n"
"&b: " << &b << "\n"
"&e: " << &e << "\n"
"&cout: " << &cout << "\n\n"
"&a: " << &a << "\n"
"&d: " << &d << "\n\n"
"&f[0]: " << &f[0] << "\n"
"&g[0]: " << &g[0] << "\n\n"
"&g: " << &g << "\n"
"&f: " << &f << "\n";
delete[] f;
}
&exit: 0x400950
&c: 0x401518
&b: 0x603090
&e: 0x603098
&cout: 0x6030a0
&a: 0x6031b8
&d: 0x6031c0
&f[0]: 0x16072b0
&g[0]: 0x1607310
&g: 0x7ffc0d449550
&f: 0x7ffc0d449570
