Program Layout

• This is Jorgensen chapter 4.
• Programs are organized in memory into different segments.
  • the .text segment
    • As we have seen, is where the program is located.
    • marked with section .text
    • The loader expects _start to be defined, at least in linux
      • This is the equivlelent to main in c/c++
    • _start, needs to be exposed to the outside via the global assembler directive.

      • [bennett]$ make regDemo2
        nasm -f elf64 -g regDemo2.asm -o regDemo2.o
        ld -g  regDemo2.o -o regDemo2
        ld: warning: cannot find entry symbol _start; defaulting to 0000000000401000 

      • But it will start at the beginning of the code.
    • reference (sort of)
  • Both segment and global are assembler directies
  • Initialized data goes in the .data segment.
    • segment .data
    • Initialized data goes into this section.
    • The assembler will provide this with initial values
  • unintialized data goes in the .bss section
    • segment .bss
    • The OS will then initailize this with 0.
• Other memory segments
  • We have discussed the stack.
    • The push instruction pushes a value onto the stack.
    • Notice what happens to the *sp register when this happens.
  • The heap
    • I hope you have discussed the heap in programming
    • But if not, this is where new or malloc grab memory.
• A generic view of how the os lays out memory
  • (Wikipedia)