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Computer Architecture - Memory: Memory Segment

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• Computer Architecture - Memory: Memory Segment
  • Memory Segments
  • Text/Code Segment
  • Data Segment
  • BSS(Block Started by Symbol) Segment
  • Heap Segment
  • Stack
  • OS Kernel Segment

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Memory Segments

• Memory Segments

  • A process’s virtual address space is typically divided into logical regions, the segments with different purposes and permissions.
• A process is an instance of a program that requires CPU and memory.
  • Individual process cannot access the physical memory directly, but access the virtual memory mapping to physical memory.
• Virtual memory is divided into pages with same size; Physical memory is divided into frames with same size.

• The virtual memory of a process is divided into regions, each of which serves a specific purpose.

  • Text/Code
  • Data
  • BSS
  • heap
  • stack
  • operating system kernel space
• Benefits of memory segments
  • Security
    • Each memory region has specific access permissions and privilege level.
    • e.g.,
      • Text/Code segment is read only and executable.
      • stack, heap is readable and writable.
  • Access pattern (sequential vs random)
    • Each segment has different access pattern
    • e.g.,
      • stack: predictable sequential pattern making memory allocation easy and fast
      • heap: in dynamic and random memory allocation,
  • Isolation of bugs (buffer overflows)
    • prevent accidential overwrites and bugs like buffer overflow

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Text/Code Segment

• text/code segment
  • the memory segment contains executable instructions of the program, which typically is loaded from the binary file that contains compiled code.
  • permission:
    • read-only + executable (r-x)
    • prevent accidental/self-modifying writes.
  • segment size: = the required space to load the entire code.

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Data Segment

• data segment
  • the memory segment that store global/static variables that have initialized values(nonzero).
  • to reduce the size of executable files and make the file load faster
    • initialized variables needs to be explictly stored in the executable files when the program starts
  • permission
    • read/write (rw-).

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BSS(Block Started by Symbol) Segment

• BSS(Block Started by Symbol) Segment
  • the memory segment that store global/static variables that have uninitialized values(zero).
  • to reduce the size of executable files and make the file load faster
    • initialuninitializedized variables does not need to be stored in the executable files when the program starts
  • permission:
    • read/write (rw-).

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Heap Segment

• heap

  • the memory segment for dynamic allocation
  • e.g., new keyword in Java.
  • use the memory allocator to manage the memory available to the program during runtime.
  • Expands upward (to higher addresses) as request more memory.

• memory allocator:

  • tracks which part of memory are free and which are in use.
    • finds the suitable block of free memory, marks it as occupied, and hands over.
    • reclaims the block when the program is done.
  • it takes time to find free memory block and reclaim unused block, making memory allocation and releases in stack are slow.

• memory leak

  • a type of programming error where a program allocates memory but fails to release it after it’s no longer needed.
  • “leaked” memory becomes unusable by other parts of the system and can accumulate over time,
    • leading to performance degradation, slow response times, and even application crashes as the system runs out of available memory.

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Stack

• Stack

  • the memory segment for simple and automatic storage
  • used to store temporary data,
    • local variables,
    • function parameters,
    • and return addresses during function calls.
  • structure:
    • stack frame
      • the more nested a function calls, the more frames are stacked on top
    • last in first out
  • Grows downward (to lower addresses)

• stack pointer

  • a small, special register in a computer’s processor that holds the memory address of the top/bottom of the stack.
    • every time a stack frame is pushed, the stack pointer advances by the size of the frame.
    • When the frame is popped, the pointer moved back by the size of the last frame.
  • It is the reason why memory allocation and releases in stack are fast.

• stack overflow

  • an error that occurs when a program attempts to store more data on the stack than its allocated capacity
    • stack memory segment is limited to a certain size(1~8MB).
  • common cause:
    • a recursive function without a proper stopping condition
    • a function with a deep function call chain.
    • a function with unusually large local variables.
  • leading to a program crash or a segmentation fault.

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OS Kernel Segment

• OS Kernel segment
  • reserved for the OS pages, mapped to the OS systems kernel