Memory in C

Memory Layout of C Programs

• text (code/instructions)
• initialized data
• uninitialized data (bss)
• heap
• stack

Text (Code/Instructions)

Initialized Data

The static variables are stored in the data segment of the memory. Different from the stack frame allocation for a certain function call, i.e., its value persists even after the function where it appears has returned.

Uninitialized Data (bss)

Heap

When you request some memory to store something on the heap using new or malloc, you don’t get a variable name and the only way to access it is through a pointer variable. Anything stored on the heap will stay on memory, unless explicitly freed using the free() function.

Stack

Stack frame allocation (correspondant to each call of a certain function) - each time the function execution finishes all the local variables are gone from the memory.

Whenever a function is invoked, some amount of memory from the stack is allocated for execution of that function, and it’s called stack frame. When a function call execution finishes, the stack frame is reclaimed.

Why use dynamic memory allocation instead of stack memory

Explained really well using linked lists.

Info: https://stackoverflow.com/questions/14588767/where-in-memory-are-my-variables-stored-in-c

MMU (Memory Management Unit)

MMUs are complex hardware engines. Their purpose is to enable an operating system to exercise a high degree of management and control over its address space and the address space it allocates to processes, i.e.:

• Access rights
• Memory translation

Their purpose is to enable an operating system to exercise a high degree of management and control over its address space and the address space it allocates to processes.

In traditional embedded systems, the OS and all the tasks had equal access rights to all resources in the system which lead to problems where a bug in a process could wipe out memory contents anywhere in the system.

Legacy embedded operating systems view and manage system memory as a single large, flat address space. That is, a microprocessor’s address space exists from 0 to the top of it’s physical address range.

Advantages of using Linux on embedded systems:

You have both the security of a protected address space that a virtual memory-based system gives you and the power and flexibility of a multiuser, multiprocessor system.

Virtual Memory Architecture

Paging

Paging is a memory management technique used by operating systems to implement virtual memory. It divides physical memory into fixed-size blocks called frames, and virtual memory into blocks of the same size called pages.

When a program accesses memory, the Memory Management Unit (MMU) translates the virtual address generated by the CPU into a physical address using a page table maintained by the operating system.

The translation works as follows:

1. The virtual address is divided into a page number and an offset.
2. The page number indexes into the page table.
3. The page table entry contains the frame number where the page is stored in physical memory.
4. The frame number combined with the offset forms the final physical address.

Paging provides several important benefits:

• Memory isolation between processes
• Efficient memory allocation without requiring contiguous physical memory
• Support for virtual memory, allowing programs to use more memory than physically available
• Memory protection through page permissions (read, write, execute)

Modern CPUs also use a Translation Lookaside Buffer (TLB), a small cache that stores recent page table translations to speed up address translation.