Dynamic Memory: Heap Allocation in Assembly
Request and release memory at run time from assembly using the OS heap interfaces (brk/mmap on Linux, VirtualAlloc on Windows) and C library malloc/free.
Dynamic Memory: Heap Allocation in Assembly is a free Assembly Language & x86 Low-Level Systems Programming lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Assembly Language & x86 Low-Level Systems Programming learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Stack vs Heap
The stack holds fixed, scope-bound data and is automatically reclaimed. The heap provides dynamic memory whose size and lifetime you control at run time.
When You Need the Heap
Use heap memory when the size is unknown at assemble time or the data must outlive the procedure that created it, e.g. growing buffers and data structures.
Linux: the brk Syscall
The brk/sbrk syscall grows the programs data segment. It is low level and mostly superseded by mmap for general allocation.
mov rax, 12 ; sys_brk
mov rdi, 0
syscallLinux: mmap
mmap maps fresh anonymous pages and is the modern way to get large heap regions directly from the kernel.
mov rax, 9 ; sys_mmap
syscallReleasing with munmap
Memory obtained from mmap is returned with munmap, telling the kernel those pages are free.
mov rax, 11 ; sys_munmap
syscallWindows: VirtualAlloc
On Windows you call VirtualAlloc to reserve and commit pages, and VirtualFree to release them.
call VirtualAllocUsing the C Library
When linking with the C runtime, it is far simpler to call malloc and free, which manage a heap on top of the OS primitives.
mov rdi, 64 ; bytes
call mallocChecking the Result
Allocation can fail. malloc returns NULL and syscalls return a negative error code, so always test the result before using the pointer.
test rax, rax
jz alloc_failedFreeing Memory
Every allocation needs a matching free. Forgetting to free leaks memory; freeing twice or using freed memory corrupts the heap.
mov rdi, rax ; pointer
call freeAlignment Considerations
Heap blocks are returned suitably aligned for any type. Manual allocators must respect alignment for SSE and atomic operations.
Common Pitfalls
Heap bugs are subtle:
- Memory leaks from missing free
- Use-after-free
- Double free
- Buffer overruns past the allocation
Quick Check
Test your understanding of heap allocation.
Recap
You learned heap allocation:
- Stack is automatic; heap is run-time managed
- Linux:
mmap/munmaporbrk - Windows:
VirtualAlloc/VirtualFree - With libc use
malloc/free; always check for NULL and free once
Frequently asked questions
Is the “Dynamic Memory: Heap Allocation in Assembly” lesson free?
Yes — the full text of “Dynamic Memory: Heap Allocation in Assembly” is free to read here on the web, and the Assembly Language & x86 Low-Level Systems Programming course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Assembly Language & x86 Low-Level Systems Programming course, upgrade to CoddyKit PRO.
What will I learn in “Dynamic Memory: Heap Allocation in Assembly”?
Request and release memory at run time from assembly using the OS heap interfaces (brk/mmap on Linux, VirtualAlloc on Windows) and C library malloc/free. You practise Assembly Language & x86 Low-Level Systems Programming with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Assembly Language & x86 Low-Level Systems Programming?
No prior experience is required. Assembly Language & x86 Low-Level Systems Programming on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Dynamic Memory: Heap Allocation in Assembly” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Assembly Language & x86 Low-Level Systems Programming lesson?
Yes. Every Assembly Language & x86 Low-Level Systems Programming lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Virtual Memory Concepts
- Linux System Calls (syscalls)
- Windows API Interaction
- Dynamic Memory: Heap Allocation in Assembly