Article by Ayman Alheraki on January 11 2026 10:37 AM
Stack manipulation instructions form a core subset of the x86-64 instruction set, enabling explicit control over the CPU stack — a region of memory that supports function calls, local variable storage, parameter passing, and interrupt handling through last-in-first-out (LIFO) semantics. Correct management of the stack is crucial for reliable subroutine linkage, exception handling, and efficient program execution.
x86-64 architecture offers dedicated instructions for pushing data onto the stack, popping data off the stack, adjusting the stack pointer, and managing call frames. The design of these instructions ensures optimized code size and execution speed while maintaining the integrity of stack data.
Before discussing stack manipulation instructions, it is important to understand the registers involved:
RSP (Stack Pointer Register):
Points to the current top of the stack in memory.
Automatically adjusted by push and pop instructions.
SS (Stack Segment Register):
Defines the segment base for stack operations, typically defaulted in flat memory models but used in segmented addressing modes.
In 64-bit mode, the stack is typically a contiguous linear address space managed by RSP, and most instructions operate relative to this register.
PUSH instructions add data onto the stack by decrementing the stack pointer (RSP) and storing the operand at the new top of the stack.
Operation:
Decrement RSP by the size of the operand (typically 8 bytes in 64-bit mode).
Store the operand value at the memory location pointed to by RSP.
Operands:
General-purpose registers (e.g., PUSH RAX)
Immediate values (e.g., PUSH 0x10)
Memory operands (e.g., PUSH [RBP-8])
Use Cases:
Saving registers before procedure calls or interrupts.
Passing parameters to functions in non-register calling conventions.
Preserving CPU state.
Encoding:
Encodes with specific opcodes depending on operand type.
For registers, there is a single-byte opcode for common registers, enabling compact code.
POP instructions remove data from the stack by loading the value at the top of the stack into a destination operand and then incrementing the stack pointer.
Operation:
Load the value at address RSP into the destination register or memory location.
Increment RSP by the size of the operand.
Operands:
General-purpose registers (e.g., POP RBX)
Memory locations (e.g., POP [RDI])
Use Cases:
Restoring saved registers after function calls or interrupts.
Retrieving return addresses manually if needed.
Encoding:
Similar compact encoding as PUSH for registers.
Supports memory operand addressing with ModR/M bytes.
PUSHA/POPGA (16-bit registers only): Pushes or pops all general-purpose registers in order.
These instructions are not available in 64-bit mode and thus rarely relevant in x86-64 assembler design. They remain supported in legacy modes for backward compatibility.
These instructions are designed to simplify the management of procedure stack frames, especially for high-level language compatibility.
ENTER
Allocates space on the stack for local variables and sets up a new stack frame.
Syntax: ENTER <size>, <nesting_level>
Operation:
Pushes the current RBP onto the stack.
Sets RBP to current RSP.
Allocates <size> bytes by subtracting from RSP.
Handles nested procedure calls with stack frame links based on <nesting_level>.
Usage: Simplifies complex stack frame setup for compilers.
Performance note: Modern compilers often avoid ENTER due to higher latency, favoring manual instructions (PUSH RBP + MOV RBP, RSP + SUB RSP, size).
LEAVE
Restores the previous stack frame and stack pointer.
Equivalent to:
MOV RSP, RBP POP RBP Usage: Cleans up stack frame at function exit.
Besides explicit stack instructions, arithmetic on the stack pointer register is common:
SUB RSP, imm
Reserves space on the stack by decrementing RSP (stack grows downward).
Used to allocate space for local variables or to prepare stack before calls.
ADD RSP, imm
Releases space by incrementing RSP, effectively deallocating stack space.
These instructions are commonly used for manual stack frame manipulation where fine control over layout and size is required.
CALL and RET instructions implicitly manipulate the stack:
CALL pushes the return address onto the stack before jumping to the target.
RET pops the return address off the stack and jumps back.
This implicit manipulation is critical for maintaining correct control flow and must be carefully coordinated with explicit stack instructions to avoid corruption.
Interrupt handlers use the stack to save processor state automatically on interrupt entry.
Instructions like IRET or IRETQ pop saved state, including flags and instruction pointer, to resume normal execution.
Stack manipulation in interrupt contexts is sensitive; improper handling leads to system crashes or undefined behavior.
Modern x86-64 ABI standards require the stack to be aligned on a 16-byte boundary before calling functions.
Assemblers must generate code that respects this alignment for interoperability and performance, especially for SIMD instructions and system calls.
Failure to maintain proper alignment can cause faults or degraded performance.
Stack-based buffer overflows are a common security vulnerability.
Modern software uses stack canaries — special values placed between buffers and control data to detect corruption.
While canary management is typically handled by compiler-generated prologues/epilogues, assembler designs for custom usage may include instructions to manipulate these canaries explicitly.
Stack manipulation instructions in x86-64 provide fundamental capabilities for managing the program's runtime environment, enabling function calls, local variable storage, parameter passing, and interrupt handling. Mastery of these instructions is essential for generating correct and efficient machine code.
An assembler must encode these instructions correctly, enforce calling conventions, maintain stack alignment, and support advanced features such as frame setup and security mechanisms. Designing a custom assembler that properly handles stack manipulation leads to reliable and optimized programs compatible with modern OS and application requirements.