Writing assembly code for x86 architecture

Introduction This document contains very brief examples of assembly language programs for the x The topic of x86 assembly language programming is messy because: There are many different assemblers out there: All use radically different assembly languages.

Writing assembly code for x86 architecture

The call instruction first pushes the current code location onto the hardware supported stack in memory see the push instruction for detailsand then performs an unconditional jump to the code location indicated by the label operand.

Unlike the simple jump instructions, the call instruction saves the location to return to when the subroutine completes. The ret instruction implements a subroutine return mechanism. This instruction first pops a code location off the hardware supported in-memory stack see the pop instruction for details.

It then performs an unconditional jump to the retrieved code location. Syntax ret Calling Convention To allow separate programmers to share writing assembly code for x86 architecture and develop libraries for use by many programs, and to simplify the use of subroutines in general, programmers typically adopt a common calling convention.

The calling convention is a protocol about how to call and return from routines. For example, given a set of calling convention rules, a programmer need not examine the definition of a subroutine to determine how parameters should be passed to that subroutine.

Furthermore, given a set of calling convention rules, high-level language compilers can be made to follow the rules, thus allowing hand-coded assembly language routines and high-level language routines to call one another.

In practice, many calling conventions are possible. We will use the widely used C language calling convention. The C calling convention is based heavily on the use of the hardware-supported stack. It is based on the push, pop, call, and ret instructions. Subroutine parameters are passed on the stack.

Registers are saved on the stack, and local variables used by subroutines are placed in memory on the stack.

Assemblers and Linkers

The vast majority of high-level procedural languages implemented on most processors have used similar calling conventions. The calling convention is broken into two sets of rules. The first set of rules is employed by the caller of the subroutine, and the second set of rules is observed by the writer of the subroutine the callee.

It should be emphasized that mistakes in the observance of these rules quickly result in fatal program errors since the stack will be left in an inconsistent state; thus meticulous care should be used when implementing the call convention in your own subroutines.

The image above depicts the contents of the stack during the execution of a subroutine with three parameters and three local variables. The cells depicted in the stack are bit wide memory locations, thus the memory addresses of the cells are 4 bytes apart.

The first parameter resides at an offset of 8 bytes from the base pointer. Above the parameters on the stack and below the base pointerthe call instruction placed the return address, thus leading to an extra 4 bytes of offset from the base pointer to the first parameter.

When the ret instruction is used to return from the subroutine, it will jump to the return address stored on the stack. Caller Rules To make a subrouting call, the caller should: Before calling a subroutine, the caller should save the contents of certain registers that are designated caller-saved.

Since the called subroutine is allowed to modify these registers, if the caller relies on their values after the subroutine returns, the caller must push the values in these registers onto the stack so they can be restore after the subroutine returns.

To pass parameters to the subroutine, push them onto the stack before the call.

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The parameters should be pushed in inverted order i. Since the stack grows down, the first parameter will be stored at the lowest address this inversion of parameters was historically used to allow functions to be passed a variable number of parameters.

To call the subroutine, use the call instruction. This instruction places the return address on top of the parameters on the stack, and branches to the subroutine code.

This invokes the subroutine, which should follow the callee rules below. After the subroutine returns immediately following the call instructionthe caller can expect to find the return value of the subroutine in the register EAX.

To restore the machine state, the caller should: Remove the parameters from stack. This restores the stack to its state before the call was performed. The caller can assume that no other registers were modified by the subroutine.Assembly language is a low-level programming language for a computer or other programmable device specific to a particular computer architecture in contrast to most high-level programming languages, which are generally portable across multiple systems.

7: Basic x86 architecture. Computer Architecture and Systems Programming. understand to write assembly code. Examples: – instruction set specification, registers. bit x86 architecture. Computer Architecture and Systems Programming.

, Herbstsemester x86 assembly language is a family of backward-compatible assembly languages, These opcodes often turn up in code writing competitions as a way to make the code smaller, faster, more elegant or just show off the author's prowess.

The x86 architecture has hardware support for an execution stack mechanism. Instructions . MASM uses the standard Intel syntax for writing x86 assembly code. The full x86 instruction set is large and complex (Intel's x86 instruction set manuals comprise over pages), and we do not cover it all in this guide.

For example, there is a bit subset of the x86 instruction set. Using the bit programming model can be quite complex. A fundamental introduction to x86 assembly programming 0. Introduction. The x86 instruction set architecture is at the heart of CPUs that power our home computers and remote servers for over two decades.

Being able to read and write code in low-level assembly language is a powerful skill to have. It enables you to write faster code, use . x86 assembly is known as a complex instruction set computing, or CISC.

This instruction set architecture allows multiple methods of addressing memory and a few instructions that are positively baroque (there's a single instruction that copies a string, for example).

writing assembly code for x86 architecture

Assembly language allows you to write code at the lowest level without.

Guide to x86 Assembly