/Teaching/System Level Programming/Assignments/A6

Pull from upstream before solving this task.

Task 6.1: Inline Assembly

Assembly Language

To be able to execute programs written in high level programming languages, they first have to be translated into CPU instructions using a compiler. Language constructs and statements of high level programming languages are CPU architecture independent and do not have any special relationship with individual CPU instructions. The compiler is responsible for selecting and emitting the appropriate instructions which are required for the task at hand.

Assembly languages are a special kind of low level programming languages. Unlike their high level counterparts, assembly languages are not architecture independent but instead target a specific instruction set. While language constructs in high level languages may be compiled into any number of instructions, each statement in an assembly language is translated into one specific CPU instruction. An assembly language can also be thought of as a translation between human readable mnemonics and binary opcodes readable by the CPU (e.g.jmp = 0xE9). A compiler for an assembly language is also called an assembler.

While it is usually a lot more efficient and practical to write code in high level languages, some things still require low level assembly instructions because they are simply not possible to accomplish in other programming languages. This is especially true when using CPU hardware features that can only be accessed using special instructions.

When using the GCC C compiler, it is possible to combine normal code written in C with assembly code in the same source file or even the same function by using GCC inline assembly.


The cpuid instruction

On the x86 architecture, the cpuid instruction can be used to retrieve information about the CPU that a program is running on. This includes information about the processor type and manufacturer, as well as information about features and instruction set extensions that are supported by the CPU.
The specific category of information to be retrieved can be selected by setting the eax register (and in some cases also the ecx register) to various ID values before executing the cpuid instruction. The CPU then provides the requested information in the eax, ebx, ecx and edx registers.

Your Task

Use the cpuid instruction to read information about the CPU (5 Points)

Write a program that uses the cpuid instruction to read the following pieces of information about the CPU:

  • Manufacturer id string (e.g. ‘GenuineIntel’, ‘AuthenticAMD’, …)
  • Processor brand string (e.g. ‘Intel(R) Core(TM) i7-4760HQ CPU @ 2.10GHz’)

You should also determine which level of SIMD instructions are supported the CPU by reading the following feature flags:

  • SSE
  • SSE2
  • SSE3
  • SSSE3
  • SSE4.1
  • SSE4.2
  • AVX
  • AVX2
  • AVX512 Foundation

Print the retrieved information to stdout via the printf calls provided as comments in the code. Do not use any compiler intrinsics or library functions to call the cpuid instruction, only inline assembly. Make sure to properly handle all inputs, outputs and clobbered registers in your inline assembly blocks.

Task 6.2: ABI – Calling Conventions

Calling Conventions

Practically all programs are split into modular functions that an application may call from anywhere in the code. In order for this to work, the caller of these functions as well as the called function (the callee) need to have a set of rules that define, e.g. where the parameters for calling the function are stored (in registers or on the stack …), where the return value is stored and which registers the function may use without having to save the previous values. These rules are called Calling Conventionsand are part of the Application Binary Interface (ABI). The ABIis similar to an API(Application Programming Interface) only on the instruction level and heavily depends on the architecture and compiler in use. On x86 64-bitLinux, the standard calling convention is the System V AMD64 ABI. (32-bit Linux and Windows use different calling conventions.)

Your Tasks

Task A: Call a function in inline assembly (4 Points)

Familiarize yourself with the System V AMD64 64-bit calling convention and implement a function call using only assembly instructions via gcc inline assembly. Take care to avoid unintentionally interfering with code outside of your inline assembly block by correctly using the clobber list to notify the compiler about potentially modified registers. (If you call a function in an inline assembly block, all side effects of that function call also need to be considered. Hint: see the calling convention for potential effects you need to take into account)

Use the provided framework in a_caller/caller.c and see the comments for details on what you need to implement.

Do not use C code for this task, only inline assembly!


Task B: Implement a function in assembly (6 Points)

In this part, your task is to implement a small function in assembly in order to get to know the receiving end of a function call.

Use x86 64-bit assembly to implement the following function in b_callee/sysv_abi.S. Follow the System V AMD64 64-bit calling convention like in the previous task and take care to e.g. save and restore registers as required.


int64_t charindex(const char* str, char c, uint64_t max_len)

    size_t index = 0;
    while(index < max_len && str[index])
        if(str[index] == c)
            return index;


    return -1;


Inline Assembly Overview

GCC Inline assembly documentation

Output / Input operands

Clobber list documentation

Input/output constraints for gcc inline assembly:


Develop your solution in the A6 folder in your git repository and use the provided files. Changes to your Makefiles won’t be included on the test system. Do not change the given source files except for the part marked with TODO.

Tag your submission with A6 and push it to the server. Your submission will be tested automatically.

Assignment Tutor

If you have any questions regarding this assignment, feel free to ask on Discord (or bs-helpline@iaik.tugraz.at as a second fallback option). If you have a more direct question regarding your specific solution, you can also ask the assignment tutor:

Florian Kargl, florian.kargl@student.tugraz.at