/Teaching/System Level

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 special kinds 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. For
instance, the rdtsc instruction, which you may use
in the Operating Systems course for precise timing, or the later discussed cpuid instruction. 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. In order to gather experience with inline assembly you will implement the first task of
this exercise.


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. ‘AuthenticAMD’)
  • Processor brand string (e.g. ‘AMD Ryzen 9 3900X 12-Core Processor’)

You should also determine which of the following features are supported by reading the corresponding feature flags
(for more information about these flags, see the resources). It is sufficient to stick to Intels layout for the
feature flags.

  • FPU
  • PAE
  • MMX
  • SSE2
  • SDBG
  • SMAP
  • SSBD

Print the retrieved information to stdout via the printf calls provided as comments in the code (see file cpuid/cpuid.c). 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.
Verify your results by comparing them with the output of cat /proc/cpuinfo.


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 Conventions and are part of the Application Binary Interface
(ABI). The ABI is 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.)
The aim of this part of the exercise is to familiarize yourself with the System V AMD64 ABI, which generates an essential understanding you will
need to start new threads later in the Operating Systems course for example.

Your Tasks

Task A: Call a function in inline assembly (5 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 syscall in assembly (5 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.

#define SIZE 4

int read_secret_number(char* filename, char* buffer, int secret)
    int fd = open(filename, O_RDONLY);
    if (fd < 0)
        return -1;
    int bytes_read = read(fd, buffer, SIZE);

    if (bytes_read != SIZE)
        return -1;

    int number = atoi(buffer);
    number ^= secret;

    return number;

Resources for GCC inline assembly

Inline Assembly Overview

GCC Inline assembly

Output / Input operands

Clobber list

Input/output constraints for gcc inline assembly:


For compiling your programs follow these steps:

  • Open the A6 folder in a terminal.
  • Create a new directory called build in
    A6 using mkdir build and enter it.
  • In build call cmake .., this will set up the build
  • Execute make. This compiles your code.
    The executables are put into the corresponding subdirectories.
  • Do not push the build directory or its
    content to your git repository.


In order to debug your submission, you can use gdb with the peda plugin. This provides a very convenient overview of register
contents, which may be helpful while solving the tasks.


Develop your solution in the A6 folder in your git repository and use the provided files. Changes to your
Makefiles won’t be included in 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 slp@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:

Simon Paul Possegger, simon.possegger@student.tugraz.at