In the course "Systemnahe Programmierung" (translates to "System-Level Programming"), we spent some time working with assembly language. Much to our dismay, this didn't just involve reading and understanding assembly code — we also had to write it ourselves. This meant creating small programs, compiling them, and actually running them.

Example Program

To understand how function calls work in assembly, we had to write a program that reads a string using fgets, converts it to uppercase, and prints it to the console.

extern puts
extern fgets
extern stdin
extern toupper
; RDI, RSI, RDX, RCX, R8

SECTION .data
    msg:	db 'Hello ',13,10,'world!',13,10,0 ; Zero is Null terminator 
    input: times 21 db 0 ;

SECTION .text
    global main
    
main:
    push rbp ; Push stack

    ;String einlesen
    mov rdi, input
    mov rsi, 20
    mov rdx, [stdin]
    call fgets

    ;loop
    mov r15, -1 ; r15 nehmen weil r9 an irgendeiner stelle überschrieben wird
    lop:
    inc r15

    ; Aktuellen Char auslesen
    mov r8, 0
    ;lea rax, [input+r15]     ; load adress of current char to rax
    mov r8b, [input+r15]                  ; buffer current character 

    ;Newlines (13 und 10) durch beep (7) ersetzen
    cmp r8b, 13
    jne weiter
    mov r8b, 7
    mov [input+r15+1], byte 7
    weiter:

    ; In Großbuchstaben konvertieren
    mov	rdi,r8
    mov	rax,0
    call toupper

    ;Zurück in String schreiben
    mov [input+r15], al

    ;loop
    cmp al, 0
    jne lop

    ;String ausgeben
    mov	rdi, input
    mov	rax,0
    call puts

    pop	rbp		; Pop stack

    mov	rax,0	; Exit code 0
    ret			; Return

This can be compiled using the following short Makefile:

CFLAGS = -Wall -Wextra -v -no-pie  -o

all: comp

comp:
nasm -f elf64 -g -F dwarf -o glibber.o glibber.S
clang $(CFLAGS) glibber glibber.o


clean:
rm -f glibber
rm -f glibber.o

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