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Library Disassembly Notes

References

• ldd(1) - print shared object dependencies
• vdso(7) - overview of the virtual ELF dynamic shared object
• libc(7) - overview of standard C libraries on Linux
• ld.so(8) - dynamic linker/loader
• nm(1) - list symbols from object files

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/* This file contains an unoptimized, recursive GCD implementation. */
/* The code assumes %rdi and %rsi will be unsigned... if signed, then cqto will be needed to sign-extend %rax to %rdx:%rax. */
/* What are some ways to optimize it? */

    .data
fmtstr:
    .ascii  "%d\n"

    .text
    .globl gcd

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%eflags

Flags

Flag	Description
CF	Carry Flag
PF	Parity Flag
AF	Adjust Flag
ZF	Zero Flag

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Interval Timer Notes

The system provides each process with three interval timers, each decrementing in a distinct time domain. When any timer expires, a signal is sent to the process, and the timer (potentially) restarts.

Timers will never expire before the requested time, but may expire some (short) time afterwards, which depends on the system timer resolution and on the system load; see time(7). Upon expiration, a signal will be generated and the timer reset.

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System Call Notes

Invoking System Calls

To invoke a system call:

1. Set %rax to the system call number (see Full Table below);
2. Set %rdi, %rsi, %rdx, %r8, %r10, %r9 (in order) according to the glibc prototype given in the manual; then
3. Perform syscall instruction.

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main.s

	.text
	.globl main
	.type main, @function
main:
	pushq 	%rbp		;
	movq	%rsp, %rbp	;
	pushq	%rbx ;

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main.s

	.text
	.globl main
	.type main, @function
main:
	pushq 	%rbp		;
	movq	%rsp, %rbp	;
	pushq	%rbx ;

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main.c

#include <unistd.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>

int main() {
 pid_t pid = fork();

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main: main.o asm.o
	gcc -o main main.o asm.o

main.o: main.s
	as --gstabs+ -o main.o main.s

main.s: main.c main.h
	gcc -std=c17 -Wall --pedantic-errors -g -O0 -S -o main.s main.c

asm.o: asm.s main.h

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pmem.c

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>

int a = 4;
int b;

void foo(void) { }