add arm syscall example

arm_asm/Makefile

+
+.PHONY: run debug clean
+
+all: main.arm32
+
+main.arm32: main.c add.S mywrite.S
+	arm-linux-gnueabihf-gcc -marm -g -O0 -o $@ $^
+
+run: main.arm32
+	qemu-arm -L /usr/arm-linux-gnueabihf/ $<
+
+debug: main.arm32
+	@echo "attach using"
+	@echo "arm-none-eabi-gdb $<\n"
+	qemu-arm -g 1234 -L /usr/arm-linux-gnueabihf/ $<
+
+clean:
+	rm -f main.arm32

arm_asm/README.md

+Simple example for a C project integrating ARM assembly.
+You need an arm toolchain to build this.
+Change the compiler to your needs (`arm-linux-gnueabihf-gcc`).
+The `Makefile` uses `qemu-user` to run the example.

arm_asm/add.S

+.section .text.add, "x"
+
+.global add
+
+add:
+  add r0, r0, r1
+  mov pc, lr

arm_asm/main.c

+#include <string.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+extern int add(int, int);
+extern int mywrite(int fd, void* buf, size_t count);
+
+int sub(int a, int b){
+  return a - b;
+}
+
+int main(int argc, char** argv){
+  int r = 0;
+  char* name = "Hello Reverser!\n";
+
+  write(STDOUT_FILENO, name, strlen(name));
+
+  r = sub(20, 10);
+  printf("sub(20, 10) = %d\n", r);
+  r = add(10, 20);
+  printf("add(10, 20) = %d\n", r);
+
+  mywrite(1, "Hello!\n", 8);
+  return EXIT_SUCCESS;
+}

arm_asm/mywrite.S

+.section .text.mywrite, "x"
+
+.global mywrite
+
+mywrite:
+  push {r7, lr}
+  mov r7, #4
+  swi 0
+  pop {r7, pc}