Ok, calling the system call through the C library was not harder than calling a normal function. Let’s try the same directly performing a Linux system call. First we have to identify the number of the system call and put it in r7. The call write has the number 4 (you can see the numbers in the file /usr/include/arm-linux-gnueabihf/asm/unistd.h). The parameters are usually the same as in the C function, so we will use registers r0, r1 and r2 likewise.