ASM-ARM

Program Documentation: hello.s

Purpose:

This assembly program is a simple example demonstrating how to use system calls on macOS to print "hello world" and another string to the console.

Instructions for Compilation and Execution:

To compile the program, open the terminal and navigate to the directory containing the source file (hello.s). Then execute the following commands:

as hello.s -o hello.o
ld hello.o -o hello -lSystem -syslibroot `xcrun -sdk macosx --show-sdk-path` -e _main -arch arm64

To run the compiled program, execute:

./hello

Program Structure:

Entry Point (_main):

• The _main function is the entry point of the program.
• It branches to the _printf subroutine to print "hello world".
• After printing, it branches to _terminate to exit the program.

Subroutine (_printf):

• The _printf subroutine uses system calls to print two strings:
  • "hello world\n"
  • "efraim\n"
• It first sets up the system call parameters:
  • mov X0, #1: Sets up the file descriptor for standard output.
  • adr X1, helloworld: Loads the address of the "hello world" string.
  • mov X2, #12: Sets the length of the string.
  • mov X16, #4: Sets the system call number for writing to the console.
  • svc 0: Issues the system call to print the string.
• Similarly, it prints the second string "efraim\n" using the same system call mechanism.

Subroutine (_terminate):

• The _terminate subroutine is responsible for terminating the program.
• It sets up the system call parameters to exit the program:
  • mov X0, #0: Sets the exit status to 0 (indicating success).
  • mov X16, #1: Sets the system call number for program termination.
  • svc 0: Issues the system call to terminate the program.

String Definitions:

• helloworld: Defines the "hello world" string to be printed.
• myname: Defines the "efraim" string to be printed.

External Resources:

• The program utilizes system calls provided by the macOS operating system.
• The syscalls.master file in the XNU source code provides documentation for system calls used in macOS.