Implement syscalls, hello world from userspace

user/hello/Makefile

@@ -16,7 +16,7 @@ all: $(TARGET)
 
 hello: $(OBJ)
 	$(LD) $^ $(LDFLAGS) -o $@
-	echo $(realpath $@) >> $(FILES)
+	echo $(realpath hello) >> $(FILES)
 
 clean:
 	rm -f $(OBJ) $(TARGET)

user/hello/hello.s

@@ -1,6 +1,18 @@
+.section .data
+STRING:
+  .string "Hello World from userspace"
+STRING_LEN:
+  .quad . - STRING
+
+.section .text
 
 .global _start
 _start:
-.spin:
-  jmp .spin
+  movq $1, %rax
+  lea STRING(%rip), %rdi
+  lea STRING_LEN(%rip), %rsi
+  syscall
+
+  movq $2, %rax
+  syscall