Add test syscall: works

usr/init.c

@@ -1,8 +1,27 @@
 #include <stdint.h>
 
+typedef uint64_t size_t;
+typedef int64_t ssize_t;
+
+#define ASM_REGISTER(name) \
+        register uint64_t name asm (#name)
+
+#define ASM_SYSCALL3(r0, r1, r2, r3)        ({ \
+        ASM_REGISTER(rax) = (uint64_t) (r0); \
+        ASM_REGISTER(rdi) = (uint64_t) (r1); \
+        ASM_REGISTER(rsi) = (uint64_t) (r2); \
+        ASM_REGISTER(rdx) = (uint64_t) (r3); \
+        asm volatile ("syscall":::"memory"); \
+        rax; \
+    })
+
+ssize_t sys_write(int fd, const void *buf, size_t count) {
+    return (ssize_t) ASM_SYSCALL3(1, fd, buf, count);
+}
+
 void _start(void) {
+    sys_write(1, "Hello!\n", 7);
+
     while (1) {
-        // XXX: I'm still allowed to do so (for testing, of course)
-        *((uint16_t *) 0xFFFFFF00000B8000) = 'A' | 0x900;
     }
 }