Improve syscall definitions

- Allow constant id specification
- Define function signature in SYSCALL macro
- Move implementation into src/kernel/syscalls/*.cpp

src/drivers/nulldrv/main.cpp

@@ -6,6 +6,7 @@ extern "C" {
 	int32_t fork();
 	void sleep(uint64_t til);
 	void debug();
+	void message(const char *msg);
 
 	int main(int, const char **);
 }
@@ -15,6 +16,7 @@ int
 main(int argc, const char **argv)
 {
 	int32_t pid = getpid();
+	message("hello from nulldrv!");
 	//int32_t child = fork();
 	//debug();
 	for (int i = 1; i < 5; ++i)

src/drivers/nulldrv/main.s

@@ -11,8 +11,8 @@ getpid:
 	push rbp
 	mov rbp, rsp
 
-	mov rax, 3  ; getpid syscall
-	syscall     ; pid is now already in rax, so just return
+	mov rax, 0x02  ; getpid syscall
+	syscall        ; pid is now already in rax, so just return
 
 	pop rbp
 	ret
@@ -22,7 +22,7 @@ debug:
 	push rbp
 	mov rbp, rsp
 
-	mov rax, 0  ; debug syscall
+	mov rax, 0x00  ; debug syscall
 	syscall
 
 	pop rbp
@@ -33,7 +33,7 @@ sleep:
 	push rbp
 	mov rbp, rsp
 
-	mov rax, 6  ; sleep syscall
+	mov rax, 0x21  ; sleep syscall
 	syscall
 
 	pop rbp
@@ -44,14 +44,26 @@ fork:
 	push rbp
 	mov rbp, rsp
 
-	mov rax, 0
-
+	mov rax, 0x00
 	syscall    ; pid left in rax
 
 	pop rbp
 	ret
 	
 
+global message
+message:
+	push rbp
+	mov rbp, rsp
+
+	; message should already be in rdi
+	mov rax, 0x10
+	syscall
+
+	pop rbp
+	ret
+
+
 global _start
 _start:
 	xor rbp, rbp	; Sentinel rbp