11.3 System Calls

11.3.1 Default Calling Convention

By default, the FreeBSD kernel uses the C calling convention. Further, although the kernel is accessed using int 80h, it is assumed the program will call a function that issues int 80h, rather than issuing int 80h directly.

This convention is very convenient, and quite superior to the Microsoft® convention used by MS-DOS®. Why? Because the UNIX® convention allows any program written in any language to access the kernel.

An assembly language program can do that as well. For example, we could open a file:

kernel:
	int	80h	; Call kernel
	ret

open:
	push	dword mode
	push	dword flags
	push	dword path
	mov	eax, 5
	call	kernel
	add	esp, byte 12
	ret

This is a very clean and portable way of coding. If you need to port the code to a UNIX system which uses a different interrupt, or a different way of passing parameters, all you need to change is the kernel procedure.

But assembly language programmers like to shave off cycles. The above example requires a call/ret combination. We can eliminate it by pushing an extra dword:

open:
	push	dword mode
	push	dword flags
	push	dword path
	mov	eax, 5
	push	eax		; Or any other dword
	int	80h
	add	esp, byte 16

The 5 that we have placed in EAX identifies the kernel function, in this case open.

11.3.2 Alternate Calling Convention

FreeBSD is an extremely flexible system. It offers other ways of calling the kernel. For it to work, however, the system must have Linux emulation installed.

Linux is a UNIX like system. However, its kernel uses the same system-call convention of passing parameters in registers MS-DOS does. As with the UNIX convention, the function number is placed in EAX. The parameters, however, are not passed on the stack but in EBX, ECX, EDX, ESI, EDI, EBP:

open:
	mov	eax, 5
	mov	ebx, path
	mov	ecx, flags
	mov	edx, mode
	int	80h

This convention has a great disadvantage over the UNIX way, at least as far as assembly language programming is concerned: Every time you make a kernel call you must push the registers, then pop them later. This makes your code bulkier and slower. Nevertheless, FreeBSD gives you a choice.

If you do choose the Linux convention, you must let the system know about it. After your program is assembled and linked, you need to brand the executable:

% brandelf -t Linux filename

11.3.3 Which Convention Should You Use?

If you are coding specifically for FreeBSD, you should always use the UNIX convention: It is faster, you can store global variables in registers, you do not have to brand the executable, and you do not impose the installation of the Linux emulation package on the target system.

If you want to create portable code that can also run on Linux, you will probably still want to give the FreeBSD users as efficient a code as possible. I will show you how you can accomplish that after I have explained the basics.

11.3.4 Call Numbers

To tell the kernel which system service you are calling, place its number in EAX. Of course, you need to know what the number is.

0	STD	NOHIDE	{ int nosys(void); } syscall nosys_args int
1	STD	NOHIDE	{ void exit(int rval); } exit rexit_args void
2	STD	POSIX	{ int fork(void); }
3	STD	POSIX	{ ssize_t read(int fd, void *buf, size_t nbyte); }
4	STD	POSIX	{ ssize_t write(int fd, const void *buf, size_t nbyte); }
5	STD	POSIX	{ int open(char *path, int flags, int mode); }
6	STD	POSIX	{ int close(int fd); }
etc...