Free call to BIOS routine

Here is a technique to remove 100% of the C language overhead on calling some of the BIOS routines.

The idea is to "cast" an address with the signature of a C function so that the calling code initializes the registers properly before calling the BIOS function.

For example, the function GTSTCK (00D5h) which returns the status of the joystick, takes its input parameter in A and returns its final value in A. Among the possible function signatures, (u8(*)(u8)) uses these same registers.

Thus, with this function definition...

inline u8 Bios_GetJoystickDirection(u8 port) { return ((u8(*)(u8))R_GTSTCK)(port); }

...the calling code will put the port number into register A, call the GTSTCK function, then read the result into register A.

Since the function is inline, we gain C typing verification, while having zero C overhead.

Obviously, this only works with BIOS routines that use registers for which there is a C function signature. There are few of them, but by playing with the sdcccall1 and z88dk_fastcall calling conventions, we still have some possibilities. In the case of a z88dk_fastcall signature you have to use a typedef to define the signature before you can use it.

Here is a summary of all possible the combinations:

	__sdcccall(0)	__sdcccall(1)	__z88dk_fastcall
RETURN
8 bits	L	A	L
16 bits	HL	DE	HL
32 bits	DE-HL	HL-DE	DE-HL
PARAMETERS
8 bits	Stack	A	L
16 bits	Stack	HL	HL
32 bits	Stack	HL-DE	DE-HL
8 + 8 bits	Stack	A + L	Invalid
8 + 16 bits	Stack	A + DE	Invalid
16 + 16 bits	Stack	HL + DE	Invalid
16 + 8 bits	Stack	HL + Stack	Invalid

You can found many examples in engine\src\bios.h.