Free call to BIOS routine - Revision history

Aoineko at 22:15, 8 July 2022

2022-07-08T22:15:23Z

Aoineko at 22:14, 8 July 2022

2022-07-08T22:14:39Z

Aoineko at 22:04, 8 July 2022

2022-07-08T22:04:09Z

Aoineko at 22:03, 8 July 2022

2022-07-08T22:03:49Z

Aoineko at 22:02, 8 July 2022

2022-07-08T22:02:21Z

Aoineko at 21:59, 8 July 2022

2022-07-08T21:59:56Z

Aoineko: Created page with "Here is a technique to remove 100% of the C language overhead on calling some of the [https://www.msx.org/wiki/Main-ROM_BIOS BIOS routines]. The idea is to "cast" an address..."

2022-07-08T21:57:28Z

Created page with "Here is a technique to remove 100% of the C language overhead on calling some of the [https://www.msx.org/wiki/Main-ROM_BIOS BIOS routines]. The idea is to "cast" an address..."

New page

Here is a technique to remove 100% of the C language overhead on calling some of the [https://www.msx.org/wiki/Main-ROM_BIOS 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 <tt>GTSTCK</tt> (<tt>00D5h</tt>) 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, <tt>(u8(*)(u8))</tt> uses these same registers (<tt>u8</tt> is <tt>unsigned char</tt>).
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 A, call the <tt>GTSTCK</tt> function, then read the result into 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 <tt>sdcccall1</tt> and <tt>z88dk_fastcall</tt> calling conventions, we still have some possibilities.
In the case of a <tt>z88dk_fastcall</tt> signature you have to use a <tt>typedef</tt> to define the signature before you can use it.
Here is a summary of all possible the combinations:
<html><img src="https://cdn.discordapp.com/attachments/574951023374761988/995082329997443184/unknown.png" /></html>

← Older revision		Revision as of 22:02, 8 July 2022
Line 13:		Line 13:

	<html><img width="900px" src="https://cdn.discordapp.com/attachments/574951023374761988/995082329997443184/unknown.png" /></html>		<html><img width="900px" src="https://cdn.discordapp.com/attachments/574951023374761988/995082329997443184/unknown.png" /></html>
		+
		+	You can found many examples in <tt>engine\src\bios.h</tt>.

@@ Line 3: / Line 3: @@
 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 <tt>GTSTCK</tt> (<tt>00D5h</tt>) 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, <tt>(u8(*)(u8))</tt> uses these same registers (<tt>u8</tt> is <tt>unsigned char</tt>).
+For example, the function <tt>GTSTCK</tt> (<tt>00D5h</tt>) 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, <tt>(u8(*)(u8))</tt> uses these same registers.
 Thus, with this function definition...

@@ Line 7: / Line 7: @@
 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 A, call the <tt>GTSTCK</tt> function, then read the result into A.
+...the calling code will put the port number into register A, call the <tt>GTSTCK</tt> function, then read the result into register A.
 Since the function is inline, we gain C typing verification, while having zero C overhead.

@@ Line 11: / Line 11: @@
 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 <tt>sdcccall1</tt> and <tt>z88dk_fastcall</tt> calling conventions, we still have some possibilities.
+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 <tt>sdcccall1</tt> and <tt>z88dk_fastcall</tt> calling conventions, we still have some possibilities. In the case of a <tt>z88dk_fastcall</tt> signature you have to use a <tt>typedef</tt> to define the signature before you can use it.
 Here is a summary of all possible the combinations: