make a Soft Reset with Gendev

[bastien]

Hi,
i search a way to do a Soft Reset( just Soft not an Hard) with Gendev but it seems no have function for it, and i don't know the ASM...
.
but i can enter piece of ASM code directly with Gendev so if anyone of you can write me just a simple code wich can do that

Thanks

[Shiru]

What you mean by Soft reset? You can just jump to start of the code, and it will reinitialize everything.

[bastien]

same thing as the Reset button of the console.

i have build a multigame cartridge with Reset function ( with a counter) , if you press reset button it will change the game ( i have make a topic and another thanks for the people who helped me).

but i would add a screen selection menu which make the reset:
-for exemple:
-if you press start with the game 1 selected the console make one reset and it launch the game.
-start with the second game : two reset
ect...

[Chilly Willy]

Just jmp through the reset vector in the exception table at the start of the rom.

[bastien]

huh ?

[Chilly Willy]

The long at address 4 is the reset vector. It points to where the MD should start when you push the reset button.

In assembly

Code: Select all

    movea.l 4.w,a0
    jmp (a0)

and in C

Code: Select all

    (*(int*)4)();

[TmEE co.(TM)]

To differentiate between Soft and Hard reset, games read controller ports to see if they're initialized or not. Soft reset does not alter controller port config, but Hard Reset puts them into default values.

[bastien]

thanks you very much Chilly Willy but i have wrote this :

Code: Select all

 if( jeu==1 && start) // lancement du jeu 1 on fait un reset
               {
                       (*(int*)4)(); // pointage sur le vecteur 4
               }

and Gendev tells me " called object is not a function "

[ElBarto]

Then declare a function pointer, init to 4 and call the function. Should work.

[HardWareMan]

M68K can push the reset button (actually, make VRES signal active) for prepare own hardware. There are "RESET" opcode (0x4E70), wich does things:

Description: Asserts the RSTO signal for 512 (124 for MC68000, MC68EC000, MC68HC000, MC68HC001, MC68008, MC68010, and MC68302) clock periods, resetting all external devices. The processor state, other than the program counter, is unaffected, and execution continues with the next instruction.

I think it usefull to use this opcode before jump to inital vector. So, do it this way:

Code: Select all

reset
movea.w (4),a0
jmp (a0)

ASM inline can be used, does it?

Code: Select all

 0x4E70, 0x2078, 0x0004, 0x4ED0

But, I don't know for sure, affects this opcode on MegaDrive hardware or not. But VRES is directly connected to RESET pin of M68K, so cartrige hardware can be reset this way.

[bastien]

i have try to write this but no effect:

Code: Select all

        if( jeu==1 && start) // lancement du jeu 1 on fait un reset
               {
                   
                  asm ("movea.l   4, %a0");
                  asm ("jmp (%a0)"); 
                  
               
               }

if have not "%" don't works.

[TascoDLX]

Note, if you want to do a proper soft reset, you'll need to reset the stack pointer.

This should work:

Code: Select all

asm volatile ( "move   #0x2700,%sr\n\t"
               "move.l 0,%a7\n\t"
               "move.l 4,%a0\n\t"
               "jmp    (%a0)" );

[bastien]

it works perfectly thanks tasco !

[Stef]

I just released version SGDK v0.7, i internally implemented a correct soft reset method (just called "reset()") which keep some internals states unchanged as ram which wasn't the case before.
You can also detect it in our main() method which now accept one parameter : main(u16 param)
(param != 0) --> hard reset
(param == 0) --> soft reset

See in the dedicated topic

[bastien]

i have a question.
is this reset do exactly the same thing as the button reset.

for exemple my cartridge with bankswitch use the Reset signal with the cartrdige port B27.
is the code do the same thing, so it will up the counter ?