swap_screen, how does it work ?

[alekmaul]

Hi alls,

I have a question regarding swap_screen and screen function.

I want to use them but don't know how to configure which name table is used to put character to vram.

To explain more, I begin my screen initialisation with screen(0x1800,0x1c00);

then I put name table values with put_vram(0x1800...

 

At least, for each frame, in nmi routine, I use swap_screen.

 

My question is : does I need to manage which screen to refresh (0x1800 or 0x1c00) with the put_vram function or is it automatically done with the swap screen function and I just have to use put_vram(0x1800 ... always ?

[Kiwi]

The syntax exactly is:

screen(name_table1,name_table2);

to show screen 1, 0x1800 and edit screen 2 at 0x1c00 by using only the Getput1 library.

 

screen(name_table2,name_table1);

will show screen 2,0x1c00 and now screen 1 at 0x1800 is now editable by using the Getput1 library.

for example,

screen(name_table1,name_table2);
put_frame(LargeObject,8,6,12,7);//will draw on name_table2
delay(1);

screen(name_table2,name_table1);//show screen 2

put_char(6,3,21);//will draw on name_table1

delay(1);

 

There's a sample that came with the newcoleco homebrew kit in the test folder that have the screen function I can post the source code of that now,

#include <coleco.h>
#include <getput1.h>

void nmi(void) {}

void main(void)
{
	screen_mode_2_text();
	screen_on();
    fill_vram0(0x2000,0x1800,0xf0);
	upload_default_ascii(BOLD_ITALIC);
    duplicate_pattern();
	cls();
	print_at(1,0,"SCREEN #1");
	screen(name_table1,name_table2);
	cls();
	print_at(1,0,"SCREEN #2");
loop:
	screen(name_table2,name_table1);
	print_at(rnd_byte(1,30),rnd_byte(1,22),"1");
	delay(5);
	screen(name_table1,name_table2);
	print_at(rnd_byte(1,30),rnd_byte(1,22),"2");
	delay(5);
	goto loop;
}

[alekmaul]

ok, thanks for reply, but if screen does all the job, what is the purpose of swap_screen ?

Edited September 13, 2015 by alekmaul

[Kiwi]

ok, thanks for reply, but if screen does all the job, what is the purpose of swap_screen ?

I just tried it by replacing screen with screen_swap() in that sample

 

 

loop:
//	screen(name_table2,name_table1);
swap_screen();
	print_at(rnd_byte(1,30),rnd_byte(1,22),"1");
	delay(5);
//	screen(name_table1,name_table2);
swap_screen();
	print_at(rnd_byte(1,30),rnd_byte(1,22),"2");
	delay(5);
	goto loop;

swap_screen(); inverts the screen() function everytime it is called.

 

I think i would have got more space back in Flappybird if I used that function.

Edited September 13, 2015 by Kiwi

[+nanochess]

ok, thanks for reply, but if screen does all the job, what is the purpose of swap_screen ?

 

I'm not using SDCC, but I suppose 'screen' setups the pointers, and swap_screen does the real swapping.

 

Probably writing directly the VDP register for the screen base.

 

In fact in Princess Quest (pure assembler) I use this register to manage the scrolling in 2 frames, half-top and then half-bottom, and then update the VDP register to the newly created screen. This allows for 30 frame per second scrolling.