From:	jsc AT lds DOT co DOT uk
Newsgroups:	comp.os.msdos.djgpp
Subject:	Gaps between my banks (vesa prog question)
Date:	Sun, 20 Dec 1998 22:54:51 GMT
Organization:	Deja News - The Leader in Internet Discussion
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To:	djgpp AT delorie DOT com
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Reply-To:	djgpp AT delorie DOT com

Hello all! This is really baffling me - I'm hoping that some kind soul can
help me out here. This is really basic vesa (1.something-or-other) pixel
plotting code using bank switching. Now, I worked out most of it on my own.
Indeed, for the most part it works, but you'll notice if you run it, that
there's a horrible black hole at the end of each bank - the first pixel of
each bank (except bank 0) jumps forward a few hundred pixels. I can't see why
or how to stop it. I tried adding "if (my_bank>0) {offset -=100 (or, the
number of places it is jumping)} but that didn't work - it simply won't write
anything in those spaces. Please help, someone - It's really bugging me and I
genuinely can't think of where to go next. Code follows below BTW I know that
my win granularity = 64 and my banks are labelled 0,1,2,3... So I left those
bits out of the code (for simplicity eg 64/win gran ==1) Thanks and Merry
Christmas! Christopher Anthony	jsc AT lds DOT co DOT uk

#include <stdio.h>
#include <dos.h>
#include <bios.h>
#include <sys/farptr.h>
#include <go32.h>
#include <dpmi.h>
#define far

void ExitGraphics(void)
	{
	union REGS regs;
	regs.x.ax = 0x3;
	int86(0x10, &regs,&regs);
	}

int set_mode (int mode)
{
union REGS regs;
regs.x.ax = 0x4F02;
regs.x.bx = mode;
int86(0x10, &regs, &regs);
return regs.x.ax; // zero if successful, non-zero otherwise
}


int bank_switch(int bank_no)
{
union REGS regs;
regs.x.ax = 0x4F05;
regs.x.bx = 0;  ///check this line - is it relevant?
regs.x.dx = bank_no;
int86(0x10, &regs, &regs);
return regs.x.ax; // zero if successful, non-zero otherwise
}


void plot_pixel (long x, long y, int colour, int rgba)
{
long offset;
int my_bank;

	// remember not y*640 as there's 4 times
	// more bytes in each line
	offset = (long)y*2560 + ((long)x * 4) + rgba;
my_bank=(int)(offset/64000);

bank_switch(my_bank);

offset=offset - (64000*(int)my_bank);

_farpokeb(_dos_ds, 0xa0000 + (long)offset, colour);

//rgba is either 1, 2, 3 or 4 and represents whether you are setting
//the b, g, r or alpha.
//It's really easy to work out why its x*4 + rgba - just draw a diagram.
}


int main(void)
{
int a, b;
long index;
set_mode(0x112);

for(b=0; b<480; b++)
	{
	for(a=0; a<640; a++)
	{
	plot_pixel(a, b, 255, 0);
	plot_pixel(a, b, 255, 1);
	plot_pixel(a, b, 255, 2);
	}
	}

/*
for(b=0; b<26; b++)
{
	plot_pixel(0, b, 255, 0);
	plot_pixel(0, b, 255, 1);
	plot_pixel(0, b, 255, 2);
}


	plot_pixel(0, 25, 255, 0);
	plot_pixel(0, 25, 255, 1);
	plot_pixel(0, 25, 255, 2);


	plot_pixel(639, 10, 255, 0);
	plot_pixel(639, 10, 255, 1);
	plot_pixel(639, 10, 255, 2);

	plot_pixel(639, 300, 255, 0);
	plot_pixel(639, 300, 255, 1);
	plot_pixel(639, 300, 255, 2);

	plot_pixel(639, 400, 255, 0);
	plot_pixel(639, 400, 255, 1);
	plot_pixel(639, 400, 255, 2);
*/
getch();
ExitGraphics();
return 0;
}

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