Thread: Help me please....

What wrong here?

Code:

#include <conio.h>
#include <dos.h>

#define WIDTH  640
#define HEIGHT 480

typedef unsigned char  byte;

byte *SCR=(byte *) MK_FP (0xA000,0); 

void svga() {
	 asm	mov ax,0x4f02;
	 asm	mov bx,0x101;
	 asm	int 0x10;
}

void textmode() {
	 asm	mov ax,0x03;
	 asm	int 0x10;
}

void putpixel (int x, int y, byte color){
		unsigned long pixel_offset = 1.0 * (y * 640 + x);
		int bank_num = (pixel_offset >> 16);
		asm mov ax, 0x4F05;
		asm mov bx, 0x0;
		asm mov dx, bank_num;
		asm int 0x10;
		SCR[(pixel_offset & 0xFFFF)] = color;
}

int main(void) {
	svga();

	byte color=0;
	for (int y=0;y<HEIGHT;y++)  {
		color=0;
		for(int x=0;x<WIDTH;x++)
			putpixel(x,y,color++);
	}

	getch();
	textmode();
	return 0;
}

compiler: turbo c
os: dos

this is the result:
http://uploaded.fresh.co.il/3f65febb34c60223.rar

....
this code do his job but very slowly

Code:

void putpixel(int x, int y, byte color) {
	 asm	mov ah,0x0c;
	 asm	mov al,color;
	 asm	mov cx,x;
	 asm	mov dx,y;
	 asm	mov bx,0x01;
	 asm	int 0x10;
}

I would say your problem is here w/o even testing the code:

unsigned long pixel_offset = 1.0 * (y * 640 + x);

Why 1.0??? This forces a conversion to double, y is double, 640 is double, and x is double. Then you are assigning a double to an unsigned long which is yet another conversion, probably one in which the result is 0 since you cannot assign a double to an unsigned long.

As well you cannot shift a double left or right but you can an unsigned long -> again, why are you using a double? There is no need.

1*(y*640+x)
or
1*((y<<9)+(y<<7)+x)

is the same as

(y*640+x)
or
((y<<9)+(y<<7)+x)


One times any number is that number. Why the 1.0????

My guess is that if you print out pixel_offset it will be 0.


Here is what you are doing:

Converting all operands to doubles.
Trying to convert the entire result into an unsigned long.


All SVGA functions can be done using integer arithmetic - no need for doubles or floats.

Code:

;put original y into eax
mov    eax,[y]

;multiply eax by 512
shl      eax,9

;put original y into eax
mov    ebx,[y]

;multiply ebx by 128
shl      ebx,7

;add result of ebx to eax - (y*512)+(y*128) = (y*(512+128))
;=(y*640)
add     eax,ebx

;add x to eax = (y*640)+x
;leave result in eax
add     eax,x

or in C:

Code:

pixel_offset=((y<<9)+(y<<7)+x);