Line of sight

[na_th_an]

I usually code in C, and I can assure you that this code works as intended

Code:

#include <stdio.h>

void main (void) {
    unsigned char lsb = 0;
    unsigned char msb = 60;
    unsigned int address;
    
    unsigned char x = 0, cx = 100;
    int dx;
    
    address = lsb + (msb << 8);
    printf ("%d\n", address);
    
    dx = x - cx;
    printf ("%d\n", dx);
}

15360 and -100 are printed on screen. This works in any C compiler I've tried so far (z88dk and gcc).

Freebasic also works this way:

Code:

Dim as uByte lsb, msb
Dim as uInteger address
Dim as uByte x, cx
Dim as Integer dx

lsb = 0: msb = 60
x = 0: cx = 100

address = lsb + (msb Shl 8)
Print address

dx = x - cx
Print dx

Also prints 15360 and -100. Usually, operands are automaticly casted to the result type. This also works when calling a function. This should print -100:

Code:

Sub printMe (a as Integer)
   Print a
End Sub

Dim as uByte x, cx

x = 0: cx = 100
printMe (x-cx)

Anyways, good to know about Cast. I should read the docs more often...

[...]as far as I know, dx variable is positive only if x is ALWAYS >= cx. I will use CAST(Uinteger, x) to avoid this problem.

It doesn't work correctly, I've just found it. Instead of

Code:

Draw x - cx, y - cy

With all variables involved typed "uByte", DRAW always draws up and right (i.e.: positive). So I changed it to:

Code:

dx = x - cx: dy = y - cy
Draw dx, dy

With dx and dy typed Integer. Stil didn't work. dx and dy were always positive. I had to do this:

Code:

            dx = x: dx = dx - cx
            dy = y: dy = dy - cy
            Draw dx, dy

And then it worked.