Slope Climber...

I managed to make Link climb half a combo, but then he stops. I have a few questions regarding this:

Is there anything wrong with my isSolid Function?
How might I optimize my isSolid Function?
Is there anything wrong with my logic?

Code:

---

import "std.zh"

bool isSolid(int x, int y) {

        int mask = Screen->ComboS[ComboAt(x,y)];
        int xc; int yc;

        xc = x % 16;
        yc = y % 16;

        if((xc >= 0) && (xc < 8)) {

                if((yc >= 0) && (yc < 8)) {

                        return mask & 0001b;

                } else {

                        return mask & 0010b;

                }

        } else {

                if((yc >= 0) && (yc < 8)) {

                        return mask & 0100b;

                } else {

                        return mask & 1000b;

                }

        }

}

global script InherentControls {

        void run() {

                while(true) {

                        //Inherent Jumping
                        if((Link->InputUp || Link->InputL) && (isSolid(Link->X,Link->Y+16) || isSolid(Link->X+8,Link->Y+16))) {

                                Link->Jump = 40;
                                Game->PlaySound(45);

                        }

                        //"Slope" Control
                        if(Link->InputRight && !isSolid(Link->X+16, Link->Y) && isSolid(Link->X+16, Link->Y+15)) {

                                Link->X ++;
                                Link->Y --;

                        }

                        if(Link->InputLeft && !isSolid(Link->X-1, Link->Y) && isSolid(Link->X-1, Link->Y+15)) {

                                Link->X --;
                                Link->Y --;

                        }

                        Link->InputUp = false;
                        Link->InputL = false;
                        Link->InputDown = false;

                        Waitframe();

                }
        }
}

---

EDIT: NOTE: I bolded the areas of concern in the code

EDIT 2: I noticed that (I think) it only doesn't work on tiles with a single unwalkable fourth in a bottom corner of the combo's mask which for some reason, Link is not allowed to jump on. It has to do with my collision detector, doesn't it?