BSR = bitwise right-shift

ORD4 = cast as 32bit integer.

BSR(x,1) simply meant x divided by 2. This is very comment coding idom back in those days when Compiler don't do any optimization and bitwise-shift is much faster than division.

The snippet in C would be:

    pt.h = (r.left + (int32_t)r.right) / 2;
    pt.v = (r.top + (int32_t)r.bottom) / 2;

    pt.h -= (width / 2);
    pt.v -= (height / 2);
  
    pt.h = max(0, min(pt.h, fDoc.fCols - width));
    pt.v = max(0, min(pt.v, fDoc.fRows - height));
  
    if (width > fDoc.fCols) {
      pt.h -= (width - fDoc.fCols - 1) / 2;
    }
  
    if (height > fDoc.fRows) {
      pt.v -= (height - fDoc.fRows - 1) / 2;
    }

Reading the full function here https://github.com/amix/photoshop/blob/2baca147594d01cf9d17d...

If I understand it correctly, it was calculating the top-left point of the bounding box.