Index: src/emu/video/pc_vga.c
===================================================================
--- src/emu/video/pc_vga.c      (revision 15818)
+++ src/emu/video/pc_vga.c      (working copy)
@@ -1245,7 +1245,7 @@
        /* calculate vblank start / end positions */
        res = 0;
        vblank_start = vga.crtc.vert_blank_start;
-       vblank_end = vga.crtc.vert_blank_start + vga.crtc.vert_blank_end;
+       vblank_end = vga.crtc.vert_blank_start + vga.crtc.vert_blank_end - 1;
        vpos = machine.primary_screen->vpos();

        /* check if we are under vblank period */
@@ -1257,11 +1257,21 @@
        }
        else
        {
-               if(vpos >= vblank_start && vpos < vblank_end)
-                       res = 1;
+               if(vblank_end < vblank_start) //Needed for Fractint's 600 scanline tweaked modes. TODO: turn this into general logic
+               {
+                       if(vpos >= vblank_start && vpos < vga.crtc.vert_total)
+                               res = 1;
+                       else if(vpos < vblank_end)
+                               res = 1;
+               }
+               else
+               {
+                       if(vpos >= vblank_start && vpos < vblank_end)
+                               res = 1;
+               }
        }

-//  popmessage("%d %d %d",vblank_start,vblank_end,vga.crtc.vert_total);
+    //popmessage("%d %d %d",vblank_start,vblank_end,vga.crtc.vert_total);

        return res;
 }
@@ -2872,4 +2882,3 @@

        mem_space->install_legacy_readwrite_handler(mem_offset + 0x00000, mem_offset + 0x1ffff, FUNC(vga_gamtor_mem_r), FUNC(vga_gamtor_mem_w), mask);
 }