diff --git a/ubb-vga/ubb-vga.c b/ubb-vga/ubb-vga.c
index 8e16d47..e753c2d 100644
--- a/ubb-vga/ubb-vga.c
+++ b/ubb-vga/ubb-vga.c
@@ -49,16 +49,19 @@ static int bad;
 
 
 static const struct mode mode_db[] = {
-    { "640x480",	 640, 480, 11, US(29.7), US(0.79+3.77-0.3) },
-    { "800x600",	 800, 600,  8, US(28.7), US(2.0+3.3+0.3) },
-    /* the next one may work after adjusting the timing in "frame" */
-    { "800x600",	 800, 600,  8, US(28.2), US(2.0+3.3+0.3-0.3) },
-    /* the 1024x768 below is not great but has good parameter tolerance */
-    { "1024x768",	1024, 768,  8, US(36.0), US(2.0+3.3) },
-    /* illustrate underruns */
-    { "1024x768ur",	1024, 768,  7, US(33.5), US(0.4+2.1+0.5) },
-    { "1024x768/53",	1024, 768,  5, US(23.1), US(0.4+2.1-0.4) },
-    { NULL }
+/* name		xres yres clkdiv vfront  hsync     hback     htotal	*/
+/*			      vsync  vback				*/
+{ "640x480",	 640, 480, 11, 2, 32, 14, US(3.47), US(0.79), US(29.7) },
+{ "800x600",	 800, 600,  8, 2, 32, 14, US(4.81), US(0.79), US(28.7) },
+/* the next one may work after adjusting the timing in "frame" */
+{ "800x600",	 800, 600,  8, 2, 32, 14, US(4.51), US(0.79), US(28.2) },
+
+/* the 1024x768 below is not great but has good parameter tolerance */
+{ "1024x768",	1024, 768,  8, 2, 32, 14, US(4.51), US(0.79), US(36.0) },
+/* illustrate underruns */
+{ "1024x768ur",	1024, 768,  7, 2, 32, 14, US(2.21), US(0.79), US(33.5) },
+{ "1024x768/53",1024, 768,  5, 2, 32, 14, US(1.31), US(0.79), US(23.1) },
+{ NULL }
 };
 
 	/*
@@ -274,7 +277,7 @@ static void line(unsigned long line)
 	DTA(DMA) = REG_PADDR(MSC_TXFIFO);	/* MUST set this each time */
 	DTC(DMA) = mode->xres >> 6;
 
-	until(US(0.79));
+	until(mode->hback_cycles);
 
 	/* HSYNC */
 
@@ -295,7 +298,7 @@ static void line(unsigned long line)
 	    (1 << 31) |         /* no descriptor */
 	    1;           
 
-	until(mode->hsync_end);
+	until(mode->hback_cycles+mode->hsync_cycles);
 
 //	MSC_TXFIFO = 0xffffffff;
 
@@ -316,7 +319,7 @@ static void hdelay(int cycles)
 	while (cycles--) {
 		TCNT(TIMER) = 0;
 		PDDATC = HSYNC;
-		until(US(3.77));
+		until(mode->hsync_cycles);
 		PDDATS = HSYNC;
 		until(mode->line_cycles);
 	}
@@ -329,12 +332,12 @@ static void frame(const unsigned long *f)
 
 	/* VSYNC */
 	PDDATC = VSYNC;
-	hdelay(2);
+	hdelay(mode->vsync_lines);
 	PDDATS = VSYNC;
 
 	/* Front porch */
 
-	hdelay(31);
+	hdelay(mode->vfront_lines-1);
 
 	/*
 	 * The horizontal back porch of the previous line is handled inside
@@ -342,9 +345,9 @@ static void frame(const unsigned long *f)
 	 */
 	TCNT(TIMER) = 0;
 	PDDATC = HSYNC;
-	until(US(3.77));
+	until(mode->hsync_cycles);
 	PDDATS = HSYNC;
-	until(mode->line_cycles-US(0.79));
+	until(mode->line_cycles-mode->hback_cycles);
 
 	/*
 	 * Note: resetting the timer just before calling "line" isn't enough.
@@ -358,7 +361,7 @@ static void frame(const unsigned long *f)
 	}
 
 	/* Back porch */
-	hdelay(14);
+	hdelay(mode->vback_lines);
 }
 
 
@@ -448,6 +451,14 @@ int main(int argc, char *const *argv)
 		usage(*argv);
 	}
 
+#if 0
+#define	CYCLES(cy)	((cy)/112.0+0.5)
+
+printf("V: %d+%d+%d+%d\n", mode->vsync_lines, mode->vfront_lines,
+  mode->yres, mode->vback_lines);
+printf("H: %4.1f+?+?+%4.1f (%4.1f)\n", CYCLES(mode->hsync_cycles),
+  CYCLES(mode->hback_cycles), CYCLES(mode->line_cycles));
+#endif
 	setup();
 	session(gen, frames);
 	cleanup();
diff --git a/ubb-vga/ubb-vga.h b/ubb-vga/ubb-vga.h
index 3fb37b5..78d2174 100644
--- a/ubb-vga/ubb-vga.h
+++ b/ubb-vga/ubb-vga.h
@@ -27,8 +27,12 @@ struct mode {
 	const char *name;	/* NULL for end of table */
 	int xres, yres;
 	int clkdiv;		/* pixel clock = 336 MHz/(clkdiv+1) */
-	int line_cycles;	/* 31.77 us for official VGA */
-	int hsync_end;		/* 0.79+3.77 us for official VGA */
+	int vsync_lines;	/* 2 lines, for official VGA */
+	int vfront_lines;	/* 32 lines */
+	int vback_lines;	/* 14 lines */
+	uint16_t hsync_cycles;	/* 3.77 us */
+	uint16_t hback_cycles;	/* 0.79 us */
+	uint16_t line_cycles;	/* 31.77 us */
 };
 
 extern const struct mode *mode;