Arne replies:

> > >If you want to change only the last color (255),
> > >then index=255, count=1, so sum=256 is valid.
> > >Actually I managed to convince myself it should be 257 once,
> > >but I've now come to my senses :-)
> > 
> > Oops, sorry.    But then surely a count of 0 is invalid,
> > and the in-range test for count should  be "count < 1" ?
>
>argh, yes of course.


I think we're heading towards a consensus here.  The acceptable
ranges are:
	0 <= index <= 255;
	1 <= count <= 256,
	1 <= count + index <= 256, 

If someone could actually *try* this on a 3100, please let me know if it works.
I expect the cfb RAMDAC driver needs a similar fix.

next on the menu: screenblank??

thanks
--Jonathan


--- src/sys/arch/pmax/dev/bt478.c.orig	Mon Dec 11 21:37:26 1995
+++ src/sys/arch/pmax/dev/bt478.c	Mon Jan 15 14:51:47 1996
@@ -266,22 +266,25 @@
 	u_char *cmap;
 	int i;
 
-	if (index > 256 || index < 0 || index + count > 256)
+	if (index < 0 || count < 1 || index + count > 256)
 		return EINVAL;
 
 	cmap_bits = (u_char *)bits;
 	cmap = (u_char *)(fi -> fi_cmap_bits) + index * 3;
 
-	vdac->mapWA = index; MachEmptyWriteBuffer();
 	for (i = 0; i < count; i++) {
-		cmap [(i + index) * 3]
-			= vdac->map = cmap_bits [i * 3];
+		vdac->mapWA = i + index; MachEmptyWriteBuffer();
+
+		cmap [i * 3] = cmap_bits [i * 3];
+		vdac->map = cmap_bits [i * 3];
 		MachEmptyWriteBuffer();
-		cmap [(i + index) * 3 + 1]
-			= vdac->map = cmap_bits [i * 3 + 1];
+
+		cmap [i * 3 + 1] = cmap_bits [i * 3 + 1];
+		vdac->map = cmap_bits [i * 3 + 1];
 		MachEmptyWriteBuffer();
-		cmap [(i + index) * 3 + 2]
-			= vdac -> map = cmap_bits [i * 3 + 2];
+
+		cmap [i * 3 + 2] = cmap_bits [i * 3 + 2];
+		vdac -> map = cmap_bits [i * 3 + 2];
 		MachEmptyWriteBuffer();
 	}
 	return 0;