Cirrus_vga: More than 1024 lines

Cirrus VGA implementation supports 10 bits to set the number of scanlines which results in a maximum of 1024 pixel for vertical resolution. For large monitors (e.g. portrait monitor) it would be nice to have more lines.

According to Cirrus technical documentation in gd5446trm.pdf (can be found e.g. here: http://www.tjd.phlegethon.org/gd5446trm.pdf.gz) page 4-55 chapter 4.44, CRTC Mode Control (CR17) bit 2 supports "Multiply Vertical Registers by Two: If this bit is '1', the scanline counter is clocked with HSYNC divided by two. This allows the number of scanlines to be doubled to 2048."

This can be used in hw/cirrus_vga.c in functions cirrus_get_offsets() and cirrus_get_resolution(). With this patch and a patch for vgabios (described in 2nd mail), I was able to create a mode 1024x1536 (from 1024x768) which I use in qemu-0.12.5 with a portrait monitor.

diff -r -u a/hw/cirrus_vga.c b/hw/cirrus_vga.c
@@ -1062,6 +1062,13 @@
     line_compare = s->vga.cr[0x18] |
         ((s->vga.cr[0x07] & 0x10) << 4) |
         ((s->vga.cr[0x09] & 0x40) << 3);
+
+    /* Multiply Vertical Registers by Two */
+    if ((s->vga.cr[0x17] & 0x04))
+    {
+        line_compare = line_compare * 2;
+    }
+
     *pline_compare = line_compare;
 }
 

@@ -1134,6 +1141,13 @@
         ((s->cr[0x07] & 0x02) << 7) |
         ((s->cr[0x07] & 0x40) << 3);
     height = (height + 1);
+
+    /* Multiply Vertical Registers by Two */
+    if ((s->cr[0x17] & 0x04))
+    {
+        height = height * 2;
+    }
+
     /* interlace support */
     if (s->cr[0x1a] & 0x01)
         height = height * 2;

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