Bare metal VGA: enable bright background colors, disable blinking (#603)

Co-authored-by: tkchia <tkchia-cosmo@gmx.com>

libc/vga/tty.c

@@ -263,8 +263,10 @@ static uint8_t TtyGetVgaAttr(struct Tty *tty)
   uint8_t attr = tty->fg | tty->bg << 4;
   if ((tty->pr & kTtyBold) != 0)
     attr |= 0x08;
+#ifdef VGA_USE_BLINK
   if ((tty->pr & kTtyBlink) != 0)
     attr |= 0x80;
+#endif
   return attr;
 }
 
@@ -637,14 +639,25 @@ static void TtyCsiN(struct Tty *tty) {
 
 /**
  * Map the given (R, G, B) triplet to one of the 16 basic foreground colors
- * or one of the 8 background colors.
+ * or one of the 8 (or 16) background colors.
  *
+ * @see VGA_USE_BLINK macro (libc/vga/vga.internal.h)
  * @see drivers/tty/vt/vt.c in Linux 5.9.14 source code
  */
 static uint8_t TtyMapTrueColor(uint8_t r, uint8_t g, uint8_t b, bool as_fg)
 {
   uint8_t hue = 0;
-  if (as_fg) {
+#ifdef VGA_USE_BLINK
+  if (!as_fg) {
+    if (r >= 128)
+      hue |= 4;
+    if (g >= 128)
+      hue |= 2;
+    if (b >= 128)
+      hue |= 1;
+  } else
+#endif
+  {
     uint8_t max = MAX(MAX(r, g), b);
     if (r > max / 2)
       hue |= 4;
@@ -656,13 +669,6 @@ static uint8_t TtyMapTrueColor(uint8_t r, uint8_t g, uint8_t b, bool as_fg)
       hue = 8;
     else if (max > 0xaa)
       hue |= 8;
-  } else {
-    if (r >= 128)
-      hue |= 4;
-    if (g >= 128)
-      hue |= 2;
-    if (b >= 128)
-      hue |= 1;
   }
   return hue;
 }
@@ -816,6 +822,13 @@ static void TtySelectGraphicsRendition(struct Tty *tty) {
                 break;
               case 100 ... 107:
                 code[0] -= 100 - 40;
+#ifndef VGA_USE_BLINK
+                /*
+                 * If blinking is not enabled in VGA text mode, then we can
+                 * use bright background colors.
+                 */
+                code[0] += 8;
+#endif
                 /* fallthrough */
               case 40 ... 47:
                 tty->bg = code[0] - 40;

libc/vga/vga-init.S

@@ -25,6 +25,7 @@
 │ OTHER DEALINGS IN THE SOFTWARE.                                              │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/macros.internal.h"
+#include "libc/vga/vga.internal.h"
 
 //	Code snippet for initializing the VGA video mode for bare metal.
 //
@@ -57,6 +58,13 @@
 	mov	$0x0500,%ax		# just make sure we are on display
 					# page 0
 2:	int	$0x10			# otherwise, change the video mode
+	mov	$0x1003,%ax		# enable/disable VGA text blinking
+#ifdef VGA_USE_BLINK
+	mov	$1,%bx
+#else
+	xor	%bx,%bx
+#endif
+	int	$0x10
 	.previous
 	.code64
 	.section .rodata,"a",@progbits

libc/vga/vga.internal.h

@@ -26,6 +26,40 @@
  * @see lkml.kernel.org/lkml/204888.1529277815@turing-police.cc.vt.edu/T/
  */
 #undef VGA_USE_WCS
+/**
+ * The VGA hardware can be configured — via the IBM BIOS, or via port I/O —
+ * to either support blinking characters, or support the use of bright
+ * background colors, but not both.  There is a hardware setting that
+ * controls whether the attribute byte for each plotted character is
+ * interpreted as
+ *
+ *           foreground────┐
+ * foreground intensity──┐ │
+ *           background┐ │ │
+ *             blinking│ │ │
+ *                   │┌┴┐│┌┴┐
+ *                   76543210
+ *
+ * or as
+ *
+ *           foreground────┐
+ * foreground intensity──┐ │
+ *           background┐ │ │
+ * background intensity│ │ │
+ *                   │┌┴┐│┌┴┐
+ *                   76543210
+ *
+ * (NOTE: QEMU 6.2.0 does not emulate the VGA blinking feature.  However,
+ * VirtualBox 6.1 does.)
+ *
+ * If VGA_USE_BLINK is defined, our VGA code will use the former mode, &
+ * will support blinking characters.  If VGA_USE_BLINK is undefined, the
+ * VGA code will instead implement bright background colors.
+ *
+ * @see Ralf Brown's Interrupt List, int 0x10, ax = 0x1003
+ *      (https://www.delorie.com/djgpp/doc/rbinter/id/22/1.html)
+ */
+#undef VGA_USE_BLINK
 
 #define kTtyFg      0x0001
 #define kTtyBg      0x0002