Scaffolding for VGA CHAIN4 support

tests/pc/vga/L42-3.C

@@ -0,0 +1,29 @@
+/* VGA mode 13h pixel-drawing and mode set functions.
+ * Tested with Borland C++ in C compilation mode and the small model.
+ */
+#include <dos.h>
+
+/* Screen dimension globals, used in main program to scale. */
+int ScreenWidthInPixels = 320;
+int ScreenHeightInPixels = 200;
+
+/* Mode 13h draw pixel function. */
+void DrawPixel(int X, int Y, int Color)
+{
+	#define SCREEN_SEGMENT	0xA000
+	unsigned char far *ScreenPtr;
+
+	FP_SEG(ScreenPtr) = SCREEN_SEGMENT;
+	FP_OFF(ScreenPtr) = (unsigned int) Y * ScreenWidthInPixels + X;
+	*ScreenPtr = Color;
+}
+
+/* Mode 13h mode-set function. */
+void SetMode()
+{
+	union REGS regset;
+
+	/* Set to 320x200 256-color graphics mode */
+	regset.x.ax = 0x0013;
+	int86(0x10, &regset, &regset);
+}