Scaffolding for VGA CHAIN4 support

tests/pc/vga/L42-5.C

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+/* Mode set and pixel-drawing functions for the 640x480 256-color mode of
+ * Tseng Labs ET4000-based SuperVGAs.
+ * 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 = 640;
+int ScreenHeightInPixels = 480;
+
+/* ET4000 640x480 256-color draw pixel function. */
+void DrawPixel(int X, int Y, int Color)
+{
+	#define SCREEN_SEGMENT		  0xA000
+	#define GC_SEGMENT_SELECT	  0x3CD /* ET4000 segment (bank) select reg */
+	unsigned char far *ScreenPtr;
+	unsigned int Bank;
+	unsigned long BitmapAddress;
+
+	/* Full bitmap address of pixel, as measured from address 0 to 0xFFFFF */
+	BitmapAddress = (unsigned long) Y * ScreenWidthInPixels + X;
+	/* Bank # is upper word of bitmap addr */
+	Bank = BitmapAddress >> 16;
+	/* Upper nibble is read bank #, lower nibble is write bank # */
+	outp(GC_SEGMENT_SELECT, (Bank << 4) | Bank);
+	/* Draw into the bank */
+	FP_SEG(ScreenPtr) = SCREEN_SEGMENT;
+	FP_OFF(ScreenPtr) = (unsigned int) BitmapAddress;
+	*ScreenPtr = Color;
+}
+
+/* ET4000 640x480 256-color mode-set function. */
+void SetMode()
+{
+	union REGS regset;
+
+	/* Set to 640x480 256-color graphics mode */
+	regset.x.ax = 0x002E;
+	int86(0x10, &regset, &regset);
+}