In 640×400, 16-color mode, page 0 runs from offset 0 to offset 31,999 (7CFFH), and page 1 runs from offset 32,000 (7D00H) to 63,999 (0F9FFH). Page 1 is selected by programming the Start Address registers (CRTC registers 0CH, the high 8 bits, and 0DH, the low 8 bits) to 7D00H. Actually, because the low byte of the start address is 0 for both pages, you can page flip simply by writing 0 or 7DH to the Start Address High register (CRTC register 0CH); this has the benefit of eliminating a nasty class of potential synchronization bugs that can arise when both registers must be set. Listing 45.3 illustrates simple 640×400 page flipping.

LISTING 45.3 L45-3.C

/* Sample program to exercise VGA 640×400 16-color mode page flipping, by
   drawing a horizontal line at the top of page 0 and another at bottom of page 1,
   then flipping between them once every 30 frames. Tested with Borland C++,
   in C compilation mode. */

#include <dos.h>
#include <conio.h>

#define SCREEN_SEGMENT  0×A000
#define SCREEN_HEIGHT   400
#define SCREEN_WIDTH_IN_BYTES 80
#define INPUT_STATUS_1  0×3DA /* color-mode address of Input Status 1
                                 register */
/* The page start addresses must be even multiples of 256, because page
   flipping is performed by changing only the upper start address byte */
#define PAGE_0_START 0
#define PAGE_1_START (400*SCREEN_WIDTH_IN_BYTES)

void main(void);
void Wait30Frames(void);
extern void Set640×400(void);

void main()
{
   int i;
   unsigned int far *ScreenPtr;
   union REGS regset;

   Set640×400();  /* set to 640×400 16-color mode */

   /* Point to first line of page 0 and draw a horizontal line across screen */
   FP_SEG(ScreenPtr) = SCREEN_SEGMENT;
   FP_OFF(ScreenPtr) = PAGE_0_START;
   for (i=0; i<(SCREEN_WIDTH_IN_BYTES/2); i++) *ScreenPtr++ = 0×FFFF;

   /* Point to last line of page 1 and draw a horizontal line across screen */
   FP_OFF(ScreenPtr) =
         PAGE_1_START + ((SCREEN_HEIGHT-1)*SCREEN_WIDTH_IN_BYTES);
   for (i=0; i<(SCREEN_WIDTH_IN_BYTES/2); i++) *ScreenPtr++ = 0×FFFF;

   /* Now flip pages once every 30 frames until a key is pressed */
   do {
      Wait30Frames();

      /* Flip to page 1 */
      outpw(0×3D4, 0×0C | ((PAGE_1_START >> 8) << 8));

      Wait30Frames();

      /* Flip to page 0 */
      outpw(0×3D4, 0×0C | ((PAGE_0_START >> 8) << 8));
   } while (kbhit() == 0);

   getch(); /* clear the key press */

   /* Return to text mode and exit */
   regset.x.ax = 0×0003;   /* AL = 3 selects 80×25 text mode */
   int86(0×10, &regset, &regset);
}

void Wait30Frames()
{
   int i;

   for (i=0; i<30; i++) {
      /* Wait until we’re not in vertical sync, so we can catch leading edge */
      while ((inp(INPUT_STATUS_1) & 0×08) != 0) ;
      /* Wait until we are in vertical sync */
      while ((inp(INPUT_STATUS_1) & 0×08) == 0) ;
   }
}

After I described 640×400 mode in a magazine article, Bill Lindley, of Mesa, Arizona, wrote me to suggest that when programming the VGA to a nonstandard mode of this sort, it’s a good idea to tell the BIOS about the new screen size, for a couple of reasons. For one thing, pop-up utilities often use the BIOS variables; Bill’s memory-resident screen printer, EGAD Screen Print, determines the number of scan lines to print by multiplying the BIOS “number of text rows” variable times the “character height” variable. For another, the BIOS itself may do a poor job of displaying text if not given proper information; the active text area may not match the screen dimensions, or an inappropriate graphics font may be used. (Of course, the BIOS isn’t going to be able to display text anyway in highly nonstandard modes such as Mode X, but it will do fine in slightly nonstandard modes such as 640×400 16-color mode.) In the case of the 640×400 16-color model described a little earlier, Bill suggests that the code in Listing 45.4 be called immediately after putting the VGA into that mode to tell the BIOS that we’re working with 25 rows of 16-pixel-high text. I think this is an excellent suggestion; it can’t hurt, and may save you from getting aggravating tech support calls down the road.

LISTING 45.4 L45-4.C

/* Function to tell the BIOS to set up properly sized characters for 25 rows of
   16 pixel high text in 640×400 graphics mode. Call immediately after mode set.
   Based on a contribution by Bill Lindley. */

#include <dos.h>

void Set640×400()
{
   union REGS regs;

   regs.h.ah = 0×11;                    /* character generator function */
   regs.h.al = 0×24;                    /* use ROM 8×6 character set for graphics */
   regs.h.bl = 2;                       /* 25 rows */
   int86(0×10, &regs, &regs);           /* invoke the BIOS video interrupt
                                           to set up the text */
}