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+Chapter 30\
+ Video Est Omnis Divisa {#Heading1}
+-----------------------
+
+### The Joys and Galling Problems of Using Split Screens on the EGA and VGA {#Heading2}
+
+The ability to split the screen into two largely independent portions
+one—displayed above the other on the screen—is one of the more
+intriguing capabilities of the VGA and EGA. The split screen feature can
+be used for popups (including popups that slide smoothly onto the
+screen), or simply to display two separate portions of display memory on
+a single screen. While it’s possible to accomplish the same effects
+purely in software without using the split screen, software solutions
+tend to be slow and hard to implement.
+
+By contrast, the basic operation of the split screen is fairly simple,
+once you grasp the various coding tricks required to pull it off, and
+understand the limitations and pitfalls—like the fact that the EGA’s
+split screen implementation is a little buggy. Furthermore, panning with
+the split screen enabled is not as simple as it might seem. All in all,
+we do have some ground to cover.
+
+Let’s start with the basic operation of the split screen.
+
+### How the Split Screen Works {#Heading3}
+
+The *operation* of the split screen is simplicity itself. A split screen
+start scan line value is programmed into two EGA registers or three VGA
+registers. (More on exactly which registers in a moment.) At the
+beginning of each frame, the video circuitry begins to scan display
+memory for video data starting at the address specified by the start
+address registers, just as it normally would. When the video circuitry
+encounters the specified split screen start scan line in the course of
+scanning video data onto the screen, it completes that scan line
+normally, then resets the internal pointer which addresses the next byte
+of display memory to be read for video data to zero. Display memory from
+address zero onward is then scanned for video data in the usual way,
+progressing toward the high end of memory. At the end of the frame, the
+pointer to the next byte of display memory to scan is reloaded from the
+start address registers, and the whole process starts over.
+
+The net effect: The contents of display memory starting at offset zero
+are displayed starting at the scan line following the specified split
+screen start scan line, as shown in Figure 30.1. It’s important to
+understand that the scan line that matches the split screen scan line is
+*not* part of the split screen; the split screen starts on the
+*following* scan line. So, for example, if the split screen scan line is
+set to zero, the split screen actually starts at scan line 1, the second
+scan line from the top of the screen.
+
+If both the start address and the split screen start scan line are set
+to 0, the data at offset zero in display memory is displayed as both the
+first scan line on the screen *and* the second scan line. There is no
+way to make the split screen cover the entire screen—it always comes up
+at least one scan line short.
+
+![](images/30-01.jpg)\
+ **Figure 30.1**  *Display memory and the split screen.*
+
+So, where is the split screen start scan line stored? The answer varies
+a bit, depending on whether you’re talking about the EGA or the VGA. On
+the EGA, the split screen start scan line is a 9-bit value, with bits
+7-0 stored in the Line Compare register (CRTC register 18H) and bit 8
+stored in bit 4 of the Overflow register (CRTC register 7). Other bits
+in the Overflow register serve as the high bits of other values, such as
+the vertical total and the vertical blanking start. Since EGA registers
+are—alas!—not readable, you must know the correct settings for the other
+bits in the Overflow registers to use the split screen on an EGA.
+Fortunately, there are only two standard Overflow register settings on
+the EGA: 11H for 200-scan-line modes and 1FH for 350-scan-line modes.
+
+The VGA, of course, presents no such problem in setting the split screen
+start scan line, for it has readable registers. However, the VGA
+supports a 10-bit split screen start scan line value, with bits 8-0
+stored just as with the EGA, and bit 9 stored in bit 6 of the Maximum
+Scan Line register (CRTC register 9).
+
+Turning the split screen on involves nothing more than setting all bits
+of the split screen start scan line to the scan line after which you
+want the split screen to start appearing. (Of course, you’ll probably
+want to change the start address before using the split screen;
+otherwise, you’ll just end up displaying the memory at offset zero
+*twice:* once in the normal screen and once in the split screen.)
+Turning off the split screen is a simple matter of setting the split
+screen start scan line to a value equal to or greater than the last scan
+line displayed; the safest such approach is to set all bits of the split
+screen start scan line to 1. (That is, in fact, the split screen start
+scan line value programmed by the BIOS during a mode set.)
+
+#### The Split Screen in Action {#Heading4}
+
+All of these points are illustrated by Listing 30.1. Listing 30.1 fills
+display memory starting at offset zero (the split screen area of memory)
+with text identifying the split screen, fills display memory starting at
+offset 8000H with a graphics pattern, and sets the start address to
+8000H. At this point, the normal screen is being displayed (the split
+screen start scan line is still set to the BIOS default setting, with
+all bits equal to 1, so the split screen is off), with the pixels based
+on the contents of display memory at offset 8000H. The contents of
+display memory between offset 0 and offset 7FFFH are not visible at all.
+
+Listing 30.1 then slides the split screen up from the bottom of the
+screen, one scan line at a time. The split screen slides halfway up the
+screen, bounces down a quarter of the screen, advances another
+half-screen, drops another quarter-screen, and finally slides all the
+way up to the top. If you’ve never seen the split screen in action, you
+should run Listing 30.1; the smooth overlapping of the split screen on
+top of the normal display is a striking effect.
+
+Listing 30.1 isn’t done just yet, however. After a keypress, Listing
+30.1 demonstrates how to turn the split screen off (by setting all bits
+of the split screen start scan line to 1). After another keypress,
+Listing 30.1 shows that the split screen can never cover the whole
+screen, by setting the start address to 0 and then flipping back and
+forth between the normal screen and the split screen with a split screen
+start scan line setting of zero. Both the normal screen and the split
+screen display the same text, but the split screen displays it one scan
+line lower, because the split screen doesn’t start until *after* the
+first scan line, and that produces a jittering effect as the program
+switches the split screen on and off. (On the EGA, the split screen may
+display *two* scan lines lower, for reasons I’ll discuss shortly.)
+
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+* * * * *
+
+Graphics Programming Black Book © 2001 Michael Abrash