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[Previous](30-02.html) [Table of Contents](index.html) [Next](30-04.html)
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------------------------ --------------------------------- --------------------
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#### VGA and EGA Split-Screen Operation Don’t Mix {#Heading5}
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You must set the **IS\_VGA** equate at the start of Listing 30.1
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correctly for the adapter the code will run on in order for the program
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to perform properly. This equate determines how the upper bits of the
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split screen start scan line are set by **SetSplitScreenRow**. If
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**IS\_VGA** is 0 (specifying an EGA target), then bit 8 of the split
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screen start scan line is set by programming the entire Overflow
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register to 1FH; this is hard-wired for the 350-scan-line modes of the
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EGA. If **IS\_VGA** is 1 (specifying a VGA target), then bits 8 and 9 of
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the split screen start scan line are set by reading the registers they
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reside in, changing only the split-screen-related bits, and writing the
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modified settings back to their respective registers.
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The VGA version of Listing 30.1 won’t work on an EGA, because EGA
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registers aren’t readable. The EGA version of Listing 30.1 won’t work on
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a VGA, both because VGA monitors require different vertical settings
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than EGA monitors and because the EGA version doesn’t set bit 9 of the
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split screen start scan line. In short, there is no way that I know of
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to support both VGA and EGA split screens with common code; separate
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drivers are required. This is one of the reasons that split screens are
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so rarely used in PC programming.
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By the way, Listing 30.1 operates in mode 10H because that’s the
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highest-resolution mode the VGA and EGA share. That’s not the only mode
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the split screen works in, however. In fact, it works in *all* modes, as
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we’ll see later.
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### Setting the Split-Screen-Related Registers {#Heading6}
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Setting the split-screen-related registers is not as simple a matter as
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merely outputting the right values to the right registers; timing is
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also important. The split screen start scan line value is checked
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against the number of each scan line as that scan line is displayed,
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which means that the split screen start scan line potentially takes
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effect the moment it is set. In other words, if the screen is displaying
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scan line 15 and you set the split screen start to 16, that change will
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be picked up immediately and the split screen will start after the next
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scan line. This is markedly different from changes to the start address,
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which take effect only at the start of the next frame.
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The instantly-effective nature of the split screen is a bit of a
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problem, not because the changed screen appears as soon as the new split
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screen start scan line is set—that seems to me to be an advantage—but
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because the changed screen can appear *before* the new split screen
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start scan line is set.
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------------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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 *Remember, the split screen start scan line is spread out over two or three registers. What if the incompletely-changed value matches the current scan line after you’ve set one register but before you’ve set the rest? For one frame, you’ll see the split screen in a wrong place—possibly a very wrong place—resulting in jumping and flicker.*
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------------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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The solution is simple: Set the split screen start scan line at a time
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when it can’t possibly match the currently displayed scan line. The easy
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way to do that is to set it when there isn’t any currently displayed
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scan line—during vertical non-display time. One safe time that’s easy to
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find is the start of the vertical sync pulse, which is typically pretty
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near the middle of vertical non-display time, and that’s the approach
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I’ve followed in Listing 30.1. I’ve also disabled interrupts during the
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period when the split screen registers are being set. This isn’t
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absolutely necessary, but if it’s not done, there’s the possibility that
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an interrupt will occur between register sets and delay the later
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register sets until display time, again causing flicker.
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One interesting effect of setting the split screen registers at the
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start of vertical sync is that it has the effect of synchronizing the
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program to the display adapter’s frame rate. No matter how fast the
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computer running Listing 30.1 may be, the split screen will move at a
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maximum rate of once per frame. This is handy for regulating execution
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speed over a wide variety of hardware performance ranges; however, be
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aware that the VGA supports 70 Hz frame rates in all non-480-scan-line
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modes, while the VGA in 480-scan-line-modes and the EGA in all color
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modes support 60 Hz frame rates.
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### The Problem with the EGA Split Screen {#Heading7}
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I mentioned earlier that the EGA’s split screen is a little buggy. How?
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you may well ask, particularly given that Listing 30.1 illustrates that
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the EGA split screen seems pretty functional.
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The bug is this: The first scan line of the EGA split screen—the scan
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line starting at offset zero in display memory—is displayed not once but
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twice. In other words, the first line of split screen display memory,
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and only the first line, is replicated one unnecessary time, pushing all
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the other lines down by one.
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That’s not a fatal bug, of course. In fact, if the first few scan lines
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are identical, it’s not even noticeable. The EGA’s split-screen bug can
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produce visible distortion given certain patterns, however, so you
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should try to make the top few lines identical (if possible) when
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designing split-screen images that might be displayed on EGAs, and you
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should in any case check how your split-screens look on both VGAs and
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EGAs.
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------------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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 *I have an important caution here: Don’t count on the EGA’s split-screen bug; that is, don’t rely on the first scan line being doubled when you design your split screens. IBM designed and made the original EGA, but a lot of companies cloned it, and there’s no guarantee that all EGA clones copy the bug. It is a certainty, at least, that the VGA didn’t copy it.*
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------------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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There’s another respect in which the EGA is inferior to the VGA when it
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comes to the split screen, and that’s in the area of panning when the
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split screen is on. This isn’t a bug—it’s just one of the many areas in
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which the VGA’s designers learned from the shortcomings of the EGA and
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went the EGA one better.
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------------------------ --------------------------------- --------------------
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[Previous](30-02.html) [Table of Contents](index.html) [Next](30-04.html)
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------------------------ --------------------------------- --------------------
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* * * * *
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Graphics Programming Black Book © 2001 Michael Abrash
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