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Convert notes into blockquotes

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James Gregory 2013-12-31 11:06:31 +11:00
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@ -24,9 +24,24 @@ sync to happen, sets the new pel panning state, and then continues
drawing. Don't worry about the details right now; page flipping will
come up again, at considerably greater length, in later chapters.
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![](images/i.jpg) *As an interesting side note, be aware that if you run DOS software under a multitasking environment such as Windows NT, timeslicing delays can make mismatched start address bytes or mismatched start address and pel panning settings much more likely, for the graphics code can be interrupted at any time. This is also possible, although much less likely, under non-multitasking environments such as DOS, because strategically placed interrupts can cause the same sorts of problems there. For maximum safety, you should disable interrupts around the key portions of your page-flipping code, although here we run into the problem that if interrupts are disabled from the time we start looking for Display Enable until we set the Pel Panning register, they will be off for far too long, and keyboard, mouse, and network events will potentially be lost. Also, disabling interrupts won't help in true multitasking environments, which never let a program hog the entire CPU. This is one reason that pel panning, although indubitably flashy, isn't widely used and should be reserved for only those cases where it's absolutely necessary.*
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> ![](images/i.jpg)
> As an interesting side note, be aware that if you run DOS software under
> a multitasking environment such as Windows NT, timeslicing delays can
> make mismatched start address bytes or mismatched start address and pel
> panning settings much more likely, for the graphics code can be
> interrupted at any time. This is also possible, although much less
> likely, under non-multitasking environments such as DOS, because
> strategically placed interrupts can cause the same sorts of problems
> there. For maximum safety, you should disable interrupts around the key
> portions of your page-flipping code, although here we run into the
> problem that if interrupts are disabled from the time we start looking
> for Display Enable until we set the Pel Panning register, they will be
> off for far too long, and keyboard, mouse, and network events will
> potentially be lost. Also, disabling interrupts won't help in true
> multitasking environments, which never let a program hog the entire CPU.
> This is one reason that pel panning, although indubitably flashy, isn't
> widely used and should be reserved for only those cases where it's
> absolutely necessary.
Waiting for the sync pulse has the side effect of causing program
execution to synchronize to the VGA's frame rate of 60 or 70 frames per