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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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@ -17,9 +17,16 @@ equal to 1 indicating where character pixels are to appear. The Data
Rotate register is set to rotate the CPU data to pixel-align it, since
without rotation characters could only be drawn on byte boundaries.
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![](images/i.jpg) *As I pointed out in Chapter 25, the CPU is perfectly capable of rotating the data itself, and it's often the case that that's more efficient. The problem with using the Data Rotate register is that the **OUT** that sets that register is time-consuming, especially for proportional text, which requires a different rotation for each character. Also, if the code performs full-byte accesses to display memory—that is, if it combines pieces of two adjacent characters into one byte—whenever possible for efficiency, the CPU generally has to do extra work to prepare the data so the VGA's rotator can handle it.*
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> ![](images/i.jpg)
> As I pointed out in Chapter 25, the CPU is perfectly capable of rotating
> the data itself, and it's often the case that that's more efficient. The
> problem with using the Data Rotate register is that the **OUT** that
> sets that register is time-consuming, especially for proportional text,
> which requires a different rotation for each character. Also, if the
> code performs full-byte accesses to display memory—that is, if it
> combines pieces of two adjacent characters into one byte—whenever
> possible for efficiency, the CPU generally has to do extra work to
> prepare the data so the VGA's rotator can handle it.
At the same time that the Data Rotate register is set, the Bit Mask
register is set to allow the CPU to modify only that portion of the