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James Gregory 2013-12-30 18:34:31 +11:00
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48-03.md
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@ -11,7 +11,7 @@ diagonal stripes, and crosshatches.
Furthermore, eight-wide patterns, which are widely used, can be drawn
with two passes, one for each half of the pattern. This principle can in
fact be extended to patterns of arbitrary multiple-of-four widths.
(Widths that aren’t multiples of four are considerably more difficult to
(Widths that aren't multiples of four are considerably more difficult to
handle, because the latches are four pixels wide; one possible solution
is expanding such patterns via repetition until they are
multiple-of-four widths.)
@ -26,15 +26,15 @@ pattern before each scan line of the actual fill is drawn. Why this
double copying process, and why is the pattern stored in that particular
area of display memory?
The double copying process is used because it’s the easiest way to load
the latches. Remember, there’s no way to get information directly from
The double copying process is used because it's the easiest way to load
the latches. Remember, there's no way to get information directly from
the CPU to the latches; the information must first be written to some
location in display memory, because the latches can be loaded *only*
from display memory. By writing the pattern to off-screen memory, we
don’t have to worry about interfering with whatever is currently
don't have to worry about interfering with whatever is currently
displayed on the screen.
As for why the pattern is stored exactly where it is, that’s part of a
As for why the pattern is stored exactly where it is, that's part of a
master memory allocation plan that will come to fruition in the next
chapter, when I implement a Mode X animation program. Figure 48.3 shows
this master plan; the first two pages of memory (each 76,800 pixels
@ -77,7 +77,7 @@ register to select the corresponding destination plane. Then, copy all
pixels in that plane, repeating for all four planes.)
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![](images/i.jpg) *Although copying through the latches is, in general, a speedy technique, especially on slower VGAs, it’s not always a win. Reading video memory tends to be quite a bit slower than writing, and on a fast VLB or PCI adapter, it can be faster to copy from main memory to display memory than it is to copy from display memory to display memory via the latches.*
![](images/i.jpg) *Although copying through the latches is, in general, a speedy technique, especially on slower VGAs, it's not always a win. Reading video memory tends to be quite a bit slower than writing, and on a fast VLB or PCI adapter, it can be faster to copy from main memory to display memory than it is to copy from display memory to display memory via the latches.*
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