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@ -6,24 +6,24 @@ Chapter 28\
Reading VGA Memory {#Heading1}
-------------------
### Read Modes 0 and 1, and the Color Don’t Care Register {#Heading2}
### Read Modes 0 and 1, and the Color Don't Care Register {#Heading2}
Well, it’s taken five chapters, but we’ve finally covered the data write
path and all four write modes of the VGA. Now it’s time to tackle the
VGA’s two read modes. While the read modes aren’t as complex as the
write modes, they’re nothing to sneeze at. In particular, read mode 1
Well, it's taken five chapters, but we've finally covered the data write
path and all four write modes of the VGA. Now it's time to tackle the
VGA's two read modes. While the read modes aren't as complex as the
write modes, they're nothing to sneeze at. In particular, read mode 1
(also known as color compare mode) is rather unusual and not at all
intuitive.
You may well ask, isn’t *anything* about programming the VGA
You may well ask, isn't *anything* about programming the VGA
straightforward? Well...no. But then, clearing up the mysteries of VGA
programming is what this part of the book is all about, so let’s get
programming is what this part of the book is all about, so let's get
started.
### Read Mode 0 {#Heading3}
Read mode 0 is actually relatively uncomplicated, given that you
understand the four-plane nature of the VGA. (If you don’t understand
understand the four-plane nature of the VGA. (If you don't understand
the four-plane nature of the VGA, I strongly urge you to read Chapters
23-27 before continuing with this chapter.) Read mode 0, the read mode
counterpart of write mode 0, lets you read from one (and only one) plane
@ -52,14 +52,14 @@ a time, so there are only four possible settings of the Read Map
register: 0, 1, 2, or 3, to select reads from plane 0, 1, 2, or 3. In
write mode 0, by contrast (in fact, in any write mode), any or all
planes may be written to at once, since the byte written by the CPU can
“fan out” to multiple planes. Consequently, there are not four but
"fan out" to multiple planes. Consequently, there are not four but
sixteen possible settings of the Map Mask register. The setting of the
Map Mask register to write only to plane 0 is 1; to write only to plane
1 is 2; to write only to plane 2 is 4; and to write only to plane 3 is
8.
As you can see, the settings of the Read Map and Map Mask registers for
accessing a given plane don’t match. The code in Listing 28.1
accessing a given plane don't match. The code in Listing 28.1
illustrates this. Listing 28.1 simply copies a sixteen-color image from
system memory to VGA memory, one plane at a time, then animates by
repeatedly copying the image back to system memory, again one plane at a