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James Gregory 2013-12-30 18:34:31 +11:00
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23-02.md
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@ -4,7 +4,7 @@
### At the Core {#Heading5}
A little background is necessary before we’re ready to examine Listing
A little background is necessary before we're ready to examine Listing
23.1. The VGA is built around four functional blocks, named the CRT
Controller (CRTC), the Sequence Controller (SC), the Attribute
Controller (AC), and the Graphics Controller (GC). The single-chip VGA
@ -23,8 +23,8 @@ makes it necessary for you to remember which registers are in which
blocks.
Most VGA registers are addressed as *internally indexed* registers. The
internal address of the register is written to a given block’s Index
register, and then the data for that register is written to the block’s
internal address of the register is written to a given block's Index
register, and then the data for that register is written to the block's
Data register. For example, GC register 8, the Bit Mask register, is set
to 0FFH by writing 8 to port 3CEH, the GC Index register, and then
writing 0FFH to port 3CFH, the GC Data register. Internal indexing makes
@ -128,7 +128,7 @@ port DX+1, this is equivalent to the first method but is faster.
However, this technique does not work for programming the AC Index and
Data registers; both AC registers are addressed at 3C0H, so two separate
byte **OUT**s must be used to program the AC. (Actually, word **OUT**s
to the AC do work in the EGA, but not in the VGA, so they shouldn’t be
to the AC do work in the EGA, but not in the VGA, so they shouldn't be
used.) As mentioned above, you must be sure which mode—Index or Data—the
AC is in before you do an **OUT** to 3C0H; you can read the Input Status
1 register at any time to force the AC to Index mode.
@ -137,11 +137,11 @@ How safe is the word-**OUT** method of addressing VGA registers? I have,
in the past, run into adapter/computer combinations that had trouble
with word **OUT**s; however, all such problems I am aware of have been
fixed. Moreover, a great deal of graphics software now uses word
**OUT**s, so any computer or VGA that doesn’t properly support word
**OUT**s, so any computer or VGA that doesn't properly support word
**OUT**s could scarcely be considered a clone at all.
------------------- ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
![](images/i.jpg) *A speed tip: The setting of each chip’s Index register remains the same until it is reprogrammed. This means that in cases where you are setting the same internal register repeatedly, you can set the Index register to point to that internal register once, then write to the Data register multiple times. For example, the Bit Mask register (GC register 8) is often set repeatedly inside a loop when drawing lines. The standard code for this is:*
![](images/i.jpg) *A speed tip: The setting of each chip's Index register remains the same until it is reprogrammed. This means that in cases where you are setting the same internal register repeatedly, you can set the Index register to point to that internal register once, then write to the Data register multiple times. For example, the Bit Mask register (GC register 8) is often set repeatedly inside a loop when drawing lines. The standard code for this is:*
------------------- ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
MOV DX,03CEH ;point to GC Index register