Misc/abrash-black-book
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James Gregory 2013-12-31 11:38:48 +11:00
commit f658bfbb40
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29-06.md
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@ -43,14 +43,16 @@ read the register, mask off the desired field, insert the desired
setting, and write the result back to the register. In the case of
setting the VGA to write mode 1, do this:
mov dx,3ceh ;Graphics controller index
mov al,5 ;Graphics mode reg index
out dx,al ;point GC index to G_MODE
inc dx ;Graphics controller data
in al,dx ;get current mode setting
and al,not 3 ;mask off write mode field
or al,1 ;set write mode field to 1
out dx,al ;set write mode 1
```nasm
mov dx,3ceh ;Graphics controller index
mov al,5 ;Graphics mode reg index
out dx,al ;point GC index to G_MODE
inc dx ;Graphics controller data
in al,dx ;get current mode setting
and al,not 3 ;mask off write mode field
or al,1 ;set write mode field to 1
out dx,al ;set write mode 1
```
This approach is more of a nuisance than simply setting the whole
register, but it's safer. It's also slower; for cases where you must set