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James Gregory 2013-12-31 17:12:29 +11:00
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@ -43,14 +43,14 @@ unmodified, AND, OR, and XOR) that we looked at in the last chapter.
The barrel shifter is powerful, but (as sometimes happens in this
business) it sounds more useful than it really is. This is because the
GC can only rotate CPU data, a task that the CPU itself is perfectly
capable of performing. Two **OUT**s are needed to select a given
capable of performing. Two `OUT`s are needed to select a given
rotation: one to set the GC Index register, and one to set the Data
Rotate register. However, with careful programming it's sometimes
possible to leave the GC Index always pointing to the Data Rotate
register, so only one **OUT** is needed. Even so, it's often easier
register, so only one `OUT` is needed. Even so, it's often easier
and/or faster to simply have the CPU rotate the data of interest CL
times than to set the Data Rotate register. (Bear in mind that a single
**OUT** takes from 11 to 31 cycles on a 486—and longer if the VGA is
`OUT` takes from 11 to 31 cycles on a 486—and longer if the VGA is
sluggish at responding to OUTs, as many VGAs are.) If only the VGA could
rotate *latched* data, then there would be all sorts of useful
applications for rotation, but, sadly, only CPU data can be rotated.