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+Chapter 25\
+ VGA Data Machinery {#Heading1}
+-------------------
+
+### The Barrel Shifter, Bit Mask, and Set/Reset Mechanisms {#Heading2}
+
+In the last chapter, we examined a simplified model of data flow within
+the GC portion of the VGA, featuring the latches and ALUs. Now we’re
+ready to expand that model to include the barrel shifter, bit mask, and
+the set/reset capabilities, leaving only the write modes to be explored
+over the next few chapters.
+
+### VGA Data Rotation {#Heading3}
+
+Figure 25.1 shows an expanded model of GC data flow, featuring the
+barrel shifter and bit mask circuitry. Let’s look at the barrel shifter
+first. A barrel shifter is circuitry capable of shifting—or rotating, in
+the VGA’s case—data an arbitrary number of bits in a single operation,
+as opposed to being able to shift only one bit position at a time. The
+barrel shifter in the VGA can rotate incoming CPU data up to seven bits
+to the right (toward the least significant bit), with bit 0 wrapping
+back to bit 7, after which the VGA continues processing the rotated byte
+just as it normally processes unrotated CPU data. Thanks to the nature
+of barrel shifters, this rotation requires no extra processing time over
+unrotated VGA operations. The number of bits by which CPU data is
+shifted is controlled by bits 2-0 of GC register 3, the Data Rotate
+register, which also contains the ALU function select bits (data
+unmodified, AND, OR, and XOR) that we looked at in the last chapter.
+
+![](images/25-01.jpg)\
+ **Figure 25.1**  *Data flow through the Graphics Controller.*
+
+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
+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
+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
+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.
+
+The drawing of bit-mapped text is one use for the barrel shifter, and
+I’ll demonstrate that application below. In general, though, don’t knock
+yourself out trying to figure out how to work data rotation into your
+programs—it just isn’t all that useful in most cases.
+
+### The Bit Mask {#Heading4}
+
+The VGA has bit mask circuitry for each of the four memory planes. The
+four bit masks operate in parallel and are all driven by the same mask
+data for each operation, so they’re generally referred to in the
+singular, as “the bit mask.” Figure 25.2 illustrates the operation of
+one bit of the bit mask for one plane. This circuitry occurs eight times
+in the bit mask for a given plane, once for each bit of the byte written
+to display memory. Briefly, the bit mask determines on a bit-by-bit
+basis whether the source for each byte written to display memory is the
+ALU for that plane or the latch for that plane.
+
+![](images/25-02.jpg)\
+ **Figure 25.2**  *Bit mask operation.*
+
+The bit mask is controlled by GC register 8, the Bit Mask register. If a
+given bit of the Bit Mask register is 1, then the corresponding bit of
+data from the ALUs is written to display memory for all four planes,
+while if that bit is 0, then the corresponding bit of data from the
+latches for the four planes is written to display memory unchanged. (In
+write mode 3, the actual bit mask that’s applied to data written to
+display memory is the logical AND of the contents of the Bit Mask
+register and the data written by the CPU, as we’ll see in Chapter 26.)
+
+The most common use of the bit mask is to allow updating of selected
+bits within a display memory byte. This works as follows: The display
+memory byte of interest is latched; the bit mask is set to preserve all
+but the bit or bits to be changed; the CPU writes to display memory,
+with the bit mask preserving the indicated latched bits and allowing ALU
+data through to change the other bits. Remember, though, that it is not
+possible to alter selected bits in a display memory byte *directly;* the
+byte must first be latched by a CPU read, and then the bit mask can keep
+selected bits of the latched byte unchanged.
+
+Listing 25.1 shows a program that uses the bit mask data rotation
+capabilities of the GC to draw bitmapped text at any screen location.
+The BIOS only draws characters on character boundaries; in 640x480
+graphics mode the default font is drawn on byte boundaries horizontally
+and every 16 scan lines vertically. However, with direct bitmapped text
+drawing of the sort used in Listing 25.1, it’s possible to draw any font
+of any size anywhere on the screen (and a lot faster than via DOS or the
+BIOS, as well).
+
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+
+* * * * *
+
+Graphics Programming Black Book © 2001 Michael Abrash