Specifications

Each pixel in mode X is represented by a on e -byte color value. Although only 256 colors are available in mode X, the
actual color space is 18-bit, with 6-bit dep th for red, green, and blue saturation. A level of indirec tion is used to map
one-byte pixel color values into this space. The table used in this mapping is called a palette, as it is analogous to a
painter’s palette, which in theory holds an infinite range of colors, but can only hold a few a t any one time. Palettes
are often used to r e duce the amount o f memory necessary to describe an image, and are thus usefu l on emb edded
devices even today. For your final projects, some of you may want to play with graphic techniqu e s such as palette
color selection to best repre sent a more d e ta iled imag e and dithering to map a more detailed image into a given palette.
The ma pping between screen pixels and video memory in mode X is a little contorted, but is no t as bad as some of the
older modes. The screen is first separated into blocks four pixels wide and one pixel high. Each block corresponds to
a single memo ry address from the processor’s point of view. You may at this point wonder how four bytes of data get
crammed into a single memory address (one byte in a byte-addressable memory). The answer, of course, is that they
don’t. For p erformance rea sons, the VGA memory was 32-bit, a nd the interface between the processor and the VGA
is used to determ ine h ow the proce ssor’s 8-bit reads and writes are mapped into the VGA’s 32-bit words. For example,
when the processor writes to a memory address, four bits of a specific VGA register are used to enable (1 bits) or
disable (0 bits) writes to 8-bit groups of the corresponding word in VGA memory. When drawing rectangles of one
color, this mask reduces the work for the pro c essor by a factor of four. When drawing images, however, it poses a
problem in that adjacent pixels must be written using different mask register setting s. As changing a VGA register
value is relatively slow, the right way to use mode X is to split the image data into four groups of interleaved pixels, set
the mask for eac h group, and write each gr oup as a wh ole using the x86 string instructions (you won’t need to wr ite
any of these, although you may want to look at how it is done in the existing code or in the x8 6 manual).
0xA12D4
address
3210
(0xA1234 + 80)
0xA1284
address
0xA1236
address
0xA1235
address
address
0xA1237
(0xA1234 + 160)
. . .
. . .
. . .
(all at 0xA1236)
plane 0
plane 1
plane 2
plane 3
0xA1234
32103210
0
address
3210
3210
321
Mode X video memory is memory-mapped and run s from (ph ysical) memory address 0xA0000 to 0xAFFFF, for a
total of 2
16
addresses and 256 kB of addressable memory (counting all four planes). The ad dresses used in your
program are virtual ra ther than physical addresses, a concept to be discussed later in the course; don’t be surprised if
they are not the same as the physical addresses, though. A full screen occupies 320 × 200/4 = 16, 000 ad dresses, so
four fit into the full memory, with a little extra room. The VGA can be directed to start the display from any address in
its memory; addresses wrap around if necessary. In the figure shown ab ove, for exam ple, the screen starts at addre ss
0xA1234.
Due to the timing behavior of emulated in te ractions with v ideo memory, the code provided to you does not use a
traditional double-buffering model in which the off-screen image is drawn directly within video memo ry. As with
double-buffering, we do m aintain two regions within the video memory for screen images. However, we use regular
memory to maintain an image o f the screen and to update that image in place. When a new image is ready for display,
we copy it into one of two regions of the video memory (the one not being displayed) and then change the VGA
registers to display the new image. Co pying an entire screen into video memory using one x86 string instruction
seems to take about as long as wr iting a small number of bytes to video memory under QEMU, thus our image display
is actually faster than try ing to draw a vertical line in video memory, which requires on e MOV instruction per vertical
pixel.
Only the modex.c file relies on mode X. The rest of the code should use a graphics for mat more convenient to C. In
particular, an image that is X pixels wide and Y pixels high should be placed in an a rray o f dimensions [Y ][X]. T he
type of the array depends on the c olor dep th of the image, and in our case is an unsigned char to store the 8-bit
color index for a pixel. The block images in blocks.s already use this format, as do the lin e gen eration r outines in
maze.c. When you write code to generate graphic images from text, as described in a later section, you should use
the same format.
Graphical Mapping in the Game