Sprites are produced by the circuitry shown in Figure 4-13. This figure shows in block form how a pair of data words becomes a set of pixels displayed on the screen. Figure 4-13: Sprite Control Circuitry The circuitry elements for sprite display are explained below. * Sprite data registers. ---------------------- The registers SPRxDATA and SPRxDATB hold the bit patterns that describe one horizontal line of a sprite for each of the eight sprites. A line is 16 pixels wide, and each line is defined by two words to provide selection of three colors and transparent. * Parallel-to-serial converters. ------------------------------ Each of the 16 bits of the sprite data bit pattern is individually sent to the color select circuitry at the time that the pixel associated with that bit is being displayed on-screen. Immediately after the data is transferred from the sprite data registers, each parallel-to-serial converter begins shifting the bits out of the converter, most significant (leftmost) bit first. The shift occurs once during each low resolution pixel time and continues until all 16 bits have been transferred to the display circuitry. The shifting and data output does not begin again until the next time this converter is loaded from the data registers. Because the video image is produced by an electron beam that is being swept from left to right on the screen, the bit image of the data corresponds exactly to the image that actually appears on the screen (most significant data on the left). * Sprite serial video data. ------------------------- Sprite data goes to the priority circuit to establish the priority between sprites and playfields. * Sprite position registers. -------------------------- These registers, called SPRxPOS , contain the horizontal position value (X value) and vertical position value (Y value) for each of the eight sprites. * Sprite control registers. ------------------------- These registers, called SPRxCTL , contain the stopping position for each of the eight sprites and whether or not a sprite is attached. * Beam counter. ------------- The beam counter tells the system the current location of the video beam that is producing the picture. * Comparator. ----------- This device compares the value of the beam counter to the Y value in the position register SPRxPOS . If the beam has reached the position at which the leftmost upper pixel of the sprite is to appear, the comparator issues a load signal to the serial-to-parallel converter and the sprite display begins. Figure 4-13 shows the following: * Writing to the sprite control registers disables the horizontal comparator circuitry. This prevents the system from sending any output from the data registers to the serial converter or to the screen. * Writing to the sprite A data register enables the horizontal comparator. This enables output to the screen when the horizontal position of the video beam equals the horizontal value in the position register. * If the comparator is enabled, the sprite data will be sent to the display, with the leftmost pixel of the sprite data placed at the position defined in the horizontal part of SPRxPOS . * As long as the comparator remains enabled, the current contents of the sprite data register will be output at the selected horizontal position on a video line. * The data in the sprite data registers does not change. It is either rewritten by the user or modified under DMA control. The components described above produce the automatic DMA display as follows: When the sprites are in DMA mode, the 18-bit sprite pointer register (composed of SPRxPTH and SPRxPTL ) is used to read the first two words from the sprite data structure . These words contain the starting and stopping position of the sprite. Next, the pointers write these words into SPRxPOS and SPRxCTL . After this write, the value in the pointers points to the address of the first data word (low word of data for line 1 of the sprite.) Writing into the SPRxCTL register disabled the sprite. Now the sprite DMA channel will wait until the vertical beam counter value is the same as the data in the VSTART (Y value) part of SPRxPOS . When these values match, the system enables the sprite data access. The sprite DMA channel examines the contents of VSTOP (from SPRxCTL , which is the location of the line after the last line of the sprite) and VSTART (from SPRxPOS ) to see how many lines of sprite data are to be fetched. Two words are fetched per line of sprite height, and these words are written into the sprite data registers. The first word is stored in SPRxDATA and the second word in SPRxDATB . The fetch and store for each horizontal scan line occurs during a horizontal blanking interval, far to the left of the start of the screen display. This arms the sprite horizontal comparators and allows them to start the output of the sprite data to the screen when the horizontal beam count value matches the value stored in the HSTART (X value) part of SPRxPOS . If the count of VSTOP - VSTART equals zero, no sprite output occurs. The next data word pair will be fetched, but it will not be stored into the sprite data registers. It will instead become the next pair of data words for SPRxPOS and SPRxCTL . When a sprite is used only once within a single display field, the final pair of data words, which follow the sprite color descriptor words , is loaded automatically as the next contents of the SPRxPOS and SPRxCTL registers. To stop the sprite after that first data set, the pair of words should contain all zeros. Thus, if you have formed a sprite pattern in memory, this same pattern will be produced as pixels automatically under DMA control one line at a time.