F motion object generator

For each motion object to be displayed. the MPU writes four bytes of data into the piayfield display RAM. As with the piayfield generator circuitry, when 4H is low, the MPU address bus (ABUS2 thru ABUS9) addresses specific RAM locations and appropriate motion object codes are written into them via the MPU data bus (DBUSOthru DBUS7). The four bytes of data stored for each motion object are: 1) the vertical position of the object. 2) the type of object it is, 3) the horizontal position of the object, and 4) the 'color" of the object. These four pieces of information are then accessed by the motion object generator as follows:

1. During the horizontal blanking period before each scan line, the vertical sync count is compared to the vertical position of each motion object stored in the RAM. This is done by adders H6 and J6 which compare vertical sync with PFD8 thru PFD15. If a motion object is to be displayed on that scan line, a MATCH signal is generated at E9 pin 8. after a delay of one 4H clock pulse.

2. Graphics ROMs address GA5 through GA8 is latched at the output of flip-flop F6 along with the MATCH signal. This part of the address byte defines the motion object to be output from the graphic ROMs. The actual line of the motion object to be scanned is latched at the output of flip-flop H7 (GAO through GA4). The MATCH pulse enables data selector/multiplexers A8, B8. C8, and D8 to pass sixteen bits of video data from graphics ROMs A/B6 and C6 to the inputs of shift registers A9 and E8. When the V1DLD pulse occurs, the graphics ROMs' data output is loaded into the shift registers

3 The third byte of RAM data determines the horizontal position of the motion object. This RAM data (PFD 16 thru PFD23) is latched by K6 and loaded into horizontal position counters L6 and K7. The horizontal location address generated by these counters then addresses high-speed VIDEO RAMs J8 and K8. These RAMs are now loaded with the video data for the particular motion object from shift registers A9 and E8 (which were loaded fiom the graphics ROM in the previous step).

4. The fourth byte of RAM data for each motion object determines the various shades of color in which that object is to be displayed. This data (PFD24 thru PFD29) is latched by L8 and M8 and used to determine the combination of video data loaded into the high-speed video RAMs (in the previous step).

In review, all four of the previous steps occur during the HBLANK period preceeding each horizontal scan line. The proper motion object picture, with the proper "color shading" was loaded into the high-speed VIDEO RAMs. The horizontal and vertical location of the motion object has been determined by the portion of this RAM into which it was loaded. The motion object is now ready to be displayed on the TV monitor. When HBLANK ends, L6 and K7 are reset and allowed to sequentially count up to their maximum count. The outputs of these counters address the high-speed video RAMs (J8 and K8). When the RAM address encounters stored video information, the motion object video is output as GRYLVLO and GRYLVL1 and sent to the video summing circuit. The combination of these two "grey level codes" determine the shape and color of the different motion objects seen on the TV monitor.

P,*9

MA&ltA'-MAtKg

t>l

00

w.

A4

(nefcT

**

At-

M

if 1

Was this article helpful?

0 0

Post a comment