Just a couple of minor sugestions about the way in which you costruct your schematic. It's your schematic, so don't be offended. It's just a matter of style.

1. Those busses don't do anything useful. Since you have to label every signal that joins a bus anyway, it's just as well to leave things that way. The busses just clutter the schematic and make it harder to read. Colors don't help much either, particularly for those of us who are "cromatically challenged." I always print 'em black an white anyway. The rule of thumb is, "when in doubt, leave it out." If there's a really compelling reason for using colors, then go ahead.

2. It makes drawings easier to read if the number of bends/corners in a line is minimized.

3. If a signal line is unlabelled and single-ended, it is presumed to be unconnected, so why include it? If it needs to be there because it has meaning, then that meaning should be specified.

4. The most popular method, at least with gates and other small devices, is inputs on the left, outputs on the right.

5. TTL devices generally have emitters on their inputs, so they represent a load. CMOS devices have capacitive inputs. If you tie multiple inputs together, their current, using TTL, or capacitance, using CMOS, add. Either way, it slows things down. For example, U2B should probably have the unused pins 13 and 12 tied to Vcc rather than the input. Careful, though, as this has been known to cause latchup (SCR-ing). It's probably best to have a pullup resistor for those unused tie-high's.

6. This circuit should probably have a data bus buffer between the memories and the MCU, since the PC will also be driving the data bus. I can't tell what's really "up" with respect to that PC interface.

RE