ie at that point in the circuit, either one of the two inputs in the LOW state will result in a HIGH at the output.
I vaguely remember this notation being referred to as "Mill Spec Logic". I am prepared to be corrected on this statement if necessary.
JimB
the first part is 'spot on'.... sometimes it's easier to follow the logic flow by using inverted input OR gates, again, looking for one zero in to give you a one out. It's drawn to see what logic level the function is looking for to yield what output. The output decoder of our fire control computers on the sub were looking for 4 unique zeros (on the 24 bit word) to enable the ouput port.
Milspec logic is basically logic gates that are rated for the full -25 to +125°C operation. Generally they lead with an M (MD8085) from some manufacturers, and a different prefix for others, ex. National Semi linear components.... LM324 is commercial spec 0 to 70°C, LM224 is industrial spec 0 to 85°C, and LM124 is mil spec -25 to +125 (or sometimes +85)°C. You would assume the mil spec parts are better, but they are not. The specs for them are wider because they have to work over a wider temperature range, but they are guaranteed to work within those specs over that range. That's not saying an LM324 won't, but it's not guaranteed to, so some might and some might not.
Also, when we put LANs on submarines in the mid 80's, we built the interface between the network and the CIP panel (course indicator panel, what they look at to drive the sub) for the new Trident Missile Subs. Our LAN board was 16 bit and used an MD8086-2 (higher speed and mil spec version of the 8086), and we paid about $160 each for them, when commercial 8086's were going for around $25. I spent three days in drafting redrawing all the schematics to the LAN interface board, some 120 chips, because when we built the prototypes, we used ANDs and ORs. Well, a mil spec NAND gate cost about $5, and a mil spec AND gate cost about $20... so it was worth it for us to convert the ANDs and ORs to inverted input NORs (ANDs) and inverted input NANDs (ORs).
Some chips weren't available in mil spec, so we had to have them tested (and dotted) at the full temperature spectrum, complete with traceable paper work. I don't know if you recall TI got into trouble in the 80's for 'gundecking' (a Navy term for cheating) on their testing and control paperwork on their mil spec parts.