D10 Initiative

AD&D uses 1 minute combat rounds broken into 10 6 second segments. A d10 then tells you which segment your action comes in, low rolls go first.
At the beginning of each round, each combatant rolls a d10. Characters with high Dexterity subtract their Reaction/Attacking Adjustment (so 18 Dexterity which is +3 R/A Adjustment has a -3 modifier). Spell casters add the casting time to their roll. So an 18 Dexterity mage rolls a 7, subtracts 3 for a result of 4. The player decides to cast Fireball which has a casting time of 3 segments, so the spell will go off on 7. Zero and Negative numbers are treated as segment 1, but those characters go first.
This also allows spell interruption. The player rolled a 7, modified it to 4, then raised it back to 7 for the casting time. That means on Segment 4 they start casting, are still casting on segments 5 and 6 and release the spell on segment 7. The DM rolls a 5 for an NPCs initiative and they shoot the mage with 2 arrows and hit. The mage now needs to make a saving throw or have their concentration broken and lose the spell and the spell points for the spell.
To be fair, this tactic works just as well against NPCs as it does the PCs.