The core program for AutoCAD has gone through 3 or 4 full re-writes in the last dozen years to keep up with hardware/software improvements and user demands.
MSTA's only re-write was between V7 and V8, when they finally understood that "connect-the-dots" was fine for kiddy coloring books, but was lunacy for CAD accuracy.
BTW, IGDS on a main frame was NOT MicroStation, But InterGraph. Two vastly different programs.
But hey, that's just me.
Randall Culp
Civil-Structural Design Technician
(aka CADaver)