It's actually an interesting question, especially relating to the mach trim. The mach trim on the 737 NG starts to function at M 0.615, but if it fails the restriction is 280 kts. / M 0.82. In other words it restricts the amount of knots (max is 340) but not Mach (max is 0.82).
I've assumed there isn't any restriction on the mach due to relatively low mach tuck tendencies and that this system exists only due to regulatory compliance, but I don't know why there's a restriction on the amount of knots.
Any smart heads in here that can enlighten me/us?