This port can be used on the 83+ SE, 84+, and 84+ SE to slow down the CPU in both the 6 and 15 MHz modes. Every bit affects the speed, but there are only two that have a significant effect. The default value for this port is 45h on the 84+ SE and 44h on the 83+ SE.

Bits 5-4	Approximate 15 MHz speed
00	14.93 MHz
01	12.18 MHz
10	14.80 MHz
11	12.09 MHz

This affects 6 MHz proportionally as well.