I have the FFU and FFL instructions in a program. After the position value stops, reaches the Length value, how do I reset this to "0"?

If the Position Value is reaching the Length Value then the .dn bit is also true for your control register.

Normally each FFL increases the position value by 1 and each FFU decreases it by 1. So if your Pos is reaching LEN then your buffer either is not big enough or your FFU's are not cycling.

You can reset the POS to zero either thru a series of FFU's or by a MOV that loads R?:$.POS to zero.

Use Res instruction with Control register RES R6:0

Thanks!