It makes it so that using small text does not erase pixels below it :) Here is how, if you are curious:
You are indeed correct that it is a text command :) Here is a breakdown:
FDCB almost always means it is going to modify a system flag
the next byte (in this case 05h) is the flag group that will be edited. This specifically happens to be textFlags (that is the name TI gave that group)
The last byte tells what is done. and can be further broken down. The first digit is:
8,9,A,B means it will reset a bit
C,D,E,F means it will set a bit
The next digit tells if it is an odd bit or an even bit. 0~7 means even, 8~F means odd. There are 8 bits in a byte, so lets look at 8E:
8 means a flag is to be reset, and it will either be bit 0 or 1
E tells us it is odd, so flag 1 is reset.
Resetting flag 1 is the flag textEraseBelow This reenables erasing the line below small text :)
The C9 just tells the calc to return. If you want to give it a try, try to make the opcode to do the opposite (erase the pixel below).