A conversion function which takes the string expression represented in a specified base to be converted to another specified base determined by the arguments passed.

Syntax: CONVBASE$(string-expression, frombase, tobase)
S$ = CONVBASE$("FFD", 16, 2) '-- Convert from base 16 to 2
S$ = CONVBASE$("287", 10, 16) '-- Convert from base 10 to 16

Use the Snakedile LIBs dated Nov. 2006 or later. CONVBASE$ is made case-insensitive and accurate to 63 bits positive, 32 bits negative. It will also accept base signifiers in input string
CONVBASE$("&HFFffFFff" , 16, 10) or
CONVBASE$("ffffFFFFh", 16, 10) will produce exactly the same results as CONVBASE$("FFFFFFFF", 16, 10).

If you use the original libraries then Negative numbers are not supported and all strings should be in UPPERCASE  and an exception Fault occurs if
string-expression =  "FFFFFFFF"