This sample include file, written in fasmw, contains a function to convert 64 bit (63 bit positive) numbers to an ascii string. It uses the FPU's floating point functions. It can also add commas to the string. (I must add, this code is a bit gunky. I have found and fixed several bugs in it, so use with caution)
I tested a function by fasm user, El Tangas, which converts 32 bit numbers to an ascii string, and it is about 4 or 5 times faster than my function, but is limited to 32 bit numbers. So, if you need 64 bit numbers this works.