You will need different version of itoa to display 32bit number from register pair DX:AX as unsigned decadic number. Maximum value of such number is 4294967295. Divide the number in DX:AX by 1000000000, giving the quotient 4. This is the 1st output digit. Subtract 4*1000000000, so now DX:AX=294967295. Divide by 100000000, giving 2. Subtract 2*100000000, so now DX:AX=94967295. Repeat until all ten digits are processed.
Twenty years ago I wrote a macro called StoDD for this, it is available for download in the file CPU.MAC at http://vitsoft.info/vsasmlib.zip Maybe it could help you.