How can you expect to buffer and transfer process to do 600 bytes when you have all of 128 bytes of internal data RAM. Seems to me that you are trying to use RAM from 0x20 up to 0xFF and when the data sheet says you have 128 bytes RAM I think that is all you get from 0x20 to 0x7F which is 127-32=95 bytes.

Maybe you should look at instead input one byte and then send it out the serial port right away. Otherwise for a 600 byte buffer you are have one of two choices....either wire on an external RAM chip or switch over to a part that has some on board eXtra RAM. Some chips have the normal DATA RAM plus a small bank of RAM that operates with the MOVX instructions.

Michael Karas