LTspice was the first PC-based SPICE program to implement its own 64bit address space on the hard disk to allow one to view waveform data files of essentially unlimited file size.
The 32-bit version of LTspice XVII(XVIIx86.exe) can address data files containing many Gigabytes of data but page in only about three Gigabytes at a time for plotting in the waveform viewer. No data trace can contain more than 2GB of data.
The 64-bit version of LTspice XVII(XVIIx64.exe) can use as much memory as you have installed but still keeps most of the data on the disk to allow you to view waveform files that exceed your physical memory. Note that current 64bit processors only have a few extra address lines over 32 bit processors.