The PX-4+ was an improved version which had the External RAM disk integrated. It passes to the kernel information about the RAM disk and its location. It also has the ability to create a RAM disk. Support for this type of RAM disk is completely optional within the kernel configuration file. Internal RAM was 64K, of which a part could be reserved as RAM disk. The Magnum could suspend and retain memory in CMOS battery-backed RAM (RAM Disk D&colon;). Additionally, many commercial programs simply overwrote the memory space occupied by the RAM disk software. Andy Bakkers sold a 1 MB RAM Disk daughter board for Big Board II. Retail version supports RAM disk size between 5MiB to 64GiB. Windows NT 4.0 Enterprise Edition thus used the PSE-36 feature essentially as a RAM disk. Unix-like systems can use a RAM disk or network shared resource as its root directory. The intelligent RAM disk module had its own Z80 processor with a backup battery. In 2007, ACard Technology produced the ANS-9010 Serial ATA RAM disk, max 64GB. AmigaOS itself would gain a Recoverable Ram Disk (called "RAD") in version 1.3. In Multiuser DOS, the RAM disk defaults to the drive letter M&colon; (for memory drive).