Linux/390
Since Linux/390 was first released for the existing 32-bit data/31-bit addressing hardware in 1999, initial mainframe Linux applications compiled in pre-z/Architecture mode are also limited to 31-bit addressing. This limitation disappeared with 64-bit hardware, 64-bit Linux on zSeries, and 64-bit Linux applications. The 64-bit Linux distributions still support 32-bit data/31-bit addressing programs. IBM's 31-bit addressing supports expanded storage, allowing 31-bit code to make use of additional memory. However, at any one instant, a maximum of 2 GiB is in each working address space. For non-64-bit Linux on processors with 31-bit addressing, it is possible to assign memory above the 2 GiB bar as a RAM disk.
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