An addressing mode which allows 2 gigabytes of main memory to be accessed. Actually it is 2 to the 31st power or 2147483648 bytes. This applies only to MVS/XA and MVS/ESA, not MVS 3.8j. The H-380 project (in Yahoo groups) is working to get 31 bit addressing for a modified version of MVS 3.8j. See Above the Line, Below the Line, and AMODE. The System 360 used 24 bit addresses and the System 370 that went on the market in the early 1970's brought in 31 bit addresses. It is 31 bits instead of 32 because the subroutine parameters list end is markd by a set 32nd bit. Most 360 programs were not upgraded to 31 bits because the LA (Load Address) instruction zeros the high-order byte of the 32 bit register that is being loaded. This led to the AMODE attribute of load modules that specifies wheter 24 bit or 31 bit addressing is being used.