Above the Line - Refers to storage above 16Mb's. As 31 bit addressing mode is required to reference storage above 16Mb it is not relevant to MVS 3.8j. The opposite is "Below The Line" which is 24 bits. Although 16 megabytes is the common phrase, this is a rounded number. 24 bits is 2 to the 24th power or 16777216 and 31 bits or 2 to the 31st power is 2147483648. The IBM System 360 was only capable of 24 bit addressing. The System 370 introduced 31 bit addressing. The older 360 programs would run on a 370 but a major rewrite was required in order to make use of the full 31 bits of a 370. The AMODE atribute of a load module specifies whether it is 24 or 31 bit addressing.