8.2. What is addressability?
One of the major developments of the MVS operating system was the implementation of 31-bit addressing. Prior to MVS/XA the highest virtual storage location that could be addressed was 16 megabytes, or hexadecimal FFFFFF. Actually, it was one byte less that 16 megabytes, because we start at zero. As applications grew larger the 24-bit architecture limitations were recognized, and 31-bit addressability was introduced. The 31-bit standard increased the amount of addressable virtual storage to 2 gigabytes. The addressing mode of a program is determined by the high order bit (bit 32 of the PSW) of the instruction address. If this bit is set to 1 the processor is running in 31-bit mode. If it is 0 then the processor is ...
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