Dual mode virtual-to-real address translation mechanism

In a computer system including a first storage means responsive to real addresses;

• second storage means for storing a virtual address;
• and a translating mechanism for translating a virtual address in accordance with either one or the other of two modes of operation to form a real address, said virtual address having first, second and third portions, said real address identifying at least a portion, such as a byte, of one of a plurality of contiguous blocks comprising a segment of storage when formed by virtual address translation in a first mode and identifying at least a portion, such as a byte, of one of a plurality of noncontiguous segments comprising a segment of storage when formed by virtual address translation in a second mode;
• said translating mechanism being comprised by:
• first table storage means for storing a table comprised of a plurality of segment descriptors, each of said segment descriptors having a first field portion and a second field portion;
• second table storage means for storing a table comprised of a plurality of subsegment descriptors, each of said subsegment descriptors having field portions,
• selecting means responsive to the contents of said first portion of said virtual address stored in said second storage means, in either of said modes of operation, to select a predetermined one of said plurality of segment descriptors from said table stored in said first table storage means;
• control means responsive to the contents of said first field portion of said selected segment descriptor, when selected, for controlling said translating mechanism to operate in one or the other of the two modes of operation for translating said virtual address into said real address;
• adder means responsive to said control means for adding said second portion of said virtual address to said second field portion of said selected segment descriptor to form a sum thereof upon said translating mechanism being controlled to operate in said first mode;
• and
• concatenating means responsive to said control means for concatenating said third portion of said virtual address onto said sum to form said real address upon said translating mechanism being controlled to operate in said first mode and responsive to said control means for concatenating said second portion of said virtual address onto said second field portion of said selected segment descriptor for selecting a predetermined one of said plurality of subsegment descriptors and for concatenating said third portion of said virtual address onto a field portion of said selected subsegment descriptor to form said real address in response to said control means controlling said translating mechanism to operate in said second mode.