The region from 1MB to 1MB+64KB is known as the A20 wrap area. Due to the segmented scheme for generating 20-bit physical addresses on an 8088, it is possible for a DOS program to generate physical addresses in the range from 1MB to 1MB + 64KB. On an 8088 system, these addresses wrap to the low 64KB of physical memory.
In the 16-bit version of OS/2, OS/2 V1.x, 80286 physical addresses are 24 bits. The twenty-first address line of the 80286 is called the A20 line, and its setting determines whether real-mode programs wrap low physical memory, or directly access the range from 1MB to 1MB + 64KB. When an 80286 is started, the A20 line is disabled, causing the 80286 to emulate the 8088 environment. When the 80286 is switched to protected mode, the A20 line is enabled, since the protected mode of the 80286 generates 24-bit physical addresses. However, the A20 wrap area can be addressed in real mode in OS/2 V1.x if the A20 line is enabled manually. OS/2 V1.x can thus use the memory in the A20 wrap area for bimodal code (for example, OS/2 V1.x device drivers) by managing the state of the A20 line. When running a DOS application in real mode, OS/2 V1.x disables the A20 line to force the 8088 segment wrapping semantics on DOS applications. When accessing bimodal code in the range from 1MB to 1MB + 64KB in real mode, the OS/2 V1.x kernel enables the A20 line.
In OS/2 V2.0 however, the area between 1MB and 1MB + 65519 bytes, cannot be accessed by a DOS program in V86 mode using 20 address lines (that is lines 0 to 19). For example, addressing 1MB + 1 byte from within a DOS application in V86 mode would access physical memory at address 0; in other words, the memory addressing would wrap around to access the extra byte of memory. This is known as wraparound. Under OS/2 Version 2.0, the addressing of memory beyond 1MB would result in an addressing exception because although address line 20 is activated, it is not valid for a V86 mode process. This is shown in Figure "A20 Line Service (64KB Wraparound)".
In order to avoid addressing exceptions in OS/2 Version 2.0, the wraparound feature must be simulated with aliased pages. The 1MB V86 address space requires a page table with 256 entries (256 x 4KB = 1MB). Sixteen additional page table entries are used for the 65519 bytes above 1MB. These 16 entries are identical to the first 16 entries for the 1MB area, thereby mapping the wraparound area to the address range 0 to 65519.
The wraparound feature requires some additional housekeeping by the OS/2 Version 2.0 kernel. When an aliased page is swapped to disk, both page table entries must be flagged as not present.