On some m68k-based Texas Instruments graphing calculators, high memory addresses that go beyond the physical memory can be used to access the physical memory (the address space folds itself over the physical memory). This range of addresses is called ghost space, because it mirrors the actual physical space.
For TI-89 programmers, ghost space starts from address 0x40000, and it is widely used to circumvent some of the protections introduced with the HW2 (hardware version 2) release of the calculator. This became a compatibility issue with the release of HW3 (the Ti-89 Titanium), where it is disabled, and patching the ROM ([http://www.tigen.org/kevin.kofler/ti89prog.htm#hw3patch HW3Patch]) becomes necessary.
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