Please refer to PSoC 6 Programming Specifications | Cypress Semiconductor section 3.1 (Organization of the hex file) for details.
In addition to the data that goes into flash, the hex files saves additional content like checksum, metadata and eFuse data as part of the hex file. Since these data do not map to any physical memory address, the 0x90xxxxxx address space is reserved for this purpose in the hex file.
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Meenakshi Sundaram R