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Designing SOCs with Configured Cores by Steve Leibson

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3.4. The Xtensa Program Counter

The Xtensa ISA employs a 32-bit program counter, which encompasses a 4-Gbyte address space. During a function call, only the lower 30 bits of the return address are saved, which restricts nested subroutines using function calls to a 1-Gbyte address space. (The other two address bits are used to store the amount of register-window translation: 0, 4, 8, or 12 entries.)

A function-call return restores the saved 30-bit return address and leaves the upper two bits of the program counter untouched. Thus related sets of function calls can operate in any of the four 1-Gbyte address spaces that comprise the overall 4-Gbyte space. Jump instructions, which use target addresses stored as 32-bit values in register-file entries, ...

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