Incorrect minstret Count Increased by ECALL

[fly-1011]

Bug Description:

When executing the ECALL instruction, the NutShell processor incorrectly incremented the minstret CSR.

As ECALL and EBREAK cause synchronous exceptions, they are not considered to retire, and should not increment the minstret CSR.

According to the RISC-V Privileged Specification, ECALL (and EBREAK) cause synchronous exceptions and must not be considered retired, therefore must not increment minstret.

Test program and log information: test_ecall.zip

Inconsistent information:

Spike reports minstret = 3 DUT reports minstret = 4

This indicates that the ECALL instruction was mistakenly counted as a retired instruction.

Environment:

NutShell: commit e315a27