@Override
	public void initialExecuteCallback(Emulate emulate, Address current_address,
			RegisterValue contextRegisterValue) throws LowlevelError {
		if (ismReg == null) {
			return;
		}
		BigInteger isaModeValue = BigInteger.ZERO;
		if (contextRegisterValue != null) {
			isaModeValue =
				contextRegisterValue.getRegisterValue(isaModeReg).getUnsignedValueIgnoreMask();
		}
		if (!BigInteger.ZERO.equals(isaModeValue)) {
			isaModeValue = BigInteger.ONE;
		}
		emu.getMemoryState().setValue(ismReg, isaModeValue);
	}
Initialize ISM register based upon context-register state before first
instruction is executed.
Ghidra/Processors/MIPS/src/main/java/ghidra/program/emulation/MIPSEmulateInstructionStateModifier.java