@Override
	public long getValue(ParserWalker walker) throws MemoryAccessException {
		long res = getContextBytes(walker);
		res >>= shift;
		if (signbit) {
			res = TokenField.signExtend(res, endbit - startbit);
		}
		else {
			res = TokenField.zeroExtend(res, endbit - startbit);
		}
		return res;
	}
Contiguous bits in the non-instruction part of the context interpreted
as an integer value
Ghidra/Framework/SoftwareModeling/src/main/java/ghidra/app/plugin/processors/sleigh/expression/ContextField.java