public long readMaskBytes(int start, int len) {
		long res = 0;
		for (int i = 0; i < len; i++) {
			res <<= 8;
			int index = start + i - offset;
			if (0 <= index && index < mask.length) {
				res |= 0xff & mask[index];
			}
		}
		return res;
	}
Decode {@code len} mask bytes in big-endian format, beginning at {@code start}

@param start the first byte to decode
@param len the number of bytes to decode
@return the decoded long
Ghidra/Framework/SoftwareModeling/src/main/java/ghidra/app/plugin/assembler/sleigh/sem/AssemblyPatternBlock.java