How does the 51 microcontroller determine the content of the text message received by SIM900A

First of all, you need to understand the Unicode encoding protocol. You can download: Text and Unicode code conversion gadget

You will understand that each Chinese character or data in Unicode encoding is composed of 4 ASCII composed of codes.

For example, 123 converted into Unicode code is: 003100320033

So you must first understand the Unicode encoding protocol.

Secondly, text message reading usually has its fixed order. You need to check the relevant information about this. Each data has its specific meaning.