Dallas DS87C550 vs C51 V3.96

I am using Keil C51 V3.96 (quite old) to develop the firmware for DS87C550. I can generate the code that runs correctly. But if I try to add some new & short codes, the progrm will run incorrectly. There is anything wrong with my additional codes. I suspect it's the problem of the compiler. Anyone can help?

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