There's certainly a stack when running under C51, for function call returns. C51 just assigns local variables into 128 byte data segment. But say, PUSH ACC work

There's certainly a stack when running under C51, for function call returns. C51 just assigns local variables into 128 byte data segment. But say, PUSH ACC works just fine when you drop into assembler. Then again, stack on 8051 might be sometimes pretty small. From 10 to 128+ bytes.