The 'C' programming language provides an excellent mechanism to perturb just about any bit of memory in the system, including the stack - it's called a Pointer.

Keil C51 enhances this facility by means of the Generic Pointer - where the pointer indicates both the Memory Space (DATA, IDATA, XDATA, etc) and the Address within that space. The key is in the fact that the IDATA m-space idicator is zero; so a null Generic Pointer addresses IDATA - which is where the Stack lives!