I look datasheet and I think maybe your don't have memory for stack. Plese look page 14 (and 13) datasheet.
The upper 1792 bytes of the internal XRAM can be configured
to be used as an extended 11-bit stack pointer. By default, the
stack will operate exactly like an 8052 in that it will roll over from
FFH to 00H in the general-purpose RAM. On the ADuC836
however, it is possible (by setting CFG836.7) to enable the 11-bit
extended stack pointer. In this case, the stack will roll over from
FFH in RAM to 0100H in XRAM. The 11-bit stack pointer is
visible in the SP and SPH SFRs. The SP SFR is located at 81H
as with a standard 8052. The SPH SFR is located at B7H. The
3 LSBs of this SFR contain the three extra bits necessary to
extend the 8-bit stack pointer into an 11-bit stack pointer. (Figure 4. Extended Stack Pointer Operation).
So result:
The 2 Kbytes of internal XRAM are mapped into the
bottom 2 Kbytes of the external address space if the CFG836.0
bit is set and CFG836.7 to enable the 11-bit
extended stack pointer.
Note:
The lower 128 bytes of
RAM can be accessed through direct or indirect addressing; the
upper 128 bytes of RAM can only be accessed through indirect
addressing as it shares the same address space as the SFR space,
which can only be accessed through direct addressing.
