3/24/97 4-17 CPU Organization
of the 64 bytes on the stack is available for handler processing, which should carefully limit
further use of the stack.
Stack Underflow
Stack underflow occurs when too many items are popped and the stack pointer value becomes
greater than its initial value, i.e., the stack top. The XA does not support stack underflow
detection.
Stack Pointer Misalignment
Pointer misalignment occurs when a pointer contains an odd value and is used by an instruction
to access a word value in memory. The same situation could occur if some program action forced
the stack pointer to an odd value. In these cases, the XA ignores the bottom bit of the pointer and
continues with a word memory access.
4.7.5 Stack Initialization
At power-on reset, both USP and SSP in all XA derivatives are initialized to 100h. Since SP is
pre-decremented, the first PUSH operation will store a word at location FEh and the stack will
grow downwards from there.
Figure 4.14 Stack-based addressing
MOV Rn, [R7+offset]
MOV [R7+offset], Rn
DS
SP (R7)
8-bit segment
identifier
16-bit pointer
0
1
SM bit in PSW
complete 24-bit
memory address
00h
[SP+2]
[SP+4]
[SP+6]
[SP+8]
[SP+0]
Data Memory
8 or 16-bit offset
(from instruction)
+
16 bits
8 bits
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