ST10R272L - CENTRAL PROCESSING UNIT
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calculated as described, but the position of the bit within the word is specified by a separate
additional 4-bit value.
The stack pointer SP
This non bit-addressable register is used to point to the top of the internal system stack
(TOS). The SP register is pre-decremented whenever data is to be pushed onto the stack,
and it is post-incremented whenever data is to be popped from the stack. Therefore, the
system stack grows from higher toward lower memory locations.
Since the least significant bit the SPregister is tied to ’0’ and bits 15 through 12 are tied to ’1’
by hardware, the SP register can only contain values from F000h to FFFEh. This makes it
possible to access a physical stack within the internal RAM of the ST10R272L. A virtual
stack (usually bigger) can be realized via software. This mechanism is supported by
registers STKOV and STKUN (see STKOV and STKUN descriptions below).
Figure 15 Implicit CP used by short GPR addressing modes