ST72104G Datasheet, PDF (16/135 Pages) STMicroelectronics – 8-BIT MCU WITH SINGLE VOLTAGE FLASH MEMORY, ADC, 16-BIT TIMERS, SPI, I2C INTERFACES

ST72104G, ST72215G, ST72216G, ST72254G
CENTRAL PROCESSING UNIT (Cont’d)
Stack Pointer (SP)
Read/Write
Reset Value: 01 7Fh
15
8
0
0
0
0
0
0
0
1
7
0
0 SP6 SP5 SP4 SP3 SP2 SP1 SP0
The Stack Pointer is a 16-bit register which is al-
ways pointing to the next free location in the stack.
It is then decremented after data has been pushed
onto the stack and incremented before data is
popped from the stack (see Figure 7).
Since the stack is 128 bytes deep, the 9th most
significant bits are forced by hardware. Following
an MCU Reset, or after a Reset Stack Pointer in-
struction (RSP), the Stack Pointer contains its re-
set value (the SP6 to SP0 bits are set) which is the
stack higher address.
Figure 7. Stack Manipulation Example
The least significant byte of the Stack Pointer
(called S) can be directly accessed by a LD in-
struction.
Note: When the lower limit is exceeded, the Stack
Pointer wraps around to the stack upper limit, with-
out indicating the stack overflow. The previously
stored information is then overwritten and there-
fore lost. The stack also wraps in case of an under-
flow.
The stack is used to save the return address dur-
ing a subroutine call and the CPU context during
an interrupt. The user may also directly manipulate
the stack by means of the PUSH and POP instruc-
tions. In the case of an interrupt, the PCL is stored
at the first location pointed to by the SP. Then the
other registers are stored in the next locations as
shown in Figure 7.
– When an interrupt is received, the SP is decre-
mented and the context is pushed on the stack.
– On return from interrupt, the SP is incremented
and the context is popped from the stack.
A subroutine call occupies two locations and an in-
terrupt five locations in the stack area.
CALL
Subroutine
@ 0100h
Interrupt
Event
PUSH Y
POP Y
IRET
RET
or RSP
SP
SP
PCH
@ 017Fh PCL
SP
CC
A
X
PCH
PCL
PCH
PCL
Stack Higher Address = 017Fh
Stack Lower Address = 0100h
SP
Y
CC
A
X
PCH
PCL
PCH
PCL
CC
A
X
PCH
PCL ^SP
PCH
PCL
PCH
SP
PCL
16/135