參數(shù)資料
型號: ST72C104G2B7
英文描述: 8-BIT MCU WITH SINGLE VOLTAGE FLASH MEMORY. ADC. 16-BIT TIMERS. SPI.
中文描述: 8位單電壓閃存微控制器。 ADC的。 16位定時器。的SPI。
文件頁數(shù): 49/140頁
文件大?。?/td> 1349K
代理商: ST72C104G2B7
ST72104G, ST72215G, ST72216G, ST72254G
16/140
6 CENTRAL PROCESSING UNIT (Cont’d)
Stack Pointer (SP)
Read/Write
Reset Value: 01 7Fh
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 9 most sig-
nificant bits are forced by hardware. Following an
MCU Reset, or after a Reset Stack Pointer instruc-
tion (RSP), the Stack Pointer contains its reset val-
ue (the SP6 to SP0 bits are set) which is the stack
higher address.
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.
Figure 7. Stack Manipulation Example
15
8
00
000
1
70
0
SP6
SP5
SP4
SP3
SP2
SP1
SP0
PCH
PCL
SP
PCH
PCL
SP
PCL
PCH
X
A
CC
PCH
PCL
SP
PCL
PCH
X
A
CC
PCH
PCL
SP
PCL
PCH
X
A
CC
PCH
PCL
SP
Y
CALL
Subroutine
Interrupt
Event
PUSH Y
POP Y
IRET
RET
or RSP
@ 017Fh
@ 0100h
Stack Higher Address = 017Fh
Stack Lower Address = 0100h
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