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参数资料
| 型号： | ST72T774S9T1 |
| 厂商： | STMICROELECTRONICS |
| 元件分类： | 微控制器/微处理器 |
| 英文描述： | 8-BIT, OTPROM, 8 MHz, MICROCONTROLLER, PQFP44 |
| 封装： | TQFP-44 |
| 文件页数： | 54/144页 |
| 文件大小： | 1280K |
| 代理商： | ST72T774S9T1 |
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ST72774/ST727754/ST72734
17/144
CPU REGISTERS (Cont’d)
Stack Pointer (SP)
Read/Write
Reset Value: 01 FFh
The Stack Pointer is a 16-bit register which is
always 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 6).
Since the stack is 256 bytes deep, the most
significant byte is forced by hardware. Following
an MCU Reset, or after a Reset Stack Pointer
instruction (RSP), the Stack Pointer contains its
reset value (the SP7 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
instruction.
Note: When the lower limit is exceeded, the Stack
Pointer wraps around to the stack upper limit,
without
indicating
the
stack
overflow.
The
previously stored information is then overwritten
and therefore lost. The stack also wraps in case of
an underflow.
The stack is used to save the return address
during 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 instructions. 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 6.
– 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
interrupt five locations in the stack area.
Figure 6. Stack Manipulation Example
15
8
00
000
00
1
70
SP7
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
@ 01FFh
@ 0100h
Stack Higher Address = 01FFh
Stack Lower Address = 0100h
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