ST10编程手册关于指令使用的问题

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SCXT Switch Context
Syntax SCXT op1, op2
Operation (tmp1) <-- (op1)
(tmp2) <--(op2)
(SP) <-- (SP) - 2
((SP)) <-- (tmp1)
(op1) <-- (tmp2)
Data Types WORD

Description
Used to switch contexts for any register. Switching context is a push and load operation. The contents of the register specified by the first operand, op1, are pushed onto the stack. That register is then loaded with the value specified by the second operand, op2.
Flags
E Z V C N
- - - - -
E Not affected
Z Not affected
V Not affected
C Not affected
N Not affected
Addressing Modes

Mnemonic Format Bytes
SCXT reg, #data16 C6 RR ## ## 4
SCXT reg, mem D6 RR MM MM 4


刚开始学汇编, 上面红色部分望大家能解释一下:
1)SCXT这条指令的详细用法;
2)其中(SP)与((SP))有什么不同.

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3) 最下面红色部分2条语句有什么不同?