2.1 ISA 的基本分类
累加器型(Accumulator) (1 register):
1 address add A acc ¬ acc + mem[A]
1+x address addx A acc ¬ acc + mem[A + x]
堆栈型（Stack）:
0 address add tos ¬ tos + next
通用寄存器型（General Purpose Register）:
2 address add A B EA[A] ¬ EA[A] + EA[B]
3 address add A B C EA[A] ¬ EA[B] + EA[C]
Load/Store:
3 address add Ra Rb Rc Ra ¬ Rb + Rc
load Ra Rb Ra ¬ mem[Rb]
store Ra Rb mem[Rb] ¬ Ra
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