Exercise 5.8 · 习题

Exercise 5.8: The following register-machine code
is ambiguous, because the label here is defined more than once:

start
(goto (label here))
here
(assign a (const 3))
(goto (label there))
here
(assign a (const 4))
(goto (label there))
there

With the simulator as written, what will the contents of register a be

when control reaches there? Modify the extract-labels procedure

so that the assembler will signal an error if the same label name is used to

indicate two different locations.

练习 5.8：下面的寄存器机器代码存在歧义，因为标号 here 被定义了两次：

start
(goto (label here))
here
(assign a (const 3))
(goto (label there))
here
(assign a (const 4))
(goto (label there))
there

按照模拟器目前的实现，当控制到达 there 时，寄存器 a 的内容是什么？修改 extract-labels 过程，使得汇编器在同一标号名被用于指示两个不同位置时发出错误信号。