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32 lines
1.1 KiB
32 lines
1.1 KiB
2 years ago
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Exercise 5.11: When we introduced save
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and restore in 5.1.4, we didn’t specify what would happen
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if you tried to restore a register that was not the last one saved, as in the
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sequence
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(save y)
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(save x)
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(restore y)
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There are several reasonable possibilities for the meaning of restore:
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(restore y) puts into y the last value saved on the stack,
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regardless of what register that value came from. This is the way our
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simulator behaves. Show how to take advantage of this behavior to eliminate
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one instruction from the Fibonacci machine of 5.1.4 (Figure 5.12).
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(restore y) puts into y the last value saved on the stack, but
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only if that value was saved from y; otherwise, it signals an error.
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Modify the simulator to behave this way. You will have to change save
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to put the register name on the stack along with the value.
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(restore y) puts into y the last value saved from y
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regardless of what other registers were saved after y and not restored.
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Modify the simulator to behave this way. You will have to associate a separate
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stack with each register. You should make the initialize-stack
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operation initialize all the register stacks.
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