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assemblyx86type-conversionattinteger-promotion

IA32 Assembly code for type casting from signed/unsigned char to unsigned/signed int

This is from Computer Systems, a Programmer's perspective (2nd edition)

In question 3.4, the student is asked to determine the assembly instruction needed to cast from a source data type to a destination that is referred to by a pointer

This answer given at the back of the chapter states that we need to sign extend the source type (left column) to the destination type (middle column).

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There isn't much information to pull a pattern from this question. Is it always the case that the type of extension is determined by the source type and not the destination type, as in this example?

Solution

  • The answer is in your book.

    When performing a cast that involves both a size change and a change of “signedness” in C, the operation should change the size first. (Section 2.2.6).

    -- CSAPP 2e, Practice Problem 3.4

    So these two conversions work like this:

    char --> int --> unsigned int
unsigned char --> unsigned int --> int