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CS485G Spring 2015 31

(b) 2 bytes (short): address ends with 0

(d) 8 bytes (double): address ends with 000

(Linux/x86 compilers

choose 00

(e) 12 bytes (long double): Linux/x86 chooses 00

4. The x86 64 is stricter

(a) 8 bytes (double, void *): address ends with 000

(b) 16 bytes (long double): Linux/x86 chooses 000

5. On some machines alignment is mandatory.

6. Motivation

(a) The CPU accesses memory in chunks of 4 or 8 bytes (architecture-

dependent).

(b) It is inefﬁcient to access a datum that crosses chunk boundaries.

ically every 4KB).

7. The compiler can add padding to accomplish this requirement:

c x

pad x+1 (pad of 3 bytes)

y x+4

pad x+12 (pad of 4 bytes)

v x+16

end x+24

8. The compiler can also re-order the ﬁelds (biggest ﬁrst, for instance)

to reduce the amount of padding.

9. The entire struct needs to be padded to a multiple of the largest

primitive data within the struct, so that arrays of such structs

work.

32 Midterm test 3/2/2015

33 Discussion of midterm test

1. Lecture 17, 3/4/2015

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Cirrus Logic CS485 Bedienungsanleitung Seite 31