lecture29-apr9

# lecture29-apr9 - Lecture 29 Announcements Monday lectures...

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4/9/10 1 Lecture 29 Announcements Monday lectures cancelled Exam 2 - back High = 99 Low = 17.5 Median = 74.5 Average = 73.5 Assignment 6 – Q&A Last time we learned how to: Shift bit values left and right Set, clear, toggle and test Single bits within a value Bit fields within a value Topics of the day More Low level programming Bit structure operations Examples – parity checker Swap the bytes example unsigned short int i , high_byte, low_byte; scanf (“%hu”, &i); high_byte = i << 8; low_byte = i >> 8; i = high_byte | low_byte; Other ways??? Rotate i by 8 bits (left or right) ?? unsigned short int rotate(unsigned short int value, int numbits, char direction); How do you swap the upper and lower bytes of a 16 bit value ? Bit-Fields in Structures A structure may contain bit-fields—members whose size is specified as some number of bits. E.g. some OS’s stores file dates as struct file_date { unsigned int day: 5; unsigned int month: 4; unsigned int year: 7; /* last 2 digits of year – e.g. 2010 -> 10 */ }; The order in which bit-fields are allocated within a storage unit depends on the implementation (e.g.Visual C/C++ allocates right to left). Basic storage units are typically bytes and words A bit-field can be used just like any other element of a structure:

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