Exercises on Bitwise operations in C In these problems, we use the term signal s

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Exercises on Bitwise operations in C In these problems, we use the term signal set rather than bit set (they are the same thing), and the term signal rather than bit (they are also the same thing). The reason for this is that one common application of Ritmasks and bitwise operations is to modify a set of signals. Rather than talking about bits 0 7, we talk about signals 0 7. If the bit in bit position 4 is set to 1, this means that signal 4 is in the signal set. If this bit is set to 0, then signal 4 is not in the signal set The types of operations on signal sets is exactly as we have been discussing. This includes determining if certain signals are in the current set (i.e., have a value of 1, adding signals to the current set, deleting signals from the current set, and determining if exactly one of a set of signals is in the current set. Thus far, we have represented the signal mask as a variable of type unsigned character and created another mask, which we have termed a Ritroask, which is also of type unsigned character. We have then set the Ritroask to a given value and applied some bitwise operation between the signal mask and bitmask that accomplishes the requested operation. For example, if we want to determine if a given signal is in the current set of signals, we would set the bitmask such that there is a 1 in the bit position corresponding to the signal of interest, set all other bit positions to 0, and perform a bitwise AND operation. If the resulting value is greater than 0, the signal of interest is in the set In some problems, you are provided with the signal(s) of interest and asked to provide the bitmask and bitwise operation (s) that would fulfill the requested operation. Thus, if you were given signal 1, and the request is to determine if signal 1 is in the current signal set, the correct response would Example Signal Bitmask: Ox 02 Bit Op: AND Other problems ask you to write a function that can fulfill a given request when the signals are unknown Problems 1 5 are asking you to determine if the given signal is in the current signal set

Explanation / Answer

Even though there are separate addressing schemes and optimized opcodes for accessing the register file and the first 64 I/O registers, all can also be addressed and manipulated as if they were in SRAM.

The very smallest of the tinyAVR variants use a reduced architecture with only 16 registers (r0 through r15 are omitted) which are not addressable as memory locations. I/O memory begins at address 000016, followed by SRAM. In addition, these devices have slight deviations from the standard AVR instruction set. Most notably, the direct load/store instructions (LDS/STS) have been reduced from 2 words (32 bits) to 1 word (16 bits), limiting the total direct addressable memory (the sum of both I/O and SRAM) to 128 bytes. Conversely, the indirect load instruction's (LD) 16-bit address space is expanded to also include non-volatile memory such as Flash and configuration bits; therefore, the LPM instruction is unnecessary and omitted.

In the XMEGA variant, the working register file is not mapped into the data address space; as such, it is not possible to treat any of the XMEGA's working registers as though they were SRAM. Instead, the I/O registers are mapped into the data address space starting at the very beginning of the address space. Additionally, the amount of data address space dedicated to I/O registers has grown substantially to 4096 bytes (000016–0FFF16). As with previous generations, however, the fast I/O manipulation instructions can only reach the first 64 I/O register locations (the first 32 locations for bitwise instructions).

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