Basic Registers In Computer Processor Essay

989 words - 4 pages

A, special, high speed storage area within the CPU. All data must be represented in a register before it can be processed. For example, if two numbers are to be multiplied, both numbers must be in registers, and the result is also placed in a register. (The register can contain the address of a memory location where data is stored rather than the actual data itself.)AccumulatorThere is a central register in every processor called the accumulator. Traditionally all mathematical and logical operations are performed on the accumulator. The word size of a processor is defined by the width of its accumulator. A 32bit processor has an accumulator of 32 bits.Pointer, Index, or Base RegisterThe name varies from manufacturer to manufacturer, but the basic distinguishing property is that it does not hold data but holds the address of data. The rationale can be understood by examining a "for" loop in a higher level language, zeroing elements in an array of ten elements located in consecutive memory cells. The location to be zeroed changes with every iteration. That is the address where the operation is performed is changing. Index register is used in such a situation to hold the address of the current array location. Now the value in the index register cannot be treated as data, but it is the address of data. In general whenever we need access to a memory location whose address is not known until runtime we need an index register. Without this register we would have needed to explicitly code each iteration separately.In newer architectures the distinction between accumulator and index registers has become vague. They have general registers which are more versatile and can do both functions. They do have some specialized behaviors but basic operations can be done on all general registers.Flags Register or Program Status WordThis is a special register in every architecture called the flags register or the program status word. Like the accumulator it is an 8, 16, or 32 bits register but unlike the accumulator it is meaningless as a unit, rather the individual bits carry different meanings. The bits of the accumulator work in parallel as a unit and each bit mean the same thing. The bits of the flags register work independently and individually, and combined its value is meaningless.An example of a bit commonly present in the flags register is the carry flag. The carry can be contained in a single bit as in binary arithmetic the carry can only be zero or one. If a 16bit number is added to a 16bit accumulator, and the result is of 17 bits the 17th bit is placed in the carry bit of the flags register. Without this 17th bit the answer is incorrect. More examples of flags will be discussed when dealing with the Intel specific register set.Program Counter or Instruction PointerEverything must translate into a binary number for our dumb processor to understand it, be it an operand or...

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