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Bit-level parallelism is a form of parallel computing based on increasing processor word size, depending on very-large-scale integration (VLSI) technology. Enhancements in computers designs were done by increasing bit-level parallelism
Increasing the word size reduces the number of instructions the processor must execute in order to perform an operation on variables whose sizes are greater than the length of the word. (For example, consider a case where an 8-bit processor must add two 16-bit integers. The processor must first add the 8 lower-order bits from each integer, then add the 8 higher-order bits, requiring two instructions to complete a single operation. A 16-bit processor would be able to complete the operation with single instruction)
Historically, 4-bit microprocessors were replaced with 8-bit, then 16-bit, then 32-bit microprocessors, then 64-bit microprocessors. 32-bit processors has been a standard in general purpose computing for about 20 years, but now 64-bit processors are taking the lead.
- David E. Culler, Jaswinder Pal Singh, Anoop Gupta. Parallel Computer Architecture - A Hardware/Software Approach. Morgan Kaufmann Publishers, 1999. ISBN 1558603433, pg 15