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The RL78 family is an accumulator-based register-bank CISC instruction set architecture (ISA). [2] Although it has eight 8-bit registers or four 16-bit register pairs, essentially all arithmetic operations are performed on a single accumulator (the A register or AX register pair).
Two additions move it away from the counter machine, however. The first enhances the machine with the convenience of indirect addressing; the second moves the model toward the more conventional accumulator-based computer with the addition of one or more auxiliary (dedicated) registers, the most common of which is called "the accumulator".
In a computer's central processing unit (CPU), the accumulator is a register in which intermediate arithmetic logic unit results are stored.. Without a register like an accumulator, it would be necessary to write the result of each calculation (addition, multiplication, shift, etc.) to cache or main memory, perhaps only to be read right back again for use in the next operation.
On an accumulator-based architecture machine, this may be a dedicated register. On a machine with multiple general-purpose registers , it may be a register that is reserved by convention, such as on the IBM System/360 through z/Architecture architecture and RISC architectures, or it may be a register that procedure call and return instructions ...
Extensions can include, for example, a memory management unit (MMU), a fast multiplier–accumulator, a Universal Serial Bus host, a Viterbi path decoder, or a user's proprietary RTL functions. The processors are synthesizable and can be implemented in any foundry or process, and are supported by a complete suite of development tools. [8]
The 6100 is a 12-bit CPU that closely emulates the PDP-8 (See PDP-8 for a more complete discussion). It has three primary registers: PC (program counter), 12-bit AC (accumulator), and MQ (Multiplier Quotient). All two-operand instructions read the AC and MQ and write back to the AC.
As computer designs have grown more complex, the central importance of a single word size to an architecture has decreased. Although more capable hardware can use a wider variety of sizes of data, market forces exert pressure to maintain backward compatibility while extending processor capability. As a result, what might have been the central ...
The 8086 [3] (also called iAPX 86) [4] is a 16-bit microprocessor chip designed by Intel between early 1976 and June 8, 1978, when it was released. The Intel 8088, released July 1, 1979, [5] is a slightly modified chip with an external 8-bit data bus (allowing the use of cheaper and fewer supporting ICs), [note 1] and is notable as the processor used in the original IBM PC design.