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两级地址编码是对微程序设计中的一级地址编码系统的改进,使其不仅具有以微指令字为单位进行地址编码的规则性,而且具有以微指令字节(即ROM模体的宽度)为单位进行地址编码的灵活性,从而使微指令的编码效率得到最大限度地提高。 通过两级地址编码选出的微指令,实际上是以微指令字节为单位的一组空间序列,与通常的微指令字序列相比,既可直接沿用机器原来的硬件结构,又可以提高运算速度和降低硬件成本。因此,在机器指令具有多重操作码,且允许在一个微指令周期中实现其全部功能等场合下,两级地址编码的方法可作为实现全面微程序控制的一种可取的方法。
The two-level address coding is an improvement on the first-level address coding system in the micro-program design so that it not only has the regularity of address coding in the micro-instruction word, but also has the characteristic of micro-instruction byte ) As a unit of address coding flexibility, so that micro-instruction coding efficiency is maximized. The microinstruction selected by the two-level address coding is actually a set of space sequences in microinstruction bytes. Compared with the usual microinstruction word sequences, the microinstruction can not only directly use the original hardware structure of the machine, but also improve Computing speed and reduce hardware costs. Thus, a two-level address encoding approach may be a desirable way to achieve full micro-program control where machine instructions have multiple opcodes and allow their full functionality to be achieved in one microinstruction cycle.