The point of this comparison should be clear. There is no universally acceptedphilosophy concerning the best way to organize processor registers [TOON81]. Aswith overall instruction set design and so many other processor design issues, it isstill a matter of judgment and taste.A second instructive point concerning register organization design is illustratedigure 12.3c. This figure shows the user-visible register organization for the Intel80386 [ELAY85], which is a 32-bit microprocessor designed as an extension of the8086.1The 80386 uses 32-bit registers. However, to provide upward compatibility forprograms written on the earlier machine, the 80386 retains the original register organizationembedded in the new organization.Given this design constraint, the architectsof the 32-bit processors had limited flexibility in designing the register organization.
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