Why the 68k CPU : Zerro Below

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The 68K CPU:

Why is the 68K series of CPU so well liked? There may be many answers to this question, though my answers are presented herein.

The 68K series of CPU's may be a CISC design, though the way the ISA and Register usage makes taking advantage of some RISC like technology much simpler than with most CISC designs.

The things that make the 68K Series well liked in my view include:

  • Well Thought Register Usage : The use of two primary register sets simplifies life in implementing a decent number of user registers while keeping good abilities. The choices on what registers are accessed with special instructions (Status Register and PC) was also well thought out to keep everything simple.
  • Well Thought out ISA : The influence of earlier Mini Computers can be seen in the 68K ISA, though it is simpler and more capable than those that came before.
  • Versitile Addressing Modes : The versitility and simplicity of the addressing modes provided helps greatly when working with data in memory.
  • Easy Assembly Lang : Almost anyone that has programmed in 68K Assembly already knows this to be true.
  • Easy Pipeline ISA : The nature of the ISA makes implementing it in a pipelined archetecture fairly straight forward.
  • ISA That Makes Multiple Issue Design Easy : The ISA and register set is implemented such that implementing a multiple issue parallel pipeline for execution is fairly easy to accomplish. This allows a simple predictable means of exceeding 1 instruction per clock.
  • Understandable : The ISA is quite easy to understand, more so than most others (possibly excepting the ARMv6 and earlier).
  • The Computers that Use it : Some of the greatest computers are based around this series of CPU's
  • Binary Compatibility : User Mode code is backwards compatible, able to run original 68000 applications all the way up to the 68080 (including the 68060, 68040, 68030, 68020, and 68010).
  • Exception Handling : When exceptions accure everything needed is on the stack, thus elliminating the need for contaminating data cache with Instructions on systems with a harvard cache archetecture.

Continuing Assembly Lang Usability:

The 680x0 series of CPU's continues to be updated by hobbyist, and others, and continues to be programmed largely in Assembly Language. To my knowledge this is the only long term series of CPU that it is still considered good practice to program in Assembly Language, at least among those CPU's still updated.


WIP : Work In Progress.

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