问题描述
VAX CPU具有可更新的微码功能。用户可以将微代码重新上传到VAX机器，然后CPU的行为就不会像VAX CPU一样，例如作为PDP11 CPU。
VAX CPUs has a updateable microcode feature. User can reupload microcode to VAX machine and then CPU will behave as not VAX CPU, but e.g. as PDP11 CPU.
可以使用VAX微代码模拟哪些CPU？它可以运行来自6502、8080、8086的机器代码吗？
What CPUs can be simulated with VAX microcode? Can it run machine code from a 6502, 8080, 8086?
我的问题不是关于软件模拟器（例如SIMH或qemu；有一个Alpha软件模拟器，通过某些微代码更新来加快了速度），而是关于硬件微代码。
My question is not about software simulators (like SIMH or qemu; there was an Alpha software simulator, speeded up by some microcode update), but about hardware microcode; both about theoretical possibility of making such microcode and practical microcode versions.
推荐答案
我相信基于RAM的可变VAX微代码存储区域（ WCS（可写的控制存储）不允许有足够的空间来定义整个新指令集。
I believe RAM-based changeable VAX microcode storage area ("WCS", writeable control store) did not allow enough space for a whole new instruction set to be defined.
充其量，它最多允许添加一些新指令或有问题的指令的错误修复-今天是X86 CPU的微代码更新。
At best it would allow a few new instructions to be added or for a bug fix for a problematic instruction - a la today's microcode updates for X86 CPUs.
要获得完整的指令集，可能需要新的微代码ROM / PLA器件，并且很可能需要特殊的门阵列，尤其是后期的VAX 8xxx CPU。
For a complete instruction set it would likely require new microcode ROM/PLA devices and that could well require special gate arrays, esp for the later VAX 8xxx CPUs.
由于（除了台式机VAXen，后来是MIPS RISC机器），它们是在IT环境中使用的昂贵的大型铁机器-我看不到它们曾经是在这样的梦想家环境中以极低的水平使用。当然，由于该设备是作为 VAX而不是通用CPU仿真器出售的，因此在计算可靠性方面将没有有用的保证能力。
Since (besides desktop VAXen which were later MIPS RISC boxes) these were expensive 'big iron' machines used in an IT context - I don't see them ever having been used in such a 'dreamer' context at this ultralow level. Certainly there would not be useful warranty ability on reliability of computation as the device was sold "as a VAX", and not a "general purpose CPU emulator".
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