问题描述
是由地址总线的大小确定的吗?如果是，那么8086是20位处理器吗?如果没有，为处理器分配8位，16位，32位之类的位的标准是什么?
Is it determined by size of the address buss; if yes then was 8086 a 20-bit processor? If no what is criteria for assigning a bit number like 8-bit, 16-bit, 32-bit to processor?
推荐答案
定义不明确.正如xtofl所指出的，广义上讲，它是原子计算单位的大小(在早期计算机中，这并不总是与寄存器"同义).因此，PDP-10是一台36位计算机，一个8080是8位，而IBM 360或Intel 80386是"32位".
It's not well defined. Broadly, as xtofl points out, it's the size of the atomic unit of computation (in early computers, this wasn't always synonymous with "register"). So the PDP-10 was a 36 bit machine, a 8080 was 8 bit, and a IBM 360 or Intel 80386 is "32 bits".
但是也有例外.摩托罗拉68000和68010 CPU实现了32位寄存器组，但是在主要是16位内部体系结构的基础上通过微代码实现了该功能.当时，它们通常以"16位" CPU的形式销售.
But there are exceptions. The Motorola 68000 and 68010 CPUs implemented a 32 bit register set, but did it via microcode on top of a mostly 16 bit internal architecture. They were usually marketed as "16 bit" CPUs at the time.
地址总线的大小几乎从来不是决定因素.例如，所有成功的"8位" CPU都实现了16位寻址(通常通过奇怪的手段来弥补缺少单个地址寄存器，参见6502的间接寻址模式或Z80的H/L寄存器).正如您所提到的，8086使用其段寄存器寻址来获得20条地址线(80286将这一技巧扩展到了24位物理地址).在另一个方向上，许多"32位" CPU的地址总线更小，以节省逻辑，而这些逻辑将不会在内存不会超过几兆字节的机器上使用:即使将68000寻址到24位，即使指针本身是32.同样，现代的64位CPU普遍实现的物理地址少于64位.
The size of the address bus is almost never the defining factor. All successful "8 bit" CPUs implemented 16 bit addressing, for example (often via odd hacks to make up for the lack of a single address register, c.f. 6502's indirect addressing modes or the Z80's H/L registers). And the 8086, as you mention, used its segment register addressing to get 20 address lines to work (the 80286 extended this trick to 24 bits of physical address). And in the other direction, many "32 bit" CPUs had smaller address buses to save logic that wouldn't be used on a machine that would never have more than a few megabytes of memory: the 68000 limited addressing to 24 bits, even though the pointers themselves were 32. Likewise modern 64 bit CPUs universally implement less than 64 bits of physical address.
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