问题
Given that the word size of a CPU allows it to address every single byte in the memory.
And given that via PAE CPUs can even use more bits than its word size for addressing.
What is the reason that a CPU cannot read an unaligned word in one step?
For example, in a 32-bit machine you can read the 4-byte chunk starting at position 0, but you cannot read the one starting at position 1 (you can but it needs several steps).
Why can CPUs not do that?
回答1:
The problem is not with the ability of the CPU to address any single byte in the memory. But it is the memory that has not the same granularity. Like Oli said, this is very architecture-specific, but memory chips are often addressed by their data bus wideness. Meaning that a given address represents a full "word" of their data bus.
Let's take the example of a 32 bits CPU, with a 32 bits-wide data bus connected to a memory device. When the CPU wants to access to the word at address 0x00000000, it really wants to access to the bytes 0, 1, 2 and 3. For the memory chip however, this is represented by the single address 0x00000000.
Now when the CPU wants to access to the word at address 0x00000001, it really wants to access to the bytes 1, 2, 3 and 4. For the memory chips however, this is represented by a piece of the word at address 0x00000000 and a piece of the word at address 0x00000001.
Hence the need for two bus cycles.
EDIT: Adding some wiring illustration
To illustrate this, here are both addressing scheme opposed:
Notice the bit shift in the addresses of the RAM chip.
Addresses will look like this:
// From the RAM point of view
@0x00000000: Bytes 0x00000000 to 0x00000003
@0x00000001: Bytes 0x00000004 to 0x00000007
To access to the dword @0x00000001, you can see that no direct addressing is possible. You need to ask the RAM chip for both dwords at addresses 0x00000000 and 0x00000001.
回答2:
The simple answer is they can't because they are designed not to.
The main reason that they are designed this way is for performance and scalability. We would lose way too many incredibly important features to support this.
A simple analogy, the humble Shipping Container. Before the days of the shipping container, freight of many different shapes and sizes were packed as efficiently as possible into the hulls of ships. Because of the infinitely variable sizes of the freight, ranging from crates, to bags of coffee, to bales of hay and cotton, the capacity of these ships was horribly and inefficiently utilized.
The shipping container changed all of that, now if you want to ship something internationally it must be in a standard-sized shipping container. It isn't that you can't just ship your bag of cat food to your friend in Hong Kong on a container ship, it's that it is just so incredibly inefficient to do that it just isn't done.
You want to get that cat food to your friend quickly, without buying a whole shipping container? Well, you can pay an express shipping company like FedEx to fly it over on a 747, but you sure as hell are going to pay for that ability.
来源：https://stackoverflow.com/questions/24228252/why-is-it-not-possible-to-read-an-unaligned-word-in-one-step