字可寻址和字节可寻址的区别 [英] Difference between word addressable and byte addressable

问题描述

有人能解释一下 Word 和 Byte 可寻址之间的区别吗?它与内存大小等有什么关系?

Can someone explain what's the different between Word and Byte addressable? How is it related to memory size etc.?

推荐答案

• 一个字节是一个内存单元strong> 用于存储
• 内存芯片充满了这样的字节.
• A byte is a memory unit for storage
• A memory chip is full of such bytes.

内存单元可寻址.这是我们可以使用内存的唯一方法.

Memory units are addressable. That is the only way we can use memory.

实际上，内存只是字节可寻址.意思是:

In reality, memory is only byte addressable. It means:

• 一个二进制地址总是只指向一个字节.
• 一个词就是一组 字节 –2、4、8 取决于数据总线的CPU大小.
• A binary address always points to a single byte only.
• A word is just a group of bytes – 2, 4, 8 depending upon the data bus size of the CPU.

了解内存操作强>完全，您必须熟悉CPU的各种寄存器和RAM的内存端口.我假设您知道它们的含义:

To understand the memory operation fully, you must be familiar with the various registers of the CPU and the memory ports of the RAM. I assume you know their meaning:

• MAR(内存地址寄存器)
• MDR(内存数据寄存器)
• PC(程序计数器寄存器)
• MBR(内存缓冲寄存器)
• MAR(memory address register)
• MDR(memory data register)
• PC(program counter register)
• MBR(memory buffer register)

RAM有两种内存端口:

1. 32-bits 用于数据/地址
2. 8-bit 用于 OPCODE.
1. 32-bits for data/addresses
2. 8-bit for OPCODE.

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假设 CPU 想要读取 a word(比如 4 bytes)从地址 xyz 开始.CPU将地址放在MAR上，发送内存读取信号给内存控制器芯片.在接收到地址和读取信号后，内存控制器会将数据总线连接到32位 port 和 4 bytes 从 address 开始的 xyz 会流出 port 到 MDR.

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Suppose CPU wants to read a word (say 4 bytes) from the address xyz onwards. CPU would put the address on the MAR, sends a memory read signal to the memory controller chip. On receiving the address and read signal, memory controller would connect the data bus to 32-bit port and 4 bytes starting from the address xyz would flow out of the port to the MDR.

如果CPU想要获取下一条指令，它会将地址放入PC寄存器并向内存控制器发送获取信号.在接收到地址并获取信号后，内存控制器会将数据总线连接到8位端口和位于地址的单个字节长 操作码接收到的数据将从 RAM 流入 CPU 的 MDR.

If the CPU wants to fetch the next instruction, it would put the address onto the PC register and sends a fetch signal to the memory controller. On receiving the address and fetch signal, memory controller would connect the data bus to 8-bit port and a single byte long opcode located at the address received would flow out of the RAM into the CPU's MDR.

这就是当我们说某个寄存器是内存可寻址或字节可寻址时的意思.现在，当您在 MAR 上的 binary 中输入 decimal 2 并打算读取 word 时会发生什么> 2，不是 (byte 没有 2)?

So that is what it means when we say a certain register is memory addressable or byte addressable. Now what will happen when you put, say decimal 2 in binary on the MAR with an intention to read the word 2, not (byte no 2)?

Word 没有 2 表示 bytes 4, 5, 6, 7 用于 32-bit 机器.在真实物理内存中，仅字节可寻址.所以有一个技巧来处理字寻址.

Word no 2 means bytes 4, 5, 6, 7 for 32-bit machine. In real physical memory is byte addressable only. So there is a trick to handle word addressing.

当MAR被放置在地址总线上时，它的32位不会映射到32地址行(分别为0-31).相反，MAR bit 0 连接到地址总线 line 2, MAR bit 1 连接到地址总线 line 3 等等.MAR的高位2 bits被丢弃，因为只有2^32以上的字地址需要它们这对我们的 32 位 机器是合法的.
使用这种映射，当MAR为1时，address 4放在总线上，当MAR是2，address 8放在总线等等.

When MAR is placed on the address bus, its 32-bits do not map onto the 32 address lines(0-31 respectively). Instead, MAR bit 0 is wired to address bus line 2, MAR bit 1 is wired to address bus line 3 and so on. The upper 2 bits of MAR are discarded since they are only needed for word addresses above 2^32 none of which are legal for our 32 bit machine.
Using this mapping, when MAR is 1, address 4 is put on the bus, when MAR is 2, address 8 is put on the bus and so forth.

一开始有点难理解.我从Andrew Tanenbaums 的结构化计算机组织.

It is a bit difficult in the beginning to understand. I learnt it from Andrew Tanenbaums's structured computer organisation.

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