Difference between word addressable and byte addressable

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

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

  • 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.

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(memory address register)
  • MDR(memory data register)
  • PC(program counter register)
  • MBR(memory buffer register)

RAM has two kinds of memory ports:

  1. 32-bits for data/addresses
  2. 8-bit for OPCODE.

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.

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.

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 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.

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.

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

This image should make it easy to understand:
http://i.stack.imgur.com/rpB7N.png

Simply put,

• In the byte addressing scheme, the first word starts at address 0, and
the second word starts at address 4.

• In the word addressing scheme, all bytes of the first word are located
in address 0, and all bytes of the second word are located in address 1.

The advantage of byte-addressability are clear when we consider applications that process data one byte at a time. Access of a single byte in a byte-addressable system requires only the issuing of a single address. In a 16–bit word addressable system, it is necessary first to compute the address of the word containing the byte, fetch that word, and then extract the byte from the two-byte word. Although the processes for byte extraction are well understood, they are less efficient than directly accessing the byte. For this reason, many modern machines are byte addressable.

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