When dealing with memory, MIPS uses a flat memory model. In reality memory that exists in the hardware of the computer is quite complex, and the memory is sliced and diced, and sent out to many different areas in memory. This is a result of how hardware actually stores data. The actual implementation in hardware of memory uses virtual memory to store programs larger than the actual amount of memory on the computer, and layers of cache to make that memory appear faster.
All of these details of how the memory is implemented are hidden by the Operating System (OS), and are not apparent to the programmer. To a MIPS programmer, memory appears to be flat; there is no structure to it. Memory consists of one byte (8 bits) stored after another, and all bytes are equal. The MIPS programmer sees a memory where the bytes are stored as one big array, and the index to the array being the byte address. The memory is addressable to each byte, and thus called byte addressable. This was shown in Figure 2.3, which is repeated here for convenience of reference.
The bytes in MIPS are organized in groups of: 1) a single byte; 2) a group of 2 bytes, called a half-word; 3) a group of 4 bytes, called a word17; and 4) a group of 8 bytes, called a double word. These groupings are not random in memory. All groupings start at 0x00000000 and then occur at regular intervals. Thus memory half words would start at addresses 0x00000002, 0x00000004, 0x00000008, 0x0000000a, 0x0000000c, and continue in that manner. Memory words would start at addresses 0x00000000, 0x00000004, 0x00000008, 0x0000000c, 0x00000010, 0x00000014, and likewise continue. Memory double words would start at addresses 0x00000000, 0x00000008, 0x00000010, 0x00000018, and continue. Where the memory groups start is called a boundary. You cannot address a group of data except at the boundary for that type. So if you want to load a word of memory, you cannot specify the address 0x15fc8232, as it is not on a word boundary (it is however a byte boundary and a half word boundary). When discussing data, a word of memory is 4 bytes large (32 bits), but it is also located on a word boundary. If 32 bits are not aligned on a word boundary, it is incorrect to refer to it as a word 18.
17 All machines define their own word size. Older computer used a 16 bit word, and some modern computers use a 64 bit word. Do not let this confuse you. In MIPS all words are 32 bits.
18 To save space, some languages, such as C Ada, and PLI, allow programmers to use unaligned data structures, where some data might not fall on the correct boundaries. Unaligned structures were used when space was a premium in computers, but these structures are slow to process. Most modern languages do not allow unaligned data, but when dealing with legacy systems programmers might have to deal with it.