A bit string of length 32 is called a word . Note that some authors refer to the 16 bit length as a word, and the 32 bit length as a long-word, but we'll stick to the book's terminology. This is because one can build the registers in small silicon area and so there is a shorter distance for the electronic signals to travel, resulting in a shorter clock cycle being possible. Work through the example on page 98 . Since the operands of arithmetic instructions can only be registers, this is what your assembly code would look like.
Registers can only store single data items.
The MIPS architecture accesses upto memory words . This introduces another problem since the data items have to be transferred between the memory and registers for the processors to execute arithmetic operations on them. Work through the example on page 99, as well as Figure 3.2 . The store instruction is for moving data from a register to a specified array element's memory location. Work through the example on page 101 .