Is there any difference between word size and size of internal register of CPU ,since "Word size" refers to the number of bits processed by a computer's CPU in one go ,so does this imply that word size is architecture dependent and ALU register's size is same as word size ?
The term "word size" is defined in different ways by different sources, but almost everybody agrees that it is architecture dependent.
In the definition you quote: "number of bits processed by a computer's CPU in one go" it is not clear what "processed" refers to.
(Sorry about using ancient example, but I have not really looked at hardware specs for ages)
For example: The 486DX processor can add 32 bit integers for a 32-bit result (plus carry). It can multiply 32 bit integers for a 64-bit result. It can add or multiply 80-bit floating-point numbers for 80-bit results.
It has 32-bit integer registers and 80-bit floating-point registers. The 64-bit int multiplication stores in result in two registers.
I won't even start on the various physical and virtual memory address sizes.
The integer multiply instruction 32x32 -> 64 is an oddity, most integer instructions can only process 32 bits.
Now, what is the word size?
It all depends on your definition.
One. Yes the size of one word is the size of a single register present in a register file.
Your memory storage may store the data in 4,8,16,32,64 bits of data, but they ultimately have to be loaded into a register before the CPU can use it for performing operations. Here MIPS reads the data from the memory in sets of the size which can be accomodated in a single register file.
e.g: If your register has 32 bits capacity and your memory stores the data in 8bit values, then MIPS will read 4 memory locations and fit it into a single register and this set is called as a "word" or "data word".
In short, one word is the data which can be fit into a single register in the register file of your computer.
Hope this helps!!