Zero Extension

Look again at the instruction:

address         machine code      assembly code

0x00400000      0x34080002        ori $8,$0,0x2

Sixteen bits of immediate operand,

0000 0000 0000 0010

are to be bitwise ORed with the thirty-two bits of register zero,

0000 0000 0000 0000 0000 0000 0000 0000

This would not ordinarily be possible because the operands are different lengths. However, MIPS zero extends the sixteen-bit operand so the operands are the same length. Sometimes this is called padding with zeros on the left.

   zero extension
---- ---- ---- ----
0000 0000 0000 0000 0000 0000 0000 0010   -- zero extended immediate operand
0000 0000 0000 0000 0000 0000 0000 0000   -- data in register $0
---------------------------------------
0000 0000 0000 0000 0000 0000 0000 0010   -- result, put in register $8

An OR operation is done in each column. The 32-bit result is placed in register $8 .