MIPS “la” pseudo instruction

Question

can someone explain to me why MARS translate the pseudo instruction into two instructions: lui & ori? Is there cases when the translation is to only one instruction?

Answer 1

This simple program

.data               
msg: .asciiz "This is a string"
.text
   la  $a0, msg    # pseudo-instruction to load the address of the label str

Translates to the instructions

 Address    Code        Basic                     Source

0x00400000  0x3c011001  lui $1,0x00001001     4       la  $a0, msg    # pseudo-instruction to load the address of the label str
0x00400004  0x34240000  ori $4,$1,0x00000000  

And its always these two instructions AFAIK. The lui instruction makes the immediate value shifted left 16 bits and stored in the register. The lower 16 bits are zeroes.

This way, you can load 32 bit address (in mips32) with 32 bit instructions.