or    $25,$0,$12     # move s into target register

$25 <-- ($0  OR $12)     or  $25,$0,$12    # wrong
$25 <-- ($25 OR $12)     or  $25,$25,$12   # right

Destination Register and Const

ori     $8,$9,0x004A
31....26	25...21	20...16	15...................0
opcode	s	d	unsigned const
0xD	0x9	0x8	0x004A

The same method is used with the bit pattern that designates the destination register. The const part is in register #13, and it is in the correct field. It needs to be copied into $25 The complete program follows. The comments on each line have been improved. Comments should describe the work done in solving the problem. They should not describe the instruction. Of course, when these notes explain an instruction the comment will often do the same. But that is a poor style, in general.

The following can be copied into the clipboard and pasted into your program editor, then saved to a file and run with SPIM.

## Program to assemble the instruction ori  $8,$9,0x004A
##
        .text
        .globl  main

main:
or    $25,$0,$0        # clear $25
ori   $11,$0,0xD       # opcode
ori   $12,$0,0x9       # operand $s
ori   $13,$0,0x8       # dest. $d
ori   $14,$0,0x004A    # immediate operand

sll   $11,$11,26       # shift opcode into position
or    $25,$25,$11      # or it into the instruction

sll   $12,$12,21       # shift operand $s into position
or    $25,$25,$12      # or it into the instruction

sll   $13,$13,16       # shift dest $d into position
or    $25,$25,$13      # or it into the instruction

or    $25,$25,$14      # or const into the instruction

## end of file