Jump Instructions

Jump instructions use direct mode addressing

If you look at "j place", it looks like a branch without the registers

However, where place would be PC-relative mode in the branch, it is direct mode in the jump

.text

        add   $11  $12  $13   # 0x00400000 is the address of this instruction
        j     place           # 0x00400004
        add   $11  $12  $13   # 0x00400008
        add   $11  $12  $13   # 0x0040000C
        add   $11  $12  $13   # 0x00400010
        add   $11  $12  $13   # 0x00400014
place:  add   $11  $12  $13   # 0x00400018
      

• Here's some code with the addresses that it assembles to
• Here's what it looks like boxed up
• The times-4 trick is in play here so the value in the address box is 0x00400018 ÷ 4

Can we convince ourselves that 0x00400018 ÷ 4 really is 0x00100006 ?

• Write 0x00400018 out in binary:
• 0000 0000 0100 0000 0000 0000 0001 1000
• Now shift it 2 places to the right:
• 0000 0000 0100 0000 0000 0000 0001 10
• regrouped, that looks like this:
• 0000 0000 0001 0000 0000 0000 0000 0110
• There you go
• Or copy the code, paste it into MARS, and assemble it
• The jump instruction will assemble to 0x08100006
• While you're in MARS, comment out the very first instruction and assemble it again
• Now the jump instruction will assemble to 0x08100005
• Notice that the distance between the jump and its target did not change
• The value in the address field of the jump changed because the absolute address of the target changed from 0x00400018 to 0x00400014
• Here's another way to see that 0x00400014 ÷ 4 is 0x00100005