Symbolic Address

How do you know what the high-order four bits of the PC are? Well, since you have the address of the instruction following the jump instruction, you know that its address is in the PC. So the high order four bits come from that address.

Usually the high order four bits of the address of the jump instruction and of the one following it are the same. But in very rare instances they might be different (when adding four to the jump instruction's address causes a carry into the high order bits).

With some trickery, a 26-bit field can specify a 32-bit address. But it is a nuisance to figure out! If you were doing machine language programming, that is what you would have to do. But the assembler does the work for you. Here is a tiny program:

## jump.asm
##
      .text
      .globl  main

main: 
      sll    $0,$0,0
      sll    $0,$0,0
      sll    $0,$0,0
      sll    $0,$0,0
      j      main
      addiu $8,$8,1 
      
## End of file

The symbolic address main stands for the address of the first instruction. The instruction   j main   tells the assembler to assemble a machine instruction with the proper 26-bit field so that control is transferred to main.

The branch delay slot is filled with an instruction that increments register $8.