The PC contains the address of the instruction that follows the jump instruction. Recall that the machine cycle is "fetch - increment PC - execute" so the PC will have been incremented.
Most jumps and branches are to nearby addresses. The target of the jump instruction and the instruction following the jump instruction are likely to be close together in memory. The high-order four bits of their addresses will be identical. So the high-order four bits of the PC are the same as needed for the target address.
Of course, an assembly language programmer must be careful to make sure that this is so. When a compiler translates a source program into machine language it also must pay attention to addresses. For the tiny programs you will write for this course the high four bits of the PC will always be the high four bits of the jump address.
A jump instruction can't jump to any arbitrary location in the full 32-bit address space. It must jump to an address within the following range:
wxyz 0000 0000 0000 0000 0000 0000 0000
.
.
.
wxyz 1111 1111 1111 1111 1111 1111 1100
Here, wxyz represents the high-order four bits of the PC. Almost always the jump instruction and the jump address are both within this range.
All these details may look terrible to you at this point. Don't worry: (1) its not as bad as it looks, and (2) usually the assembler does all the work. (But for now, you get to do the work).
Some 32-bit processors have instructions of several lengths. Such processors can include the full 32-bit address in a jump instruction thats long enough to hold it. But MIPS instructions are always 32 bits. Some sort of trick must be used with its jump instruction.
Here is a great idea! Why not implement the jump instruction without using an explicit op-code? Build the processor so that when execution encounters a 32-bit address it automatically jumps to that address.
Will this scheme work?