Question

How are the control lines extended in the control unit to include the jump/branch instructions?

How is the program counter incremented for jump/branch as well as other instructions? state of objects.

Answer 1

1. The branch datapath (jump is an unconditional branch) uses instructions such as beq $t1, $t2, offset, where offset is a 16-bit offset for computing the branch target address via PC-relative addressing. The beq instruction reads from registers $t1 and $t2, then compares the data obtained from these registers to see if they are equal. If equal, the branch is taken. Otherwise, the branch is not taken.

By taking the branch, the ISA specification means that the ALU adds a sign-extended offset to the program counter (PC). The offset is shifted left 2 bits to allow for word alignment (since 22 = 4, and words are comprised of 4 bytes). Thus, to jump to the target address, the lower 26 bits of the PC are replaced with the lower 26 bits of the instruction shifted left 2 bits.

2. In most processors, the Program Counter is incremented after fetching an instruction, and holds the memory address of ("points to") the next instruction that would be executed. (In a processor where the incrementation precedes the fetch, the PC points to the current instruction being executed.) Processors usually fetch instructions sequentially from memory, but control transfer instructions change the sequence by placing a new value in the PC. These include branches (sometimes called jumps), subroutine calls, and returns. A transfer that is conditional on the truth of some assertion lets the computer follow a different sequence under different conditions.