Question

The following code segment is stored in memory starting at memory location 0x00445670.

What are the two possible values for the contents of the PC after the branch instruction has executed?      

bgez $a0, skip                      # mem location: 0x00445670

subu $s2, $s1, $t0 # branch NOT taken (false)

ori    $v0, $t1, 0x0003 #      

skip:

addi $t0, $t1, 2 # branch taken (true)

if taken:

if not taken:

Hint: Remember how many bytes each instructions takes.

Answer 1

bgez $a0, skip                      # mem location: 0x00445670

subu $s2, $s1, $t0 # branch NOT taken (false)     # mem location: 0x00445674

ori    $v0, $t1, 0x0003 # mem location: 0x00445678     

skip:                    # mem location: 0x004457c

addi $t0, $t1, 2 # branch taken (true)   # mem location: 0x00445680

BGEZ : Branch if the register is greater than or equal to zero.

in this instruction the operation will be perform are :

instruction syntax : BGEZ $a0 , offset

here we have skip as offset

if ( $a0 >= 0)

    advance_pc(offset<<2)

    pc = pc + (4<<2)

    pc = 0x00445670 + (in binary 4 = 100 << 2)

    pc = 0x00445670 + (in binary = 10000 = 0x10)

    pc = 0x00445680

else

    advance_pc(4)

    meaning pc += 4

    pc = 0x00445674