Question

Hand assemble the following unconditional BRA instruction: HERE BRA HERE. Remember to show your work including the branch calculation. (Motorolla 68k)

Answer 1

Greetings!!

BRA instruction is an unconditional branch instruction which stands for Branch Always.

HERE BRA HERE instruction is equivalent to NOP since it loops on the same label itself.

Assume that a given set of instructions are givenL

  

         ORG $2000

         LDAB #5

         ABA

         STAA $0000

         LDAA #2

         STAA $0001

HERE BRA HERE

After loading to memory it looks like:

ORG $2000

$2000                         LDAB #5

$2002                         ABA

$2003                         STAA $0000

$2005                         LDAA #2

$2007                         STAA $0001

$2009    HERE BRA HERE

Opcode for this instruction is 20 and it loops to PC+offset and in this case, PC is 200B at the time of executing this instruction and hence to loop back to the address 2009, the offset should be -2 and which is represented as FE. Hence the machine code of this instruction would be 20 FE.

Hope this helps