- Recall:
b LABELin cause the CPU to "branch" to the program instruction stored at the label LABEL
- In other words:
- The next instruction that is fetched (and executed) by the CPU is at the address LABEL
This will happen when we update the Program Counter in the CPU with the address value of LABEL !!!
- So the
instruction
"b LABEL"
is
implemented as follows:
- Store the value of the address LABEL into the Program Counter
Because the Program Counter is set equal to the address LABEL, the next instruction execution cycle (see: click here) will fetch the next instruction at the memory address LABEL !!!
- BTW, we can update
the Program Counter with
a move instruction in ARM !!!
Here is a program that shows that you cab branch with a mov instruction:
main: movw r0, #:lower16:there movt r0, #:upper16:there // r0 = addr(there) mov pc, r0 // pc = addr(there) // The CPU will branch to "there" !!! mov r1, #4444 // These instructions are SKIPPED over ! mov r2, #4444 mov r3, #4444 there: mov r4, #4444 mov r5, #4444 mov r6, #4444
- Example Program:
(Demo above code)
- Prog file: /home/cs255001/demo/asm/6-if/impl-branch.s
How to run the program:
- To compile: as255 impl-branch
- To run: use EGTAPI