Test Information
Description Read the instructions and answer the questions.
Instructions Objective
The main objective of this lab is to understand control structures in ARMv8. Before proceeding with this lab, you should be familiar with the different branch
and conditional branch instructions in ARMv8 (Sections 2.7, 2.8 and 2.10 of the textbook).
Flow Control
In previous labs, your programs have mostly been purely sequential (each instruction is executed in a sequential order); most programs, however, require
flow control. ARMv8 contains control instructions, which allow programs to execute in a non-sequential manner. In a high-level language, such as C, control
structures have the form if-then-else, goto, for,or do-while. In assembly language, the flow control statements are generally simpler and are of the following
forms: branch, call, and return. Each has the ability to alter the flow of the program.
The program counter (PC) register contains the address of current instruction to be executed. In sequential flow, the PC is incremented to the next
instruction (by 4) each time an instruction starts its execution. The PC register itself cannot be accessed directly by the program, but ARMv8 provides other
means to alter its value, and thus change the location where the program will continue its execution. There are 2 types of control instructions:
Conditional Branches - These control instructions may or may not change the PC depending on a certain condition. These instructions are generally
referred to as branch instructions, because the program flow will follow one of two possible paths (branches). A branch instruction must both specify
the condition on which it will branch, as well as the target address if the conditions are satisfied. If the conditions are not satisfied,the program
continues execution at the next instruction.
Unconditional Branches - These control instructions always change the PC without any conditions. An unconditional branch instruction must always
specify where to get it's the target address.
Conditional Branches
Conditional branch instructions are typically used to form loops. Since most loops tend to be small in size, the target address tends to be close to the branch
instruction. When the target instruction is close enough to the current instruction, then the target can be specified relative to the current instruction with a
small offset. A branch instruction could be described as the equivalent of “If X0 is not 0, then skip the next 5 instructions.” In addition to the offset being
positive (forward branch), the branch instruction also allows a negative offset (backwards branch). Such a branch would be useful in an instruction that does
the equivalent of “If the loop is not done, go back 100 instructions and execute it again.” This is called PC-relative addressing.
Note that all PC-relative addressing is in terms of instructions rather than bytes, so the offset encoded is always right shifted by two versus the true byte
offset of the target from the current PC.
Read More
