- In CS255
(assembler programming),
the terms subroutines and
functions are
synonmous to
methods in
Java
So whenever, I use subroutine or function or method, they mean the same thing
- Subroutine:
- Subroutine is
a program unit that
can be called (= invoked)
from anywhere in the computer program
- As you will see soon, a
subroutine will
consist of a sequence of
assembler instructions
without any special structure
--- just like the
main program that
you have been writing so far in assembler....
- The start (= the first instruction) of the subroutine is marked with a label -- this label is always the name of the subroutine
- As you will see soon, a
subroutine will
consist of a sequence of
assembler instructions
without any special structure
--- just like the
main program that
you have been writing so far in assembler....
- When a subroutine is
called (= invoked),
the program execution is
transfered to
the instructions that
comprise the subroutine
- Program execution will
continue in the
subroutine until
the subroutine
returns
- When the subroutine returns, the program execution will continue at the next statement after the subroutine call instruction
- Subroutine is
a program unit that
can be called (= invoked)
from anywhere in the computer program
- Example:
- Consider the following
computer program
in "pseudo assembler instructions"
(I don't want to use
ARM subroutine call and return instructions
yet, I only specify the
"high level programming language"
instructions
that you are familiar):
The pseudo instruction call-sub will call a subroutine
The pseudo instruction sub-return will return to the caller
The arrows show you the program execution order
I.e.:
- The first part of the
main function is
executed first
- Then the execution continues
in the
subroutine sub1
But only the first part of the sub1 function is executed
- Then the execution continues
in the
subroutine sub2
The complete sub2 function is executed and then the execution returns back to the instruction sub1-instr3
- Then the execution continues
in the 2nd half of
subroutine sub1
and return to
the instruction
main-instr3
- Then the execution continues
in the 2nd half of
the main function
- The first part of the
main function is
executed first
- Consider the following
computer program
in "pseudo assembler instructions"
(I don't want to use
ARM subroutine call and return instructions
yet, I only specify the
"high level programming language"
instructions
that you are familiar):
- In order to implement subroutine,
you will learn
2 new assembler instructions:
- An instruction used by the
caller function to
call a
subroutine
(branch and link)
- An instruction used by the called subroutine to return to its caller (in ARM, this is a mov or a pop instruction - depending where you store the return address (explain later)
- An instruction used by the
caller function to
call a
subroutine
(branch and link)
- The difficult part of
the implementation of
subroutines is:
- Storing the parameters of a subroutine
- Storing the local variables of a subroutine
In CS170, you have learned that:
- Parameter variables are created when a subroutine starts its execution
- Local variables are
created when
a subroutine
starts its
execution
- Parameter variables are destroyed when a subroutine finishes its execution
- Local variables are destroyed when a subroutine finishes its execution
When you write a subroutine in assembler:
- You are the one
that need to
create the
parameter variables and the
local variables when
a subroutine starts
- You are also the one that need to destroy the parameter variables and the local variables when a subroutine finishes.
So we will spend a lot of time learning how to create variables and destroying variables in the discussion on how to implement subroutines
- But first:
- We must learn how to:
- How to
call a
subroutine
(with no parameters and local variables)
- How a subroutine returns to its caller
- How to
call a
subroutine
(with no parameters and local variables)
- We must learn how to: