Programming Lab 3A Functions & Parameters


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Programming Lab 3A
Functions & Parameters
Topics: Passing parameters, function return values, nested functions, preserving and restoring
registers across function calls, calling C functions from assembly.
Prerequisite Reading: Chapters 1-3
Revised: January 19, 2021
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Assignment: The main program contains each of the four functions shown below. The program may be compiled
and executed without writing any assembly. However, your task is to create assembly language replacements for
each of these C functions. The original C functions are defined as “weak”, so that the linker will automatically
replace them in the executable image by those you create in assembly; you do not need to remove the C versions.
Functions Square and SquareRoot are provided in the main program; do not recreate them in assembly – just
call them from your assembly language code for functions Square2x and Last.
int32_t Add(int32_t a, int32_t b)
return a + b ;
int32_t Less1(int32_t a)
return a – 1 ;
int32_t Square2x(int32_t x)
return Square(x + x) ;
int32_t Last(int32_t x)
return x + SquareRoot(x) ;
Code and test your functions one at a time using the main program downloaded using the link above. If your
code works correctly, the display should look like the image shown. Press the blue pushbutton to cycle through
all the test cases to verify that everything is correct. Color is used to indicate the status of a function:
Gray Function is never called.
Yellow Function is provided by the main program.
Orange Function never returns from being called.
Green Function returns the correct value.
Red Function returns an incorrect value.
IMPORTANT – The .thumb_func directive:
The “.thumb_func” assembler directive specifies that the next label is the entry point of a function that contains instructions from the Thumb
subset of the ARM processor and causes the binary representation of instructions that branch to
that label to be generated somewhat differently.
Thus in a source code file that contains more than
one function, it is imperative that you place a
.thumb_func directive immediately before the
entry point label of every function.

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