V22.0201 (sec. 1) - Computer Systems Organization (Honors)
Assignment 5 - Combining C and Assembler
As we have mentioned repeatedly in class, there are few occasions any
more when it is worthwhile to program in assembler. As we have
seen from looking at the output of the Microsoft C compiler, compilers
can produce very efficient code -- about as good as a skilled assembly
language programmer. However, there are still times when we may
be tempted to use assembly code because the machine has some
capabilities which we cannot easily use from a higher-level
language. In this assignment, we will look at two such cases.
Part I: string operations
(1) Prepare your own versions, in C, of the string functions strcpy,
strlen, and strchr. You need not be original; for example, the
Kernighan and Ritchie book gives several versions of the first two
(2) The x86 machines have special instructions for string copy and
compare which were meant to be faster than the 'standard'
instructions. Study section 5.2 of the Carter text.
Prepare assembly language versions of the same functions (
strcpyand strlenare given by
Carter; you have to do a bit of your own
coding for strchr).
(3) Observe the time required by these two sets of functions for
strings of at least two different lengths. Try the C code both in
debug mode and in maximal-speed mode. How much gain do you get from
using the special string instructions (over optimized C)?
Part II: extra long integers
Applications such as cryptography and tracking the national debt
require large integers. Java and some C implementations provide
64-bit integers (_int64
type in Visual C++, for example). For this part of the
assignment, you are to write a function for adding 128-bit
integers. Each 128-bit integer is stored as an array of 4 ints
(32-bit integers). If a 128-bit integer is stored in int
num, the most significant 32 bits are in int and the least
significant 32 bits are in int.
(1) Write a C function for performing this addition:
longlongadd (int* a, int* b, int* sum)
(2) Write an assembly language function for performing the same
(3) Validate your code by checking that the two implementations
return the same results for a range of inputs. Observe the time
required by the C function (unoptimized and optimized) and the assembly
code. How much gain do you get from using assembly code?
(Note: the optimization level of the Visual C/C++ compiler is
under Project ... Settings ... C/C++ tab ... Optimizations pull-down
You must also remove the /ZI and /GZ options under "Project Options" at
the bottom of that tabbed pane. The speed of the program can be
using clock(); doing
puts in duration the time required for blahblahblah.)
clock_t start, finish;
start = clock();
finish = clock();
duration = (double)(finish - start) / CLOCKS_PER_SEC;
Tabulate, as accurately as you can, the time required
(in nanoseconds) for a calls to the different functions.
It is recommended that you run the program several times to make sure you
are getting (roughly) consistent times; you may find the times for
the first run somewhat different from later runs.
This assignment is due on November 19th and is worth 10
points. As usual, there is a penalty of 1/2 point for each day
late. For 1 point of extra credit, measure the speed of the library string functions
with the optimizing C, and explain why they are faster or
slower than the functions you coded in C or assembler.
Email your assignment to the grader, with a copy to the instructor
(firstname.lastname@example.org). This includes all your
and a 1-2 page write-up (in Word
format) describing your results. Show
the raw data you obtained and the computed times. Please include the
and speed of the CPU you are running. Include your name and "Asgn
5" in the subject line.