Description

1 Overview
The objective is to start to prepare you to write functions in assembly. Assembly has a different vocabulary from C and Java (eg, instructions on registers instead of expressions on variables), and a different structure (eg, branches and labels instead of conditionals and blocks).
2 Context: Use of Assembly in Practice
Assembly language is used in several key situations. First, when a machine first starts up, the code that reads in the operating system is written in assembly. You might call this the “boot loader”. Second, when software needs to interact with hardware (eg, IO adaptors), it uses special instructions (eg, in, out) or special registers that aren’t exposed to C code. Third, “hand tuned” assembly is used when a routine needs to be optimized for size, speed, or to ensure specific timing. Only extremely important or frequently used code gets this treatment, but on small microcontrollers without much memory for instructions, optimization for size is common.
In practice, assembly routines are called from C, but we won’t do that here (because our testing platforms currently don’t have a C compiler that generates MIPS assembly).
3 Grading Criteria
Your assignment grade will be determined with the following weights: Public tests 100%
IMPORTANT: We will look at your code. You will lose most of the points if you use a C-compiler to generate MIPS code, or if you do not follow conventions on stack accessing and function calling (see section 7).
5 Starter files
This description is in the 216public exercise_descriptions directory. Copy the directory assembly_basic from the 216public exercises directory to your 216 directory. The .submit file for this exercise is in that folder.
6 Specifications
You will implement three functions in MIPS assembly. Their prototypes, in C syntax, are given below.
• int max(int x, int y): Parameters x and y are passed in registers. Returns the max of x and y in a register. Eg, max(8, 9) returns 9.
• int mash(int x, int y): Parameters x and y are passed in the stack. Returns the value of 10*x + y in a register. Eg, mash(8, 9) returns 89. Assume inputs do not cause overflow.
• int strlen(char* str): Parameter str is passed in a register. Assume that str points to a nul-terminated string. Returns the length of the string in a register.
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We supply template files max.s, mash.s and strlen.s, one for each function. We also supply drivers for these functions, in files max_driver.s, mash_driver.s and strlen_driver.s. Consult the drivers to determine the precise locations of arguments and return values, ie, which registers and/or where in stack.
The key reference documentation for the SPIM instruction set is HP_AppA.pdf, pages 48–72 (also available in the class page resources).
7 Important
1. To run spim and qtspim in a shell in Grace, you must first run the command “module load spim” in that shell. To avoid typing this for each new shell, add a line with this command to your ~/.cshrc.mine file (and then run “source ~/.cshrc.mine” or logout and login).
2. Your code must be written in the template files max.s, mash.s and strlen.s.
4. Do not remove the existing comments in the template files. Add comments to your code as you see fit.
5. Do not modify the driver files.
6. To test your function, concatenate the driver file and your file and execute that resulting file. For example, do cat max_driver.s max.s > max_prog.s
and execute file max_prog.s in qtspim or in command-line spim. The latter is illustrated below (see max_test1); the first three lines are typed in, the last line is output.
% spim -file max_prog.s
8
9
9
%
7. Your grade will be based on the results obtained from the submit server. It is your responsibility to verify that your program generates the expected results on the submit server.
8. Obey conventions for accessing quantities on the stack, otherwise you will lose points. In particular, access arguments and local variables on stack via offsets to the frame pointer.
8 Submitting your assignment
In the assignment directory (assembly_basic) execute the command submit. For a web submission, submit a zip of max.s, mash.s and strlen.s.
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