Ex 4: A small function in assembly

Using the online GCC/GAS Assembler, write an assembly program which computes 3x³ - 2x² + x - 1 using just the general purpose 32-bit registers, EAX, EBX, ECX, and EDX. You can save and restore registers on the stack if necessary. Your code must work for both positive and negative integers. As an example, if x is 4, the result will be 163, while the result for -4 will be -229.

Your program will end by moving the computed result to %eax and returning. However, the return value will typically be too large to display. If you put the result in %eax and set a breakpoint for the ret instruction, stepping one step from that will show as the function's result just after executing the return instruction (so you will need to step one step after getting to the breakpoint). For example, executing the code

        mov $9876, %rbx
        mov $12345, %rax

up to the ret statement, then clicking once on step over gives this result where the red circle on the right shows the returned value is 12345. As an aside, if you run this program to completion, it will say "Program finished with exit code 57". The 57 is because the C runtime does some processing on return results (for example, masking off various bits). So the printed return value is often different from the value you have placed into %rax or %eax. In OnlineGDB, The only way to see the actual value returned (in %rax) is to take one step after reaching ret.

Notes

• Put the value in %ecx and leave the value in that register. This simulates passing the value as a parameter to a function. In particular, one should be able to change the value of x by changing the value of %ecx in one place in your code.
• Be sure your screen shots show just the relevant material. The blue menu on the left and your browser title bar are not relevant, but all of the code is. If you cannot capture the full listing in the screen shot, include the full code as a separate item.
• You can save your project in onlinegdb, but it's not clear those files will be there when you revisit the site. Save the files, then click the download arrow from the onlinegdb menu to save your code locally.
• Capture screen shots for when x is 5 and -6.
• Edit the screen shots to highlight the answers in the debugger, say by circling them. The windows screenshot tool allows you to annotate screen shots.
• Create a document containing your screenshots (showing the code as well) and upload a PDF to show you completed the exercise.