Assembler Lab

Assembly is not an easy language to pick up; usually I have no real issues learning new programming and scripting languages. But assembly has proven to be difficult. Many times I have to look at things backwards than I normally would when trying to determine the logic of program. I feel I have to juggle more pieces of the puzzle, which normally I would not have to concern myself with.

I still feel overwhelmed when I look at the simplest assembly, but I am beginning to understand some of it. I think most of my confusion comes from how assembly changes depending on the architecture, the manufacturer or if it’s gas or nasm. I also got to see some inline assembly in C, which also had tags that were foreign to me; it seems the learning curve is going to be steep and unforgiving.

In this lab, we introduced loops, and a little logic to our program. Beginning with X86_64 architecture, I took the hello, world! program and added some looping components to output hello, world ten times. With that amazing feat accomplished, I was able to add an increment to number the loops 0-9.

I had an issue getting the proper output. To append the increment value to the end of the text I used:

movq %r14,msg+6

Thinking this would add the single ascii character the 6th field, but apparently this is not so. My output kept coming out on a single line, and I was forced to add several empty spaces between the loop: and \n that gave me the new line. After adding 7 additional blank spaces did the new line finally show. This is the assembly code I created to get the following output:

Loop: 0
Loop: 1
Loop: 2
Loop: 3
Loop: 4
Loop: 5
Loop: 6
Loop: 7
Loop: 8
Loop: 9

This was after many failed attempts and watching the dreaded endless loop while spamming ctrl-c in hopes of stopping the loop before it gets out of hand.

The lab continues by asking us to do the same in aarch64, but when trying to compile the example assembly in that folder, I received the following error:

hello.s: Assembler messages:
hello.s:5: Error: too many memory references for `mov'
hello.s:6: Error: no such instruction: `adr x1,msg'
hello.s:7: Error: too many memory references for `mov'
hello.s:9: Error: too many memory references for `mov'
hello.s:10: Error: no such instruction: `svc 1'
hello.s:12: Error: too many memory references for `mov'
hello.s:13: Error: too many memory references for `mov'
hello.s:14: Error: no such instruction: `svc 0'
make: *** [hello] Error 1

I attemped to resolve the issue, and it seemed like a common problem from posts found on the internet, but being inexperienced in assembly, I felt lost in how to resolve this issue. If someone has some input, feel free to let me know. I will speak with people in class and get this resolved, and post the aarch64 parts of this lab within a few days.

Continuing on with the lab (x86_64 portions), we are asked to expand on the loop to 30, and to display two characters (00-30). This was a bit tricky. At first, I wanted to try something different than what was suggested, and I failed miserably.

Instead of doing division (which I admit was much simpler), I tried to do it with logic in the attempt to learn more about jumps and compares through trial. After spending some time on it, and failing into the late hours of the night, I completed it as it was recommended in the lab. Doing it this way was much simpler, and provided some insight into the div command and how some of the registers work. This is my final code and output:

Loop:00
Loop:01
Loop:02
Loop:03
Loop:04
Loop:05
Loop:06
Loop:07
Loop:08
Loop:09
Loop:10
Loop:11
Loop:12
Loop:13
Loop:14
Loop:15
Loop:16
Loop:17
Loop:18
Loop:19
Loop:20
Loop:21
Loop:22
Loop:23
Loop:24
Loop:25
Loop:26
Loop:27
Loop:28
Loop:29
Loop:30


I am still getting the hang of using the registries; having such finite control over memory adds a few more balls I need to juggle as I work. I will have to review my notes on register use; understanding why I should use register x over register y (performance?).

The lab was interesting and not as daunting as I first imaged. Sitting down and looking though some of the documentation helped. I look forward to completing the aarch64 once I resolve my errors.

-Richard K.