Solving a Quora programming homework question in ARM Assembly

It makes me sad when students ask the online communities to ‘please write me a program that does the following’. Not only is this flat out dishonest to ask someone else to do your homework for you, the opportunity for learning is in the process of solving the problem and writing the code yourself. If someone else writes the code, you’ve missed that opportunity. There’s little to be learned if someone else does the hard work and gives you the final result.

Most communities police these types of questions pretty well. Reviewers on Stackoverflow for example are quick to respond to these types of questions to help developers restructure a generic request for help into a specific question about a specific problem that the developer need help with. The guides that are usually referred to on suggestions to restructure these questions are actually very good advice and reminders for us all on how to ask good questions:

On other community sites, the community response goes in a different direction. This question for help on how to write a program was responded to be some of the funniest and bizarre approaches to solve the askers problem in all sorts of obscure language from Brainfuck to Whitespace and plenty of other weirdness inbetween.

Not to be left out but a little later to the party, I realized I hadn’t done any ARM assembly for a while, so here’s my solution in ARM Assembly that I developed on my Raspberry Pi:

.global main

MOV R4, #3 @ init outer line counter =3

MOV R3, R4 @ init word loop counter with current value of outer counter

MOV R7, #4 @ syscall 4: output to stdout
MOV R0, #1 @ stdout
MOV R2, #6 @ length of string
LDR R1, =output

SUB R3, R3, #1 @ decrement word loop counter
CMP R3, #0
BNE _wordloop

@print newline
MOV R7, #4 @ syscall 4: output to stdout
MOV R0, #1 @ stdout
MOV R2, #1 @ length of string
LDR R1, =eol

SUB R4, R4, #1 @ decrement outer counter
CMP R4, #0
BNE _outerloop

MOV R1, #0
MOV R7, #1

.asciz "Smile!"
.asciz "\n"

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