Program 3: Find the Otter

Program 3: Find the Otter

ADD operation #

You will focus on math heavy concepts later, but for now you do need one operator to increment for things like memory addresses and loops. Also, you may have guessed that the ADD opcode adds two numbers. Both of these formats are exactly the same.

    add     r0, r1, r2      @ Add r1 and r2 and store in r0 
    add     r1, r2          @ Add r1 and r2 and store in r1 

STR(-B) operation #

STR is the opposite of LDR. As such, STR takes a value in a register and inserts it into memory. Remember that registers are 32-bit values and that STR, by default, will write the complete 32-bit value to memory. In the next exercise we only should be moving a single byte as opposed to the full 4 bytes. We can do this; both with LDR and STR, by appending a -B suffix, we can work on single bytes at a time.1

Accessing Values in Memory #

When you used a memory address in the “Hello World!” exercise, we took the address and provided it to the supervisor (the operating system) and it read the data in memory for us and wrote it to STDOUT. If YOU want to access the data at an address, you need to use the register but with brackets around the register number.2

    mov         r0, #'A'        @ load the value of A into r0 
    ldr         r1, =outstr     @ load address of outstr into r1 
    strb        r0, [r1]        @ store the first byte in r0 into the address 
                                @ starting at r1 
    strb        r0, r1          @ Does not work  

Reserving Space, Pt. 1 #

The last thing you need to know before going into this exercise is how to ask the computer to put a little space aside for the program to use. For now, use the data section and create a string full of spaces. You can even put the line ending there BEFORE writing the program as we already know the length of the output before you start.

outstr:     .ascii      "     \n" 

  1. Worth noting, although not used throughout the overall course, that you can also use -H for moving a half word worth of data. (2 bytes) ↩︎

  2. I think about it this way. Your friend gives you the address of their house. You find the address but you still need to go into the house. You can’t just go into the address. The brackets are what allow you to go to the house. Eccentric? Yes, but it helps me. ↩︎

Program 3: Find the Otter Video

Description of Program
Do you like otters? I like otters. Using the below starter code, write ‘OTTER’ to stdout.
Completed Code
Expected Output