Program 2: Hello World
From the exiting program, you can add a couple more lines before it to actually output the “Hello World!”
Labels #
You have already used the _start: label but what does that actually do? A label
simply names a place in the program that you can reference to later. In the data
section, it’s common to label the beginning part of a piece of data. When this
happens, that label and all references to it, will become the same address that
points to a place in the data loaded into memory.
The data section #
In the data section, the format is:
label:? .datatype data
In this example you are going to use the ascii datatype which takes the data between the two double quotes and converts them to their ASCII equivalence. You will see other types of data in the near future.
LDR operation #
Like the mov opcode, ldr loads a value into a register. However, unlike
mov, ldr loads a value from memory. This is a small but very important
difference. But in order to load a value from memory, you need to know where in
memory it is located. Luckily for us, the compiler will do the calculation for
use if you reference a label in the data section.
ldr r1, =hello @ Load the address that hello points to
@ into register 1
.data
@ The next line labels the location our string starts as `hello`
hello: .ascii "Hello World!\n"