Program 18: Add the Inputs

Reading input and the docs #

As you dig in deeper and deeper into your programming journey, it becomes more and more important to be able to read the docs. As boring and monotonous as that sounds, this is the best way to learn. Start by looking at the table of syscalls that you saw before. You can see that read is #3.

To learn more about the read operation, you can use the man pages by looking at the 2 (syscall) entry. In the terminal type: man 2 read This tells you everything you need to know about how to make this syscall and what variables it is expecting.

As you read this man page, you can see that read takes three arguments. For this exercise, you will read from stdin. To give them to the supervisor, you will put them in r0, r1, and r2 – very much in the same way you call write!

1. int fd: 0 = stdin
2. void *buf: The * tells you that is is expecting an address. So this is where the output buffer space is in memory. Since the largest unsigned 32-bit number is 4,294,967,296, this needs to be 10. You can make this a null terminated area for easier processing. (Just change the number of bytes to 11 as opposed to 10)
3. size_t count: This is the max number of characters you are willing to read. Again, since the largest number is 10 digits, you only want to accept 10 digits.

Automatic length calculation #

Imagine you have a long string in the data section and you don’t want to calculate the length. Or possible a string where the length may change. You can use the following form to automatically calculate the length on compile. The dot acts as representation of “this memory address here” and then you subtract the address at the given label. This results in the length:

.data
prompt:     .asciz      "Input a number to add or Enter to sum:\n"
promptlen:  .word       .-prompt

Another quick note about errors #

Again, I feel like I need to stress the absence of error checking in these examples. Especially when it comes to reading user input: things will go wrong. If the numbers added overflow, there is no check there. If the number entered is negative, there is no check there. If the input length is > 10, there isn’t really a check there. Please, please, please, stand on the shoulders of those who came before you and always be aware of all the ways something can go wrong. This is more true in assembly than any other programming language.

Program 18: Add the Inputs Video - Coming Soon

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