Group Project 1: Syscall input and output

This assignment will run through the second program everyone writes, prompting for the user’s name and then greeting them by name. When you run the final program, it should work like this:

What is your name: Andy                                             
Hello, Andy, nice to meet you!     

(Where Andy is what I typed in; the rest was printed by the program.)

In order to do this, you will need to add another syscall to your repertoire: syscall code 0 is SYS_READ, which reads from a stream up to a newline or a maximum length (whichever comes first) and stores the resulting bytes into a buffer.

• rax = 0, SYS_READ

• rdi =the stream to read from. Standard input (stdin) is stream 0.

• rsi =the address of the beginning of the buffer to save the read-in bytes into

• rdx =the maximum number of bytes to read in (i.e. the length of the buffer). Reading will stop after this many bytes, or after a newline character, whichever comes first.

Note that the characters stored in the buffer are not NUL-terminated; instead, the syscall will “return” the number of bytes read in, including the newline, by storing it into rax. Because you need to use rax for other things, you’ll want to copy this value into one of the registers that is not used by syscalls. This length will include the newline that ended the input, so you’ll probably want to subtract 1 to prevent that from appearing in the output.

There’s no way to “concatenate” strings in assembly, so to print the final message you will need three syscalls: one to print Hello,, a second to print the contents of the buffer, and a third to print , nice to meet you!.

When you print the contents of the buffer, the length you should use is not the BUFFER_LEN, because it’s unlikely that the user typed in a name that filled the entire buffer; instead, use the length returned by the SYS_READ syscall (which you should have saved into a different register).

Like all syscall-style programs, this one must end with a SYS_EXIT = 60 syscall, to correctly exit the program.

Here’s a skeleton that you can use to start:

;;;
;;; greet.s
;;; Prompt the user for their name, then greet them by name.
;;; At runtime, the program should work like this:
;;; 
;;;     What is your name: Andy                     <--- Andy is user input
;;;     Hello, Andy, nice to meet you!
;;;
section .data

prompt:         db          "What is your name: "
PROMPT_LEN:     equ         $ - prompt

greet1:         db          "Hello, "
GREET1_LEN:     equ         $ - greet1

greet2:         db          ", nice to meet you!",  10
GREET2_LEN:     equ         $ - greet2

; For user input, we need an "empty" buffer of space to store the input into.
; This reserves 256 bytes of space, filled with 0s, for that purpose.
BUFFER_LEN:     equ         256
buffer:         times BUFFER_LEN    db 0

section .text

global _start
_start:

     ; Your code here

Save your submission in cs241/group1/. You only need one submission per group, if you want.

You can find a copy of this skeleton on the server in /usr/local/class/src/cs241_assign1.s. To copy it into your current directory, run the command

cp /usr/local/class/src/cs241_assign1.s .

(NOTE: The . at the end of the command is part of the command!)