Unit #4-04

Learning Outcome

At the end of this lesson, you will be able to:

• understand how to use functions

• understand how to use the stack

Review

• printing out characters

Lesson

• go over functions

  • call and return

• go over the stack

  • push and pop

Activity

• nil

Daily Assignment

• recreate the program below

  • the first 7 numbers in the fibonacci sequence

  • use this algorithm

  • use my PrintSingleDigitInt function to print the numbers out

• HINT: I used registers for all the variables, made my life a lot easier!

; --------------------------------------------------------------

; Created by: Mr Coxall

; Created on: Sep 2022

; Writes out a number to STDOUT.

; Runs on 64-bit x86 Linux only.

; --------------------------------------------------------------

section .bss

someNumber: RESD 1 ; 4 bytes

section .data

; constants here

newLine: db 10 ; UNICODE 10 is new line character

done: db 10, "Done.", 10 ; string to print

doneLen: equ $-done ; length of string

section .text

global _start ; entry point for linker

_start: ; start here

mov r8, -1 ; move the integer 0 into r8

mov r9, 9 -1 ; move the integer 9 into r9

IncrementLabel:

; doing a do ... while loop!

inc r8

push r8

call PrintSingleDigitInt ; call our print single digit function

add rsp, 4 ; pop but throw away the value

cmp r8, 9-1 ; compare r8 and ascii 9

jle IncrementLabel ; jump if <= goto "LoopLable"

; write done to screen

mov rax, 1 ; system call for write

mov rdi, 1 ; file handle 1 is stdout

mov rsi, done ; address of string to output

mov rdx, doneLen ; number of bytes

syscall ; invoke operating system to do the write

mov rax, 60 ; system call for exit

mov rdi, 0 ; exit code 0

syscall ; invoke operating system to exit

PrintSingleDigitInt:

; takes in a single digit int and prints the assci equivalent

; when a function is called, the return value is placed on the stack

; we need to keep this, so that we can return to the corret place in our program!

pop r14 ; pop the return address to r9

pop r15 ; pop the "parameter" we placed on the stack

add r15, 48 ; add the ascii offset

push r15 ; place it back onto the stack

; write value on the stack to STDOUT

mov rax, 1 ; system call for write

mov rdi, 1 ; file handle 1 is stdout

mov rsi, rsp ; the string to write popped from the top of the stack

mov rdx, 1 ; number of bytes

syscall ; invoke operating system to do the write

;add $8, %rsp # restore sp -> not sure this is neede!

; https://stackoverflow.com/questions/8201613/printing-a-character-to-standard-output-in-assembly-x86

; print a new line

mov rax, 1 ; system call for write

mov rdi, 1 ; file handle 1 is stdout

mov rsi, newLine ; address of string to output

mov rdx, 1 ; number of bytes

syscall ; invoke operating system to do the write

push r14 ; put the return address back on the stack to get back

ret ; return