80x86 program crashes after declaring second variable

Question

I started learning assembly a few weeks ago and I wrote this program to get user input. I'm hung up because the program freezes dos box after I declare msgOut. However it will work fine if I leave it commented out along with the code to print it out. Any help would be appreciated.

; This program gets a character from the user and prints it out 

    org 100h        ; program start point
section .data
    msgIn:  DB  "Enter a character: $"
    msgOut: DB  13, 10, "Character value: $"

section .bss
char resb 1         ; storage for input character

section .txt
; print enter message
    mov dx, msgIn   ; offset address of message to display
    mov ah, 9       ; print string function
    int 21h

; get user input
    mov ah, 1       ; keyboard input sub-program
    int 21h         ; read character into al

; store character in char variable
    mov [char], al  ; move entered char into char variable

; print second message
    mov dx, msgOut  ; offset of second message
    mov ah, 9       ; print string function
    int 21h         ; display message

; display character
    mov dl, [char]  ; char to display
    mov ah, 2       ; print char function
    int 21h

; exit program
    mov ah, 4ch     ; exit to DOS function
    int 21h         ; see you later!

Answer 1

org 100h is for a COM file. You have your code section named .txt, that is wrong; it should be .text

; This program gets a character from the user and prints it out 

    org 100h        ; program start point
section .data
    msgIn:  DB  "Enter a character: $"
    msgOut: DB  13, 10, "Character value: $"

section .bss
char resb 1         ; storage for input character

section .text ; <<<<<<< notice the name!!!

; print enter message
    mov dx, msgIn   ; offset address of message to display
    mov ah, 9       ; print string function
    int 21h

; get user input
    mov ah, 1       ; keyboard input sub-program
    int 21h         ; read character into al

; store character in char variable
    mov [char], al  ; move entered char into char variable

; print second message
    mov dx, msgOut  ; offset of second message
    mov ah, 9       ; print string function
    int 21h         ; display message

; display character
    mov dl, [char]  ; char to display
    mov ah, 2       ; print char function
    int 21h

; exit program
    mov ah, 4ch     ; exit to DOS function
    int 21h         ; see you later!

nasm -f bin DOSTest.asm -o DOSTest.com

@Tim, it does not matter where your data is, NASM will place the code section in the proper place

The bin format puts the .text section first in the file, so you can declare data or BSS items before beginning to write code if you want to and the code will still end up at the front of the file where it belongs.

Answer 2

Look at section 8.2.1 of the nasm documentation.

I'm pretty sure you need to move your .text section to be first in the file, before the other sections. I suspect it's trying to execute your data segment, and that's why the extra variable breaks it.