CODEWITHSUNDEEP
assemblyx86-16dosboxvga
CaseyK
CaseyK

Reputation: 37

How do you output to video screen in dosbox?

This code does not output anything.
How can I output the character 'A' on left corner of a new screen?

assume cs: code, ds: code  
code segment  
org 100h  

start:  

mov ax, 0B800h  
mov es, ax  
mov byte ptr es:[0], 'A'   

int 20h      ; exit program
code ends  
end start

I would like there to the letter 'A' on the left corner of an otherwise blank screen.

Upvotes: 1

Views: 474

Answers (1)

Brendan
Brendan

Reputation: 37232

This code does not output anything.

Possibilities include:

  • the video mode is not using display memory at 0xB800:0x0000 (e.g. a graphics video mode).

  • the code did write 'A' but the attribute makes it invisible (e.g. black foreground with black background).

  • the code did write 'A' and the attribute makes it visible; but it's overwritten or scrolled off the top of the screen as soon as the program exists (before you have a chance to see it)

To guard against all these potential problems:

  • set the video mode to make sure it's using text mode

  • do mov ax,(0xF0 << 8) | 'A' and mov [es:0],ax to set the attribute while also writing the character. Note: This can be optimized to a single mov word [es:0],(0xF0 << 8) | 'A' instruction.

  • have some kind of delay before you exit (wait for time to pass, wait for user to press a key, ...).

Upvotes: 2

Related Questions