I'm using a 1366x768 screen with a 16:9 aspect ratio. Whenever I run Dosbox in fullscreen mode, the DOS 4:3 image is stretched out to fill up my whole screen. I would like Dosbox's fullscreen mode to use a 4:3 aspect ratio, using up the whole screen but leaving 171 columns of pixels black on each side (left and right) of the image. How can I do this?

I tried editing the configuration file (~/.dosbox/dosbox-0.74.conf) by changing "fullscreenresolution" from "original" to "1024x768", but it didn't help: I still got the same aspect ratio; only, now Dosbox placed black areas above, below, to the left of and to the right of the Dosbox image.

Dosbox prints a couple of lines to my terminal, but I assume that the information is of no relevance here:

DOSBox version 0.74 Copyright 2002-2010 DOSBox Team, published under GNU GPL. --- CONFIG:Loading primary settings from config fi […]
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DOSBox version 0.74
Copyright 2002-2010 DOSBox Team, published under GNU GPL.
---
CONFIG:Loading primary settings from config file /home/username/.dosbox/dosbox-0.74.conf
MIXER:Got different values from SDL: freq 44100, blocksize 512
ALSA:Can't subscribe to MIDI port (65:0) nor (17:0)
MIDI:Opened device:none

I start Dosbox with a plain "dosbox" call without any additional flags.

Apart from the playing with the "fullscreenresolution" setting, the only changes I've done to the configuration file are in the autoexec section: I'm using "mount" to mount a C drive and I'm using "keyb" to get the correct keyboard layout. I've also set fullscreen mode to default.

Window mode gives me the correct aspect ratio, but since the window can't be resized, it doesn't help much: the image is way too small.