anchor.diff:            -drive file=$NIX_DISK_IMAGE,if=virtio,boot=on \
             -kernel ${config.system.build.toplevel}/kernel \
             -initrd ${config.system.build.toplevel}/initrd \
  +          ${qemuGraphics} \
             $QEMU_OPTS \
  -          -append "$(cat ${config.system.build.toplevel}/kernel-params) init=${config.system.build.bootStage2} systemConfig=${config.system.build.toplevel} $QEMU_KERNEL_PARAMS"
  +          -append "$(cat ${config.system.build.toplevel}/kernel-params) init=${config.system.build.bootStage2} systemConfig=${config.system.build.toplevel} ${kernelConsole} $QEMU_KERNEL_PARAMS"
       '';
   
   in
  
positive.code:   services.xserver.defaultDepth = mkOverride 50 {} 0;
    services.xserver.resolutions = mkOverride 50 {} [];
  
    # Wireless won't work in the VM.
    networking.enableWLAN = mkOverride 50 {} false;
  }
  
negative.code:             exist.
            '';
        };
      virtualisation.graphics =
        mkOption {
          default = true;
          description =
            ''
              Whether to run qemu with a graphics window, or access
              the guest computer serial port through the host tty.
            '';
        };
  
    };
  
    cfg = config.virtualisation;
  
    qemuGraphics = if (cfg.graphics) then "" else "-nographic";
    kernelConsole = if (cfg.graphics) then "" else "console=ttyS0";
    ttys = [ "tty1" "tty2" "tty3" "tty4" "tty5" "tty6" ];
  
    # Shell script to start the VM.
    startVM =
  