DOS specifics

Drivers JOY_TYPE_*/DOS
The DOS library supports the following type parameters for the install_joystick() function:

Drivers GFX_*/DOS
The DOS library supports the following card parameters for the set_gfx_mode() function:

Drivers DIGI_*/DOS
The DOS sound functions support the following digital soundcards:

      DIGI_AUTODETECT      - let Allegro pick a digital sound driver
      DIGI_NONE            - no digital sound
      DIGI_SB              - Sound Blaster (autodetect type)
      DIGI_SB10            - SB 1.0 (8 bit mono single shot dma)
      DIGI_SB15            - SB 1.5 (8 bit mono single shot dma)
      DIGI_SB20            - SB 2.0 (8 bit mono auto-initialised
      DIGI_SBPRO           - SB Pro (8 bit stereo)
      DIGI_SB16            - SB16 (16 bit stereo)
      DIGI_AUDIODRIVE      - ESS AudioDrive
      DIGI_SOUNDSCAPE      - Ensoniq Soundscape
      DIGI_WINSOUNDSYS     - Windows Sound System

Drivers MIDI_*/DOS
The DOS sound functions support the following MIDI soundcards:

      MIDI_AUTODETECT      - let Allegro pick a MIDI sound driver
      MIDI_NONE            - no MIDI sound
      MIDI_ADLIB           - Adlib or SB FM synth (autodetect type)
      MIDI_OPL2            - OPL2 synth (mono, used in Adlib and SB)
      MIDI_2XOPL2          - dual OPL2 synths (stereo, used in
                             SB Pro-I)
      MIDI_OPL3            - OPL3 synth (stereo, SB Pro-II
                             and above)
      MIDI_SB_OUT          - SB MIDI interface
      MIDI_MPU             - MPU-401 MIDI interface
      MIDI_DIGMID          - sample-based software wavetable player
      MIDI_AWE32           - AWE32 (EMU8000 chip)

void split_modex_screen(int line);
This function is only available in mode-X. It splits the VGA display into two parts at the specified line. The top half of the screen can be scrolled to any part of video memory with the scroll_screen() function, but the part below the specified line number will remain fixed and will display from position (0, 0) of the screen bitmap. After splitting the screen you will generally want to scroll so that the top part of the display starts lower down in video memory, and then create two sub-bitmaps to access the two sections (see examples/exscroll.c for a demonstration of how this could be done). To disable the split, call split_modex_screen(0).

extern int i_love_bill;
When running in clean DOS mode, the timer handler dynamically reprograms the clock chip to generate interrupts at exactly the right times, which gives an extremely high accuracy. Unfortunately, this constant speed adjustment doesn't work under most multitasking systems (notably Windows), so there is an alternative mode that just locks the hardware timer interrupt to a speed of 200 ticks per second. This reduces the accuracy of the timer (for instance, rest() will round the delay time to the nearest 5 milliseconds), and prevents the vertical retrace simulator from working, but on the plus side, it makes Allegro programs work under Windows. This flag is set by allegro_init() if it detects the presence of a multitasking OS, and enables the fixed rate timer mode.

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