SDL2 port for DOS (DJGPP and VBE 2.0+) #13906

glebm · 2025-09-08T17:45:38Z

@AJenbo and I took @jayschwa's SDL2 DOS port from a few years ago, rebased it onto the current SDL2, and got palettized output and keyboard fully working.

I understand that the current SDL development focuses on SDL3. I'm posting this PR here for visibility, in case someone might be interested in using it, improving it further, or forward-porting it to SDL3.

Supported

Toolchain: This port requires the DJGPP toolchain, tested with the latest one based on GCC 14.2.
Graphics: The minimum requirement for graphics is VBE 2.0. Only double-buffered graphics modes are supported. An equivalent of vsync is always on.
Mouse: Mouse is supported but custom hardware cursors are not. This is not a big problem as most software that would be appropriate to port to DOS already supports software cursor rendering. Mouse wheel is not supported.
Keyboard: Text input events assume en-US layout.
File system: Works out of the box, courtesy of DJGPP toolchain's POSIX support. Base/pref paths are empty strings (= current directory).

Not supported

Threads, audio, gamepad.

Here is a screenshot of DevilutionX running in DOSBox. We also tested on real hardware (hit and miss depending on the graphics card).

The performance is pretty good when using the 640x480 8-bit palettized mode.
Building instructions are here in the "DOS" section: https://github.com/diasurgical/DevilutionX/blob/master/docs/building.md

Timers remain unsupported due to lack of multithreading.

A few other minor style things were tweaked too.

This implementation is probably wrong, but it lays the foundation for doing it better, eventually.

Windows are now always created with the fullscreen flag.

It ain't pretty though!

Key release events do not seem to be visible via _bios_keybrd. This will probably change to use a keyboard interrupt handler.

It doesn't seem to be necessary.

Example toolchain file: ```bash set(CMAKE_SYSTEM_NAME Generic) set(CMAKE_SYSTEM_VERSION 1) if($ENV{DJGPP_PREFIX}) set(DJGPP_PREFIX "$ENV{DJGPP_PREFIX}") else() set(DJGPP_PREFIX "/opt/i386-pc-msdosdjgpp-toolchain") endif() set(CMAKE_C_COMPILER "${DJGPP_PREFIX}/bin/i386-pc-msdosdjgpp-gcc") set(CMAKE_CXX_COMPILER "${DJGPP_PREFIX}/bin/i386-pc-msdosdjgpp-g++") set(CMAKE_STRIP "${DJGPP_PREFIX}/bin/i386-pc-msdosdjgpp-strip") set(PKG_CONFIG_EXECUTABLE "${DJGPP_PREFIX}/bin/i386-pc-msdosdjgpp-pkg-config" CACHE STRING "Path to pkg-config") set(CMAKE_EXE_LINKER_FLAGS_INIT "-static") set(DJGPP_ROOT "${DJGPP_PREFIX}/i386-pc-msdosdjgpp") set(CMAKE_FIND_ROOT_PATH "${DJGPP_ROOT}") set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER) set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY) set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY) set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY) link_directories("${DJGPP_ROOT}/lib") include_directories(BEFORE SYSTEM "${DJGPP_ROOT}/sys-include" "${DJGPP_ROOT}/include") ``` Example invocation: ```bash cmake -S. -Bbuild-dos -DCMAKE_TOOLCHAIN_FILE="path/to/djcpp.toolchain.cmake" -DDOS=ON ```

Makes the arrow keys and page up/down work.

Basic TEXTINPUT event firing that assumes US keyboard layout.

This allows S3 trio video cards to work

Calling out via real-mode bridge would kill performance when color cycling. Luckily, VBE 2.0 provides protected mode interface for some of the functions, and setting the palette is one of them. The asm code is based on Allegro 4.2 `vesa_set_palette_range`. Luckily, Allegro license is extremely permissive.

icculus · 2025-09-08T19:54:13Z

I have a from-scratch DOS port of SDL3 sitting in https://github.com/icculus/SDL/tree/sdl3-dos ... we should compare notes.

glebm · 2025-09-08T21:25:08Z

@icculus That's awesome! I've had a look at your branch:

Audio is supported in your branch, this is amazing.
Keyboard: this version supports extended input events and sends SDL_TEXTINPUT events. Can be ported pretty much verbatim into your branch.
Video:
- This version supports palettized 8bpp, which is very important for DevilutionX (non-palette modes are too slow for software rendering on most hardware from the era). Should be straightforward to port over, unless SDL3 removed support for palettized 8bpp (this would be very bad for the source ports community, I really hope it's still there).
- The vsync implementation is a bit different. This branch uses double-buffered modes together SetDisplayStart with 0x80 in ebx (via a protected-mode interface), which tells the hardware to wait for a vertical refresh. Your branch seems to do some CPU waits in a loop instead. I think the approach here might be better?
Mouse: more work has gone into custom cursor support but still incomplete (not that important though).

AJenbo · 2025-09-08T21:58:45Z

palettized 8bpp is still supported in SDL3

jayschwa · 2025-09-09T04:19:01Z

What a bunch of mad lads 🤠. I know it's not relevant for your Diablo port, but I also had a branch off my VBE work that was for plain old VGA mode 13h. It could serve as a fallback if VBE isn't available for increased compatibility. It keeps its back buffer in RAM (not VGA) though, so it's not as optimal as page-flipping with VBE.

I recently migrated the game I'm working on from SDL 2 to 3, so I guess I'll have to check out @icculus's branch now. 👀

AJenbo · 2025-09-09T06:02:52Z

A similar solution could be used for supporting vba 2.0 or systems with no double buffer modes

icculus · 2025-09-09T13:27:45Z

So which one of us is going to be the first to hook the timer interrupt and implement threading? :)

ccawley2011

I might as well share my own efforts at a DOS port of SDL in case it's useful: https://github.com/ccawley2011/SDL-1.2/tree/dos

ccawley2011 · 2025-09-11T15:53:35Z

CMakeLists.txt

+  set(SDL_SSE OFF)
+  set(SDL_SSE2 OFF)
+  set(SDL_SSE3 OFF)
+  set(SDL_MMX OFF)
+  set(SDL_3DNOW OFF)


Could this be detected at runtime for DOS?

ccawley2011 · 2025-09-11T15:54:43Z

configure.ac

+
+    case "$host" in
+    *-*-msdosdjgpp*)
+        AC_MSG_NOTICE(skipping unfinished fseeko64 in DJGPP)


Should this apply when building with CMake as well?

ccawley2011 · 2025-09-11T15:57:50Z

src/timer/dos/SDL_systimer.c

+#ifdef HAVE_UCLOCK
+    return uclock();
+#else
+    return SDL_GetTicks();


Is it possible to call clock() directly here to avoid extra multiplication and division when uclock() is unavailable?

ccawley2011 · 2025-09-11T16:05:17Z

src/video/svga/SDL_svga_framebuffer.c

+        return -1;
+    }
+
+    /* Populate color palette for indexed pixel formats. */


It would be useful to formally standardise the behaviour of paletted framebuffers so that it can be used on other platforms as well (especially ones that have limitations regarding changing the palette).

ccawley2011 · 2025-09-11T16:10:11Z

test/testfile.c

@@ -152,7 +152,11 @@ int main(int argc, char *argv[])
        RWOP_ERR_QUIT(rwops);
    }
    if (0 != rwops->read(rwops, test_buf, 1, 1)) {
+        #ifdef __DJGPP__
+        SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "DJGPP allowed read on write only file\n");


Is it worthwhile to manually check this in the RWops callback?

ccawley2011 · 2025-09-11T16:18:16Z

src/video/svga/SDL_svga_framebuffer.c

+    movedata(_my_ds(), (uintptr_t)surface->pixels, windata->framebuffer_selector,
+        framebuffer_offset, surface_size);


How practical would it be to allow direct access to the framebuffer memory by applications to avoid extra memory copying on every frame?

Changes to the framebuffer show up on the next vertical refresh, you really have to blit it to VRAM in one block at the end.

The port currently requires a mode that supports double buffering, which should avoid that particular issue.

I wonder if a similar buffer could be used on vbe 1.2 where a linear buffer isn't available.

Direct access would require the "Fat DS hack", which has all sorts of issues https://djgpp.mirror.garr.it/v2faq/faq18_6.html

Fwiw, I think the warnings about the Fat DS hack are way overblown.

My SDL3 port is using it, Quake 1 also used it. It's fine, whatever, if you wanted all your bad memory accesses to segfault, don't write DOS programs. :)

AJenbo · 2025-09-17T02:37:47Z

So which one of us is going to be the first to hook the timer interrupt and implement threading? :)

Diablo for the most part doesn't require multi threading so it might well be up to you 😅

jayschwa added 30 commits August 31, 2025 21:06

Add DJGPP to configure scripts

dbf7d2b

Do not use MMX or SSE on x86 hosts by default

593979d

djgpp: Get testfile (mostly) passing

8c297b5

dos: Add support for basic timing functions

63f0003

Timers remain unsupported due to lack of multithreading.

Create SVGA video driver (cloned from dummy driver)

1ae45fe

svga: Add VBE types

3d40fe7

svga: Tweak file names

0b4fd9d

svga: Read VBE info and require VBE 2.0

a55ec2b

svga: Use structs for special VBE fields

773715c

svga: Assert VBE struct sizes at compile time

8236e8a

svga: Implement GetVBEModeInfo

58edca8

svga: Add GetCurrentVBEMode

7906114

svga: Implement GetDisplayModes

4e0797a

svga: Remove SDL prefix from VBE functions

3d99753

A few other minor style things were tweaked too.

svga: Only use color graphics modes with linear memory

3dcab18

svga: Convert from VBE to SDL pixel format

b6943f3

This implementation is probably wrong, but it lays the foundation for doing it better, eventually.

svga: Add SetVBEMode

c65fdad

WIP: svga: Implement framebuffer functions

c070749

svga: Use color masks to derive pixel format

63c27e8

svga: Calculate surface size with pitch field

c01008a

svga: Save framebuffer ptr in device data

e10b1f2

svga: Add VBE_FLAT_PTR macro function

d8b2e85

svga: Add window functions and data

58df679

Windows are now always created with the fullscreen flag.

svga: Modes must have 4-byte aligned scan lines

0ff763e

svga: Use safer form of memory mapping for framebuffer

56f214a

It ain't pretty though!

svga: Implement double buffering

6b21e8a

WIP: svga: Support indexed color formats

630ef22

svga: Naively generate key press events

6fe697e

Key release events do not seem to be visible via _bios_keybrd. This will probably change to use a keyboard interrupt handler.

svga: Restore original video state on quit

8689844

dos: Write logs to a file

dbe5c10

jayschwa and others added 21 commits August 31, 2025 21:06

svga: Invalidate window surface after freeing it

88d8427

dos: Move keyboard handling out of svga

bd0280e

dos: Use interrupt service routine for keyboard events

1c3335b

dos: Use PS/2 status register to poll keyboard

cd0e976

dos: Do not set pm_selector

360dddd

It doesn't seem to be necessary.

svga: Add some debug logs

e8c6797

svga: Remove obsolete SVGA_Available

58863eb

WIP: dos: Add mouse support

0dca333

Add SDL_mouse.h include to SDL_svga_framebuffer.c

0a8e2bc

dos: fix functions for latest SDL2

643bd9b

dos: Do not bother attempting framebuffer texture

5f92a76

dos: #if SDL_VIDEO_DRIVER_SVGA -> #ifdef

8048fa3

dos: implement some extended keys

6529694

Makes the arrow keys and page up/down work.

dos: handle text input events

d9cf906

Basic TEXTINPUT event firing that assumes US keyboard layout.

dos: modernize fullscreen-only handling

027fb04

dos: Implement palette support

b55cac0

dos: Only draw the cursor if it is enabled

6e5751f

Change return to continue on SVGA mode info error

4862825

This allows S3 trio video cards to work

dos: Use protected-mode interface to request flip

20b47a6

slouken added this to the 2.x milestone Sep 8, 2025

ccawley2011 reviewed Sep 11, 2025

View reviewed changes

		movedata(_my_ds(), (uintptr_t)surface->pixels, windata->framebuffer_selector,
		framebuffer_offset, surface_size);

SDL2 port for DOS (DJGPP and VBE 2.0+) #13906

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SDL2 port for DOS (DJGPP and VBE 2.0+) #13906

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icculus commented Sep 8, 2025

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AJenbo commented Sep 17, 2025

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glebm commented Sep 8, 2025 •

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