Add DOS platform support (DJGPP) (#15377)

* dos: Some initial work.

* dos: Turn off buffer on stdio SDL_IOStreams.

Seeking breaks otherwise. We might be able to just fflush() before or seeking
instead?

* dos: Audio implementation using the Sound Blaster 16.

* dos: remove audio Pump interface.

Turns out DosBox-X was having trouble with the Sound Blaster or something;
standard DosBox works correctly directly from the interrupt handler, and
without doubling the buffer size.

* dos: just dump and restore the stdio buffer when seeking.

This is MUCH faster than just leaving buffering disabled, and also works
around getting bogus reads after an fseek. SDL_LoadWAV on test/sample.wav
no longer takes several seconds to finish, and comes up with the correct
data.

I wonder if we're triggering this in LoadWAV because we're malloc'ing data
between seeks/reads, and it's causing the djgpp transfer buffer to change. Or
maybe the Fat DS trick is confusing it? I don't know, I haven't had time to
debug it, it might just be a legit libc bug in djgpp too, for all I know.

* dos: Protect audio device "thread" iterations when streams are locked.

This uses an old trick we used in SDL 1.2 for MacOS Classic, which did its
audio callback in a hardware interrupt. If the audio is locked when the
interrupt fires, make a note of it and return immediately. When the lock is
released, if the interrupt has been fired, run the audio device iteration
right then.

Since there isn't a big device lock in SDL3 (available to the app, at least),
this keeps a counter of when any SDL_AudioStream is locked, which is probably
good enough.

* dos: Implemented initial video subsystem.

This uses VESA interfaces to manage the display and works with the software
renderer.

Events aren't hooked up yet, so prepare to close DosBox on each run.  :)

* dos: Whoops, forgot to add these to revision control. Core and Main support.

* dos: Wired up basic filesystem support.

This gets most of the rendering examples, which use SDL_GetBasePath() to
find textures to load, working.

* dos: Fixed compiler warning.

* dos: Initial mouse support!

* dos: Move interrupt hooking code into core/dos.

* dos: Initial keyboard support!

* dos: Use a simple ring buffer for keyboard events.

Of course Quake 1 solved this better, haha. It's smart: less memory, dirt
simple, and you don't even have to worry about synchronizing with the
interrupt handler, because it's safe for both sides no matter when an
interrupt fires.

* ci: add djgpp job

[sdl-ci-filter djgpp]
[sdl-ci-artifacts]

* dos: Fix build issues after rebase onto current main

- SDL_runapp.c: Add SDL_PLATFORM_DOS to the exclusion list so the
  generic
  SDL_RunApp() is disabled when the DOS-specific one is compiled.
- SDL.c: Exclude SDL_Gtk_Quit() on DOS. DJGPP defines __unix__ which
  sets
  SDL_PLATFORM_UNIX, but DOS has no GTK/display server. The GTK source
  is not compiled (CMake UNIX is false for DOS) so this was a link
  error.
- sdlplatform.cmake: Add DOS case to SDL_DetectCMakePlatform so the
  platform is properly detected from CMAKE_SYSTEM_NAME=DOS.
- i586-pc-msdosdjgpp.cmake: Add i386-pc-msdosdjgpp-gcc as a fallback
  compiler name, since some DJGPP toolchain builds use the i386 prefix.

* Add 8-bit palette support to DOS VESA driver

* Add VBE page-flipping, state restore, and robust keyboard handling

- Implement double-buffered page-flipping for VBE modes with >1 image
  page
- Save and restore full VBE state on video init/quit for clean mode
  switching
- Improve DOS keyboard handling: support extended scancodes and Pause
  key
- Lock ISR code/data to prevent page faults during interrupts
- Always vsync when blitting in single-buffered modes to reduce tearing

* Refactor Sound Blaster audio mixing to main loop

Move audio mixing out of IRQ handler to main loop for improved
stability and to avoid reentrancy issues. Add SDL_DOS_PumpAudio
function, update DMA buffer handling, and adjust sample rate to 22050
Hz.
Silence stale DMA buffer halves to prevent stutter during load.

* Add DOS timer support and update build config

* Add support for pre-SB16 8-bit mono Sound Blaster audio

Detect SB version and select 8-bit mono or 16-bit stereo mode.
Handle DMA and DSP setup for both SB16 and pre-SB16 hardware.
Add FORCE_SB_8BIT option for testing in DOSBox.

* Add SB Pro stereo support and simplify IRQ handler

* Add DOS joystick driver support

* Improve DOS hardware handling and clarify memory allocation

- Poll Sound Blaster DSP status instead of fixed delay after speaker-on
- Clarify DPMI conventional memory is always locked; update comments
- Document and justify DMA memory allocation strategy
- Free IRET wrapper after restoring interrupt vector to avoid leaks
- Throttle joystick axis polling to ~60 Hz to reduce BIOS timing loop
  cost
- Always poll joystick buttons directly for responsiveness

* Query and use mouse sensitivity from INT 33h function 0x1B

* Add support for VESA banked framebuffer modes

Implement banked framebuffer access for VBE 1.2+ modes without LFB.
Detect and initialize banked modes, copy framebuffer data using bank
switching, and blank the framebuffer on mode set. Page-flipping is
disabled in banked mode.

* Add optional vsync to page flipping in DOS VESA driver

* Add cooperative threading support for DOS platform

* Move SoundBlaster audio mixing to SDL audio thread

* Fix DOS platform comments and workarounds for DJGPP support

* Fix SoundBlaster IRQ handling and DMA setup for DOS

- Pass IRQ number to DOS_EndOfInterrupt and handle slave PIC EOI
- Validate DMA channel from BLASTER variable
- Correct DMA page register selection for SB16
- Improve BLASTER variable parsing and error messages
- Unmask/mask IRQs on correct PIC in DOS_HookInterrupt
- Rename SDL_dosjoystick.c to SDL_sysjoystick.c
- Include SDL_main_callbacks.h in SDL_sysmain_runapp.c
- Add include guard to SDL_systhread_c.h

* Add DOS platform options and preseed cache for DJGPP

Disable unsupported SDL features when building for DOS. Add
PreseedDOSCache.cmake to pre-populate CMake cache variables for DJGPP.

* cmake: use a 8.3 naming scheme for tests on DOS

* Apply code style

* Update include/SDL3/SDL_platform_defines.h

Co-authored-by: Anonymous Maarten <madebr@users.noreply.github.com>

* Code review clean up

- Split DOS VESA mode-setting into its own file
- Replace magic numbers with named constants
- Update copyright dates to 2026
- Substract time taken by other threads form delays

* Fix DOS bugs and improve compatibility

- Disable fseeko64 for DJGPP due to broken implementation
- Refactor DOS timer delay to always yield and avoid busy-waiting
- Fix animated cursor rendering in DOS VESA backend
- Always set display mode when creating DOS VESA window
- Work around DJGPP allowing invalid file access in testfile.c
- Bump max threads to 16
- Apply workarounds for threading tests

* Add DOS platform documentation and fix a few issues

- Fix fullscreen default resolution
- Improve best mode matching
- Fix builds on GCC older than 7.0
- Fix text input events

* Fix keyboard mapping of "*"

* Fix running, and existing, under PCem

* Apply suggestions from code review

Co-authored-by: Cameron Cawley <ccawley2011@gmail.com>

* Pre-mix audio in ring buffer and copy to DMA via IRQ thread

* Video fixes and optimizations

* DOS: Fix Intel 740 and VGA compatability

* DOS: Update readme

* DOS: Fix thread ID, get GPU name

* DOS: Cap mouse range

* DOS: Map test resources to 8.3 names

* DOS: Skip unsupported WM color modes

* Fix "windowed" resolution selection

* DOS: Hide INDEX8 modes behind SDL_DOS_ALLOW_INDEX8_MODES

* Remove SDL_HINT_DOS_ALLOW_INDEX8_MODES and order modes logically

* Don't convert cursor if dest is not INDEX8

---------

Co-authored-by: Ryan C. Gordon <icculus@icculus.org>
Co-authored-by: Anonymous Maarten <anonymous.maarten@gmail.com>
Co-authored-by: Cameron Cawley <ccawley2011@gmail.com>
Co-authored-by: Gleb Mazovetskiy <glex.spb@gmail.com>
Co-authored-by: Jay Petacat <jay@jayschwa.net>
Tested-by: Cameron Cawley <ccawley2011@gmail.com>

src/thread/SDL_thread_c.h

@@ -38,6 +38,8 @@
 #include "vita/SDL_systhread_c.h"
 #elif defined(SDL_THREAD_N3DS)
 #include "n3ds/SDL_systhread_c.h"
+#elif defined(SDL_THREAD_DOS)
+#include "dos/SDL_systhread_c.h"
 #else
 #error Need thread implementation for this platform
 #include "generic/SDL_systhread_c.h"

src/thread/dos/SDL_sysmutex.c

@@ -0,0 +1,122 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2026 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+#include "SDL_internal.h"
+
+#ifdef SDL_THREAD_DOS
+
+/* Mutex implementation for DOS cooperative threading.
+   Uses cli/sti for atomicity and cooperative yielding for contention. */
+
+#include "../../core/dos/SDL_dos.h"
+#include "../../core/dos/SDL_dos_scheduler.h"
+
+#define MUTEX_NO_OWNER -1
+
+struct SDL_Mutex
+{
+    volatile int owner;     /* Thread ID of owner, or MUTEX_NO_OWNER if unlocked */
+    volatile int recursive; /* Recursion count */
+};
+
+SDL_Mutex *SDL_CreateMutex(void)
+{
+    SDL_Mutex *mutex = (SDL_Mutex *)SDL_malloc(sizeof(*mutex));
+    if (mutex) {
+        mutex->owner = MUTEX_NO_OWNER;
+        mutex->recursive = 0;
+    }
+    return mutex;
+}
+
+void SDL_DestroyMutex(SDL_Mutex *mutex)
+{
+    if (mutex) {
+        SDL_free(mutex);
+    }
+}
+
+void SDL_LockMutex(SDL_Mutex *mutex) SDL_NO_THREAD_SAFETY_ANALYSIS
+{
+    if (!mutex) {
+        return;
+    }
+
+    int tid = DOS_GetCurrentThreadID();
+
+    for (;;) {
+        DOS_DisableInterrupts();
+        if (mutex->owner == MUTEX_NO_OWNER) {
+            mutex->owner = tid;
+            mutex->recursive = 1;
+            DOS_EnableInterrupts();
+            return;
+        }
+        if (mutex->owner == tid) {
+            mutex->recursive++;
+            DOS_EnableInterrupts();
+            return;
+        }
+        DOS_EnableInterrupts();
+        DOS_Yield();
+    }
+}
+
+bool SDL_TryLockMutex(SDL_Mutex *mutex)
+{
+    if (!mutex) {
+        return true;
+    }
+
+    int tid = DOS_GetCurrentThreadID();
+
+    DOS_DisableInterrupts();
+    if (mutex->owner == MUTEX_NO_OWNER) {
+        mutex->owner = tid;
+        mutex->recursive = 1;
+        DOS_EnableInterrupts();
+        return true;
+    }
+    if (mutex->owner == tid) {
+        mutex->recursive++;
+        DOS_EnableInterrupts();
+        return true;
+    }
+    DOS_EnableInterrupts();
+    return false;
+}
+
+void SDL_UnlockMutex(SDL_Mutex *mutex) SDL_NO_THREAD_SAFETY_ANALYSIS
+{
+    if (!mutex) {
+        return;
+    }
+
+    DOS_DisableInterrupts();
+    if (mutex->recursive > 1) {
+        mutex->recursive--;
+    } else {
+        mutex->owner = MUTEX_NO_OWNER;
+        mutex->recursive = 0;
+    }
+    DOS_EnableInterrupts();
+}
+
+#endif /* SDL_THREAD_DOS */

src/thread/dos/SDL_syssem.c

@@ -0,0 +1,115 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2026 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+#include "SDL_internal.h"
+
+#ifdef SDL_THREAD_DOS
+
+/* Semaphore implementation for DOS cooperative threading.
+   Uses cli/sti for atomicity and cooperative yielding for waits. */
+
+#include "../../core/dos/SDL_dos.h"
+#include "../../core/dos/SDL_dos_scheduler.h"
+
+struct SDL_Semaphore
+{
+    volatile Uint32 count;
+};
+
+SDL_Semaphore *SDL_CreateSemaphore(Uint32 initial_value)
+{
+    SDL_Semaphore *sem = (SDL_Semaphore *)SDL_malloc(sizeof(*sem));
+    if (sem) {
+        sem->count = initial_value;
+    }
+    return sem;
+}
+
+void SDL_DestroySemaphore(SDL_Semaphore *sem)
+{
+    if (sem) {
+        SDL_free(sem);
+    }
+}
+
+bool SDL_WaitSemaphoreTimeoutNS(SDL_Semaphore *sem, Sint64 timeoutNS)
+{
+    if (!sem) {
+        return true;
+    }
+
+    /* Try-wait (poll): check and decrement if possible */
+    if (timeoutNS == 0) {
+        bool acquired = false;
+        DOS_DisableInterrupts();
+        if (sem->count > 0) {
+            sem->count--;
+            acquired = true;
+        }
+        DOS_EnableInterrupts();
+        return acquired;
+    }
+
+    /* Indefinite wait (-1) or timed wait */
+    Uint64 deadline = 0;
+    if (timeoutNS > 0) {
+        deadline = SDL_GetPerformanceCounter() +
+                   (Uint64)((double)timeoutNS * (double)SDL_GetPerformanceFrequency() / 1e9);
+    }
+
+    for (;;) {
+        DOS_DisableInterrupts();
+        if (sem->count > 0) {
+            sem->count--;
+            DOS_EnableInterrupts();
+            return true; /* Acquired */
+        }
+        DOS_EnableInterrupts();
+
+        /* Check timeout */
+        if (timeoutNS > 0 && SDL_GetPerformanceCounter() >= deadline) {
+            return false; /* Timed out */
+        }
+
+        /* Yield to other threads instead of busy-spinning */
+        DOS_Yield();
+    }
+}
+
+Uint32 SDL_GetSemaphoreValue(SDL_Semaphore *sem)
+{
+    if (!sem) {
+        return 0;
+    }
+    return sem->count;
+}
+
+void SDL_SignalSemaphore(SDL_Semaphore *sem)
+{
+    if (!sem) {
+        return;
+    }
+
+    DOS_DisableInterrupts();
+    sem->count++;
+    DOS_EnableInterrupts();
+}
+
+#endif /* SDL_THREAD_DOS */

src/thread/dos/SDL_systhread.c

@@ -0,0 +1,87 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2026 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+#include "SDL_internal.h"
+
+#ifdef SDL_THREAD_DOS
+
+/* DOS thread management routines for SDL — cooperative threading via
+   the DOS mini-scheduler (no OS-level threads on DOS). */
+
+#include "../../core/dos/SDL_dos_scheduler.h"
+#include "../SDL_systhread.h"
+#include "../SDL_thread_c.h"
+
+static int ThreadEntry(void *arg)
+{
+    SDL_Thread *thread = (SDL_Thread *)arg;
+    SDL_RunThread(thread);
+    return 0;
+}
+
+bool SDL_SYS_CreateThread(SDL_Thread *thread,
+                          SDL_FunctionPointer pfnBeginThread,
+                          SDL_FunctionPointer pfnEndThread)
+{
+    /* Ensure the scheduler is initialized (idempotent) */
+    DOS_SchedulerInit();
+
+    size_t stack_size = thread->stacksize;
+
+    int tid = DOS_CreateThread(ThreadEntry, thread, stack_size);
+    if (tid < 0) {
+        return SDL_SetError("DOS_CreateThread() failed — no free thread slots");
+    }
+
+    thread->handle = tid;
+    return true;
+}
+
+void SDL_SYS_SetupThread(const char *name)
+{
+    /* Nothing to do on DOS */
+}
+
+SDL_ThreadID SDL_GetCurrentThreadID(void)
+{
+    return (SDL_ThreadID)DOS_GetCurrentThreadID();
+}
+
+void SDL_SYS_WaitThread(SDL_Thread *thread)
+{
+    DOS_JoinThread(thread->handle);
+    DOS_DestroyThread(thread->handle);
+}
+
+void SDL_SYS_DetachThread(SDL_Thread *thread)
+{
+    /* For cooperative threads, detach is a no-op. The thread will clean
+       itself up when it finishes. In practice, SDL's thread code handles
+       the detach lifecycle via atomics in SDL_RunThread. */
+}
+
+bool SDL_SYS_SetThreadPriority(SDL_ThreadPriority priority)
+{
+    /* DOS cooperative scheduler uses round-robin — priority is not
+       meaningful. Accept any value without error. */
+    return true;
+}
+
+#endif /* SDL_THREAD_DOS */

src/thread/dos/SDL_systhread_c.h

@@ -0,0 +1,28 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2026 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+
+#ifndef SDL_systhread_c_h_
+#define SDL_systhread_c_h_
+
+/* DOS thread handle is an integer thread ID from the DOS scheduler */
+typedef int SYS_ThreadHandle;
+
+#endif /* SDL_systhread_c_h_ */

src/thread/dos/SDL_systls.c

@@ -0,0 +1,63 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2026 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+#include "SDL_internal.h"
+
+#ifdef SDL_THREAD_DOS
+
+/* DOS thread-local storage — uses per-thread static array indexed by
+   the scheduler's thread ID. With only DOS_MAX_THREADS (8) threads,
+   a simple static array is efficient and avoids any dynamic allocation. */
+
+#include "../../core/dos/SDL_dos_scheduler.h"
+#include "../SDL_thread_c.h"
+
+static SDL_TLSData *tls_data[DOS_MAX_THREADS];
+
+void SDL_SYS_InitTLSData(void)
+{
+    SDL_memset(tls_data, 0, sizeof(tls_data));
+}
+
+SDL_TLSData *SDL_SYS_GetTLSData(void)
+{
+    int tid = DOS_GetCurrentThreadID();
+    if (tid < 0 || tid >= DOS_MAX_THREADS) {
+        return NULL;
+    }
+    return tls_data[tid];
+}
+
+bool SDL_SYS_SetTLSData(SDL_TLSData *data)
+{
+    int tid = DOS_GetCurrentThreadID();
+    if (tid < 0 || tid >= DOS_MAX_THREADS) {
+        return SDL_SetError("Invalid thread ID for TLS");
+    }
+    tls_data[tid] = data;
+    return true;
+}
+
+void SDL_SYS_QuitTLSData(void)
+{
+    SDL_memset(tls_data, 0, sizeof(tls_data));
+}
+
+#endif /* SDL_THREAD_DOS */