SDL/src/core/dos/SDL_dos.c

/*
  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"

#if defined(SDL_PLATFORM_DOS)

#include "SDL_dos.h"

void *DOS_AllocateConventionalMemory(const int len, _go32_dpmi_seginfo *seginfo)
{
    seginfo->size = (len + 15) / 16; // this is in "paragraphs"
    if (_go32_dpmi_allocate_dos_memory(seginfo) != 0) {
        SDL_OutOfMemory();
        return NULL;
    }
    // No need to lock: DPMI 0.9 §3.3 guarantees the first megabyte is always
    // committed and locked. CWSDPMI (the DJGPP DPMI host) doesn't support
    // virtual memory at all, so conventional memory is never paged out.
    return DOS_PhysicalToLinear(seginfo->rm_segment * 16);
}

void *DOS_AllocateDMAMemory(const int len, _go32_dpmi_seginfo *seginfo)
{
    // ISA DMA transfers cannot cross a 64 KB physical page boundary (hardware
    // limitation of the 8237 DMA controller). Allocating 2× the requested size
    // guarantees at least one contiguous `len`-byte region that doesn't straddle
    // a boundary. This is the standard technique used by Allegro, MIDAS, and
    // every other DOS audio library; allocate-check-retry would add complexity
    // for zero benefit.
    uint8_t *ptr = (uint8_t *)DOS_AllocateConventionalMemory(len * 2, seginfo);
    if (!ptr) {
        return NULL;
    }

    // if we're past the end of a page, use the second half of the block.
    const uint32_t physical = (seginfo->rm_segment * 16);
    if ((physical >> 16) != ((physical + len) >> 16)) {
        ptr += len;
    }
    return ptr;
}

void DOS_FreeConventionalMemory(_go32_dpmi_seginfo *seginfo)
{
    _go32_dpmi_free_dos_memory(seginfo);
}

char *DOS_GetFarPtrCString(const Uint32 segoffset)
{
    if (!segoffset) { // let's just treat this as a NULL pointer.
        return NULL;
    }

    const unsigned long ofs = (unsigned long)(((segoffset & 0xFFFF0000) >> 12) + (segoffset & 0xFFFF));
    size_t len;

    for (len = 0; _farpeekb(_dos_ds, ofs + len) != '\0'; len++) {
    }

    len++; // null terminator.
    char *retval = SDL_malloc(len);
    if (!retval) {
        return NULL;
    }

    for (size_t i = 0; i < len; i++) {
        retval[i] = (char)_farpeekb(_dos_ds, ofs + i);
    }
    return retval;
}

void DOS_HookInterrupt(int irq, DOS_InterruptHookFn fn, DOS_InterruptHook *hook)
{
    SDL_assert(irq >= 0 && irq <= 15);
    SDL_assert(fn != NULL);
    SDL_assert(hook != NULL);
    hook->fn = fn;
    hook->irq = irq;
    hook->interrupt_vector = DOS_IRQToVector(irq);
    hook->irq_handler_seginfo.pm_selector = _go32_my_cs();
    hook->irq_handler_seginfo.pm_offset = (uint32_t)fn;
    _go32_dpmi_get_protected_mode_interrupt_vector(hook->interrupt_vector, &hook->original_irq_handler_seginfo);
    _go32_dpmi_chain_protected_mode_interrupt_vector(hook->interrupt_vector, &hook->irq_handler_seginfo);

    // enable interrupt on the correct PIC
    if (irq > 7) {
        outportb(PIC2_DATA, inportb(PIC2_DATA) & ~(1 << (irq - 8))); // unmask on slave PIC
        outportb(PIC1_DATA, inportb(PIC1_DATA) & ~(1 << 2));         // ensure cascade (IRQ2) is unmasked
    } else {
        outportb(PIC1_DATA, inportb(PIC1_DATA) & ~(1 << irq)); // unmask on master PIC
    }
}

void DOS_UnhookInterrupt(DOS_InterruptHook *hook, bool disable_interrupt)
{
    if (!hook || !hook->fn) {
        return;
    }

    SDL_assert(hook->interrupt_vector > 0);
    SDL_assert(hook->irq > 0);

    if (disable_interrupt) {
        if (hook->irq > 7) {
            outportb(PIC2_DATA, inportb(PIC2_DATA) | (1 << (hook->irq - 8))); // mask on slave PIC
        } else {
            outportb(PIC1_DATA, inportb(PIC1_DATA) | (1 << hook->irq)); // mask on master PIC
        }
    }

    _go32_dpmi_set_protected_mode_interrupt_vector(hook->interrupt_vector, &hook->original_irq_handler_seginfo);

    // Note: _go32_dpmi_chain_protected_mode_interrupt_vector internally
    // allocates a wrapper, but it is NOT safe to free it with
    // _go32_dpmi_free_iret_wrapper — that function expects a wrapper
    // allocated by _go32_dpmi_allocate_iret_wrapper, and calling it on
    // a chain-allocated wrapper causes a page fault on exit.  The wrapper
    // is a few bytes of conventional memory that the OS reclaims when the
    // process terminates, so the leak is harmless.

    SDL_zerop(hook);
}

#endif // defined(SDL_PLATFORM_DOS)