Atlas - SDL_dos.c
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1/*
2  Simple DirectMedia Layer
3  Copyright (C) 1997-2026 Sam Lantinga <[email protected]>
4
5  This software is provided 'as-is', without any express or implied
6  warranty.  In no event will the authors be held liable for any damages
7  arising from the use of this software.
8
9  Permission is granted to anyone to use this software for any purpose,
10  including commercial applications, and to alter it and redistribute it
11  freely, subject to the following restrictions:
12
13  1. The origin of this software must not be misrepresented; you must not
14     claim that you wrote the original software. If you use this software
15     in a product, an acknowledgment in the product documentation would be
16     appreciated but is not required.
17  2. Altered source versions must be plainly marked as such, and must not be
18     misrepresented as being the original software.
19  3. This notice may not be removed or altered from any source distribution.
20*/
21#include "SDL_internal.h"
22
23#if defined(SDL_PLATFORM_DOS)
24
25#include "SDL_dos.h"
26
27void *DOS_AllocateConventionalMemory(const int len, _go32_dpmi_seginfo *seginfo)
28{
29    seginfo->size = (len + 15) / 16; // this is in "paragraphs"
30    if (_go32_dpmi_allocate_dos_memory(seginfo) != 0) {
31        SDL_OutOfMemory();
32        return NULL;
33    }
34    // No need to lock: DPMI 0.9 §3.3 guarantees the first megabyte is always
35    // committed and locked. CWSDPMI (the DJGPP DPMI host) doesn't support
36    // virtual memory at all, so conventional memory is never paged out.
37    return DOS_PhysicalToLinear(seginfo->rm_segment * 16);
38}
39
40void *DOS_AllocateDMAMemory(const int len, _go32_dpmi_seginfo *seginfo)
41{
42    // ISA DMA transfers cannot cross a 64 KB physical page boundary (hardware
43    // limitation of the 8237 DMA controller). Allocating 2× the requested size
44    // guarantees at least one contiguous `len`-byte region that doesn't straddle
45    // a boundary. This is the standard technique used by Allegro, MIDAS, and
46    // every other DOS audio library; allocate-check-retry would add complexity
47    // for zero benefit.
48    uint8_t *ptr = (uint8_t *)DOS_AllocateConventionalMemory(len * 2, seginfo);
49    if (!ptr) {
50        return NULL;
51    }
52
53    // if we're past the end of a page, use the second half of the block.
54    const uint32_t physical = (seginfo->rm_segment * 16);
55    if ((physical >> 16) != ((physical + len) >> 16)) {
56        ptr += len;
57    }
58    return ptr;
59}
60
61void DOS_FreeConventionalMemory(_go32_dpmi_seginfo *seginfo)
62{
63    _go32_dpmi_free_dos_memory(seginfo);
64}
65
66char *DOS_GetFarPtrCString(const Uint32 segoffset)
67{
68    if (!segoffset) { // let's just treat this as a NULL pointer.
69        return NULL;
70    }
71
72    const unsigned long ofs = (unsigned long)(((segoffset & 0xFFFF0000) >> 12) + (segoffset & 0xFFFF));
73    size_t len;
74
75    for (len = 0; _farpeekb(_dos_ds, ofs + len) != '\0'; len++) {
76    }
77
78    len++; // null terminator.
79    char *retval = SDL_malloc(len);
80    if (!retval) {
81        return NULL;
82    }
83
84    for (size_t i = 0; i < len; i++) {
85        retval[i] = (char)_farpeekb(_dos_ds, ofs + i);
86    }
87    return retval;
88}
89
90void DOS_HookInterrupt(int irq, DOS_InterruptHookFn fn, DOS_InterruptHook *hook)
91{
92    SDL_assert(irq >= 0 && irq <= 15);
93    SDL_assert(fn != NULL);
94    SDL_assert(hook != NULL);
95    hook->fn = fn;
96    hook->irq = irq;
97    hook->interrupt_vector = DOS_IRQToVector(irq);
98    hook->irq_handler_seginfo.pm_selector = _go32_my_cs();
99    hook->irq_handler_seginfo.pm_offset = (uint32_t)fn;
100    _go32_dpmi_get_protected_mode_interrupt_vector(hook->interrupt_vector, &hook->original_irq_handler_seginfo);
101    _go32_dpmi_chain_protected_mode_interrupt_vector(hook->interrupt_vector, &hook->irq_handler_seginfo);
102
103    // enable interrupt on the correct PIC
104    if (irq > 7) {
105        outportb(PIC2_DATA, inportb(PIC2_DATA) & ~(1 << (irq - 8))); // unmask on slave PIC
106        outportb(PIC1_DATA, inportb(PIC1_DATA) & ~(1 << 2));         // ensure cascade (IRQ2) is unmasked
107    } else {
108        outportb(PIC1_DATA, inportb(PIC1_DATA) & ~(1 << irq)); // unmask on master PIC
109    }
110}
111
112void DOS_UnhookInterrupt(DOS_InterruptHook *hook, bool disable_interrupt)
113{
114    if (!hook || !hook->fn) {
115        return;
116    }
117
118    SDL_assert(hook->interrupt_vector > 0);
119    SDL_assert(hook->irq > 0);
120
121    if (disable_interrupt) {
122        if (hook->irq > 7) {
123            outportb(PIC2_DATA, inportb(PIC2_DATA) | (1 << (hook->irq - 8))); // mask on slave PIC
124        } else {
125            outportb(PIC1_DATA, inportb(PIC1_DATA) | (1 << hook->irq)); // mask on master PIC
126        }
127    }
128
129    _go32_dpmi_set_protected_mode_interrupt_vector(hook->interrupt_vector, &hook->original_irq_handler_seginfo);
130
131    // Note: _go32_dpmi_chain_protected_mode_interrupt_vector internally
132    // allocates a wrapper, but it is NOT safe to free it with
133    // _go32_dpmi_free_iret_wrapper — that function expects a wrapper
134    // allocated by _go32_dpmi_allocate_iret_wrapper, and calling it on
135    // a chain-allocated wrapper causes a page fault on exit.  The wrapper
136    // is a few bytes of conventional memory that the OS reclaims when the
137    // process terminates, so the leak is harmless.
138
139    SDL_zerop(hook);
140}
141
142#endif // defined(SDL_PLATFORM_DOS)
143
[FILE END]
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