[MS-DOS] Preliminary audio support using PC-Speaker

platforms/msdos/Makefile

@@ -1,8 +1,10 @@
 CC := ia16-elf-gcc
-SOURCES := extras.c video.c input.c ../../common/main.c ../../common/minefield.c ../../common/video-tiles.c
+
+# extras.c implements _start and must be the first one here.
+SOURCES := extras.c audio.c video.c input.c ../../common/main.c ../../common/minefield.c ../../common/video-tiles.c
 BUILDDIR := build
-CFLAGS := -nodefaultlibs -nostdlib -mcmodel=tiny -mregparmcall -march=i186 -Os -flto -Wall -pedantic -I../../common -I. -DNO_UNUSED_MACRO
-LDFLAGS := -flto
+CFLAGS :=  -nodefaultlibs -nostdlib -mcmodel=tiny -mregparmcall -march=i186 -Os -Wall -I../../common -I. -DNO_UNUSED_MACRO
+LDFLAGS :=
 SCREEN_RESOLUTION := -DSCREEN_WIDTH=320/8 -DSCREEN_HEIGHT=200/8 -DMINEFIELD_X_OFFSET=10 -DMINEFIELD_Y_OFFSET=2
 
 PYTHON := python3

platforms/msdos/audio.c

@@ -0,0 +1,102 @@
+#include <stdint.h>
+#include <stdlib.h>
+
+static union {
+    uint32_t dword;
+    struct {
+        uint16_t low_word, high_word;
+    };
+} orig_bios_irq0_isr;
+static uint16_t sample_rate_counter;
+
+static inline void outb(uint16_t port, uint8_t value)
+{
+    asm __volatile("outb %0, %1" : : "Ral"(value), "Nd"(port));
+}
+
+static inline uint8_t inb(uint16_t port)
+{
+    uint8_t value;
+    asm __volatile("inb %1, %0" : "=Ral"(value) : "Nd"(port));
+    return value;
+}
+
+static inline void audio_set_sample_rate(const uint16_t sample_rate)
+{
+    uint32_t corrected = sample_rate ? (0x1234dc / (uint32_t)sample_rate) : 0;
+
+    sample_rate_counter = corrected & 0xfffe;
+
+    asm __volatile("cli");
+    outb(0x43, 0x36);
+    outb(0x40, sample_rate_counter & 0xff);
+    outb(0x40, sample_rate_counter >> 8);
+    asm __volatile("sti");
+}
+
+static inline void audio_play_sample(uint8_t sample)
+{
+    outb(0x43, 0xb0);
+    outb(0x42, sample >> 1);
+    outb(0x42, 0);
+}
+
+__attribute__((used)) static void audio_tick(void)
+{
+    static uint16_t tick;
+
+    audio_play_sample(rand() & 255);
+
+    tick += sample_rate_counter;
+
+    if (sample_rate_counter > tick) {
+        asm __volatile("pushf");
+        asm __volatile("lcall *(%0)" : : "b"(&orig_bios_irq0_isr));
+    } else {
+        outb(0x20, 0x20); /* acknowledge IRQ */
+    }
+}
+
+void audio_tick_isr(void);
+__asm__(".global audio_tick_isr\n"
+        "audio_tick_isr:\n"
+        "pusha\n"
+        "call audio_tick\n"
+        "popa\n"
+        "iret\n");
+
+void audio_init(void)
+{
+    /* Save original BIOS IRQ0 handler */
+    asm __volatile("int $0x21\n"
+                   : "=b"(orig_bios_irq0_isr.low_word),
+                     "=e"(orig_bios_irq0_isr.high_word)
+                   : "a"(0x3508)
+                   : "cc", "memory");
+
+    /* Replace it with ours */
+    asm __volatile("int $0x21"
+                   :
+                   : "d"(audio_tick_isr), "a"(0x2508)
+                   : "cc", "memory");
+
+    /* Connect speaker to PIT channel 2 */
+    outb(0x61, inb(0x61) | 0x3);
+
+    /* Reprogram PIT Channel 0 to fire IRQ0 at 16KHz */
+    audio_set_sample_rate(16000);
+}
+
+void audio_shutdown(void)
+{
+    audio_set_sample_rate(0);
+
+    /* Restore original BIOS IRQ0 routine */
+    asm __volatile("int $0x21\n"
+                   :
+                   : "Rds"(orig_bios_irq0_isr.high_word),
+                     "d"(orig_bios_irq0_isr.low_word), "a"(0x2508)
+                   : "cc", "memory");
+
+    outb(0x61, inb(0x61) & 0xfc);
+}

platforms/msdos/extras.c

@@ -36,15 +36,21 @@ time_t time(time_t *t)
 
 void *malloc(size_t v)
 {
-    static char heap[4096];
+    static char heap[5000];
     static char *heap_ptr = heap;
     char *old_heap_ptr;
 
-    old_heap_ptr = heap_ptr;
-    heap_ptr += v;
+    if (sizeof(heap) - (heap_ptr - heap) < v) {
+        asm __volatile("int $16\n"
+                       "jmp exit"
+                       :
+                       : "a"((uint16_t)0));
 
-    if (heap_ptr > heap + sizeof(heap))
         return NULL;
+    }
+
+    old_heap_ptr = heap_ptr;
+    heap_ptr += v;
 
     return old_heap_ptr;
 }

platforms/msdos/video.c

@@ -207,7 +207,7 @@ static inline void set_tile_full(uint8_t dst_x, uint8_t dst_y, uint8_t tile, int
 {
     static uint8_t __far *video_seg = (uint8_t __far *)0xa0000000;
     uint8_t __far *video_seg_start =
-        &video_seg[320 * (int)dst_y * 8 + (int)dst_x * 8];
+        &video_seg[320 * (unsigned int)dst_y * 8 + (unsigned int)dst_x * 8];
     const uint16_t offs = get_tile_offset(tile);
     const uint16_t off_high = 8 * (offs >> 4);
     const uint16_t off_low = 8 * (offs & 0xf);
@@ -261,18 +261,16 @@ void highlight_current_cell(minefield *mf)
     old_y = y;
 }
 
-
 #ifdef USE_DEBUG_MODE
-/* Thanks to OSDev Wiki for the serial initialization sequence! */
-
-#define SERIAL_PORT_ADDR 0x3f8
 static inline uint8_t inb(uint16_t port)
 {
     uint8_t value;
     asm __volatile("inb %1, %0" : "=Ral"(value) : "Nd"(port));
     return value;
 }
 
+/* Thanks to OSDev Wiki for the serial initialization sequence! */
+#define SERIAL_PORT_ADDR 0x3f8
 
 static void serial_debug_init(void)
 {
@@ -336,6 +334,7 @@ void debug_mode(uint8_t mode)
 #endif
 
 
+
 void platform_init()
 {
 #ifdef USE_DEBUG_MODE
@@ -346,6 +345,9 @@ void platform_init()
     srand(time(NULL));
 
     video_init();
+
+    void audio_init(void);
+    audio_init();
 }
 
 
@@ -356,6 +358,11 @@ void platform_shutdown()
 {
     free(mines_xpm[0]);
     free(mines_xpm);
+
     set_mode(VGA_TEXT_MODE);
+
+    void audio_shutdown(void);
+    audio_shutdown();
+
     exit(0);
 }