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

This uses the PIT to control the PC speaker frequency, and play a
random sample in the ISR.  For some reason, this works fine as long
as we don't get into graphical mode.  I don't yet know why.

Author: lpereira

platforms/msdos/Makefile

@@ -1,7 +1,7 @@
 CC := ia16-elf-gcc
 SOURCES := extras.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
+CFLAGS := -nodefaultlibs -nostdlib -mcmodel=tiny -mregparmcall  -march=i186 -Os -flto -Wall -I../../common -I. -DNO_UNUSED_MACRO
 LDFLAGS := -flto
 SCREEN_RESOLUTION := -DSCREEN_WIDTH=320/8 -DSCREEN_HEIGHT=200/8 -DMINEFIELD_X_OFFSET=10 -DMINEFIELD_Y_OFFSET=2
 

platforms/msdos/video.c

@@ -260,18 +260,16 @@ 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;
 }
 
+#ifdef USE_DEBUG_MODE
+/* Thanks to OSDev Wiki for the serial initialization sequence! */
+#define SERIAL_PORT_ADDR 0x3f8
 
 static void serial_debug_init(void)
 {
@@ -334,6 +332,92 @@ void debug_mode(uint8_t mode)
 }
 #endif
 
+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 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);
+    outb(0x42, 0);
+}
+
+__attribute__((used)) static void audio_tick(void)
+{
+    static uint16_t tick;
+    uint8_t sample = rand() & 255;
+
+    outb(0x43, 0xb0);
+    outb(0x42, sample >> 1);
+    outb(0x42, 0);
+
+    uint32_t v = (uint32_t)tick + sample_rate_counter;
+    tick = (uint16_t)v;
+
+    if ((uint32_t)tick != v) {
+        asm __volatile("pushf");
+        void (*far_void_func)(void) __far =
+            (void (*)(void) __far)orig_bios_irq0_isr.dword;
+        far_void_func();
+    } else {
+        outb(0x20, 0x20); /* acknowledge IRQ */
+    }
+}
+
+extern void audio_tick_isr(void);
+__asm__(".global audio_tick_isr\n"
+        "audio_tick_isr:\n"
+        "call audio_tick\n"
+        "iret\n");
+
+static inline 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));
+
+    /* Replace it with ours */
+    asm __volatile("int $0x21" : : "d"(audio_tick_isr), "a"(0x2508));
+
+    /* 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);
+}
+
+static inline void audio_shutdown(void)
+{
+    audio_set_sample_rate(0);
+
+    /* Restore original BIOS IRQ0 routine */
+    asm __volatile("int $0x21\n"
+                   :
+                   : "Rds"(orig_bios_irq0_isr.low_word),
+                     "d"(orig_bios_irq0_isr.high_word), "a"(0x2508));
+
+    outb(0x61, inb(0x61) & 0xfc);
+}
 
 void platform_init()
 {
@@ -345,6 +429,8 @@ void platform_init()
     srand(time(NULL));
 
     video_init();
+    audio_init();
+
     draw_scenario();
 }
 
@@ -357,5 +443,6 @@ void platform_shutdown()
     free(mines_xpm[0]);
     free(mines_xpm);
     set_mode(VGA_TEXT_MODE);
+    audio_shutdown();
     exit(0);
 }