Minimal audio implementation

I investigated various implementations of sound in other emulators,
which tended to be a bit tricky. After brainstorming with @ladyada
we hit upon the idea of simply assuming samples are written as
bytes in order and trapping on the write to the RAM address of the
final sample to collect a full 370 sample buffer.

This is good enough to play the system beep. It also works with some
games (dark castle) but not others (glider) and still does not
successfully play samples in hypercard. This may have to do with
the incomplete implementation of the VIA timer2, because debug
messages about timer2 are printed when trying to play sounds in
hypercard.

Makefile

@@ -27,6 +27,7 @@
 
 DEBUG ?= 0
 MEMSIZE ?= 128
+ENABLE_AUDIO ?= 1
 
 SOURCES = $(wildcard src/*.c)
 
@@ -54,7 +55,7 @@ endif
 # Basic support for changing screen res (with the MacPlusV3 ROM)
 DISP_WIDTH ?= 512
 DISP_HEIGHT ?= 342
-CFLAGS_CFG = -DDISP_WIDTH=$(DISP_WIDTH) -DDISP_HEIGHT=$(DISP_HEIGHT)
+CFLAGS_CFG = -DDISP_WIDTH=$(DISP_WIDTH) -DDISP_HEIGHT=$(DISP_HEIGHT) -DENABLE_AUDIO=$(ENABLE_AUDIO)
 
 all:	main
 

include/machw.h

@@ -81,6 +81,8 @@ extern int overlay;
  * But, that should never happen post-boot.
  */
 #define CLAMP_RAM_ADDR(x) ((x) >= RAM_SIZE ? (x) % RAM_SIZE : (x))
+/* Is the address (previously clamped with CLAMP_RAM_ADDR) the audio trap address? (last byte of audio data) */
+#define IS_RAM_AUDIO_TRAP(x) (x == umac_get_audio_offset() + 2 * 369)
 
 #define IS_VIA(x)       ((ADR24(x) & 0xe80000) == 0xe80000)
 #define IS_IWM(x)       ((ADR24(x) >= 0xdfe1ff) && (ADR24(x) < (0xdfe1ff + 0x2000)))

include/umac.h

@@ -53,4 +53,11 @@ static inline unsigned int      umac_get_fb_offset(void)
         return RAM_SIZE - ((DISP_WIDTH * DISP_HEIGHT / 8) + 0x380);
 }
 
+#if ENABLE_AUDIO
+#define umac_get_audio_offset() (RAM_SIZE - 768)
+
+void umac_audio_trap();
+void umac_audio_cfg(int volume, int sndres);
+#endif
+
 #endif

src/main.c

@@ -40,6 +40,7 @@
 #include <errno.h>
 #include <setjmp.h>
 
+#include "umac.h"
 #include "machw.h"
 #include "m68k.h"
 #include "via.h"
@@ -70,6 +71,9 @@ static unsigned int g_int_controller_highest_int = 0;  /* Highest pending interr
 uint8_t *_ram_base;
 uint8_t *_rom_base;
 
+#if ENABLE_AUDIO
+static int umac_volume, umac_sndres;
+#endif
 int overlay = 1;
 static uint64_t global_time_us = 0;
 static int sim_done = 0;
@@ -155,6 +159,14 @@ static void     via_ra_changed(uint8_t val)
                 update_overlay_layout();
         }
 
+#if ENABLE_AUDIO
+        uint8_t vol = val & 7;
+        if (vol != umac_volume) {
+                umac_volume = val & 7;
+                umac_audio_cfg(umac_volume, umac_sndres);
+        }
+#endif
+
         oldval = val;
 }
 
@@ -166,7 +178,15 @@ static void     via_rb_changed(uint8_t val)
         // 4 = mouse4 (in, mouse X2)
         // 3 = mouse7 (in, 0 = button pressed)
         // [2:0] = RTC controls
+#if ENABLE_AUDIO
+        uint8_t sndres = val >> 7;
+        if(sndres != umac_sndres) {
+                umac_sndres = sndres;
+                umac_audio_cfg(umac_volume, umac_sndres);
+        }
+#else
         (void)val;
+#endif
 }
 
 static uint8_t  via_ra_in(void)
@@ -437,7 +457,13 @@ unsigned int    cpu_read_long_dasm(unsigned int address)
 void    FAST_FUNC(cpu_write_byte)(unsigned int address, unsigned int value)
 {
         if (IS_RAM(address)) {
-                RAM_WR8(CLAMP_RAM_ADDR(address), value);
+                address = CLAMP_RAM_ADDR(address);
+                RAM_WR8(address, value);
+#if ENABLE_AUDIO
+                if(IS_RAM_AUDIO_TRAP(address)) {
+                        umac_audio_trap();
+                }
+#endif
                 return;
         }
 

src/rom.c

@@ -116,6 +116,9 @@ static void     rom_patch_plusv3(uint8_t *rom_base)
          * allowing the ROM checksum to fail (it returns failure, then
          * we carry on).  This avoids wild RAM addrs being accessed.
          */
+
+        /* Fix up the sound buffer as used by BootBeep */
+        ROM_WR32(0x292, RAM_SIZE - 768);
 #endif
 
 #if DISP_WIDTH != 512 || DISP_HEIGHT != 342

src/unix_main.c

@@ -27,6 +27,7 @@
  * SOFTWARE.
  */
 
+#include <assert.h>
 #include <stdio.h>
 #include <fcntl.h>
 #include <getopt.h>
@@ -35,6 +36,9 @@
 #include <sys/time.h>
 #include <string.h>
 #include <unistd.h>
+#if ENABLE_AUDIO
+#include <stdatomic.h>
+#endif
 #include "SDL.h"
 
 #include "rom.h"
@@ -78,6 +82,42 @@ static void     copy_fb(uint32_t *fb_out, uint8_t *fb_in)
         }
 }
 
+#if ENABLE_AUDIO
+// audio_callback is called from a thread, so pending_v_retrace can't be a regular variable
+atomic_int pending_v_retrace;
+static int volscale;
+uint8_t *audio_base;
+
+int16_t audio[370];
+
+void umac_audio_cfg(int umac_volume, int umac_sndres) {
+        volscale = umac_sndres ? 0 : 65536 * umac_volume / 7;
+}
+
+void umac_audio_trap() {
+        int32_t  offset = 128;
+        uint16_t *audiodata = (uint16_t*)audio_base;
+        int scale = volscale;
+        if (!scale) {
+                memset(audio, 0, sizeof(audio));
+                return;
+        }
+        int16_t *stream = audio;
+        for(int i=0; i<370; i++) {
+                int32_t a = (*audiodata++ & 0xff) - offset;
+                a = (a * scale) >> 8;
+                *stream++ = a;
+        }
+}
+
+static void audio_callback(void *userdata, Uint8 *stream_in, int len_bytes) {
+        (void) userdata;
+        assert(len_bytes == sizeof(audio));
+        atomic_store(&pending_v_retrace, 1);
+        memcpy(stream_in, audio, sizeof(audio));
+}
+#endif
+
 /**********************************************************************/
 
 /* The emulator core expects to be given ROM and RAM pointers,
@@ -232,7 +272,11 @@ int     main(int argc, char *argv[])
         SDL_Renderer *renderer;
         SDL_Texture *texture;
 
-        SDL_Init(SDL_INIT_VIDEO);
+        SDL_Init(SDL_INIT_VIDEO
+#if ENABLE_AUDIO
+                | SDL_INIT_AUDIO
+#endif
+);
         SDL_Window *window = SDL_CreateWindow("umac",
                                               SDL_WINDOWPOS_UNDEFINED,
                                               SDL_WINDOWPOS_UNDEFINED,
@@ -264,18 +308,47 @@ int     main(int argc, char *argv[])
                 return 1;
         }
 
+#if ENABLE_AUDIO
+        SDL_AudioSpec desired, obtained;
+
+        audio_base = (uint8_t*)ram_base + umac_get_audio_offset();
+
+        SDL_zero(desired);
+        desired.freq = 22256; // wat
+        desired.channels = 1;
+        desired.samples = 370;
+        desired.userdata = (uint8_t*)ram_base + umac_get_audio_offset();
+        desired.callback = audio_callback;
+        desired.format = AUDIO_S16;
+
+        SDL_AudioDeviceID audio_device = SDL_OpenAudioDevice(NULL, 0, &desired, &obtained, 0);
+        if (!audio_device) {
+                char buf[500];
+                SDL_GetErrorMsg(buf, sizeof(buf));
+                printf("SDL audio_deviceSDL_GetError() -> %s\n", buf);
+                return 1;
+        }
+#endif
+
         ////////////////////////////////////////////////////////////////////////
         // Emulator init
 
         umac_init(ram_base, rom_base, discs);
         umac_opt_disassemble(opt_disassemble);
 
+#if ENABLE_AUDIO
+        // Default state is paused, this unpauses it
+        SDL_PauseAudioDevice(audio_device, 0);
+#endif
+
         ////////////////////////////////////////////////////////////////////////
         // Main loop
 
         int done = 0;
         int mouse_button = 0;
+#if !ENABLE_AUDIO
         uint64_t last_vsync = 0;
+#endif
         uint64_t last_1hz = 0;
         do {
                 struct timeval tv_now;
@@ -321,11 +394,14 @@ int     main(int argc, char *argv[])
                 uint64_t now_usec = (tv_now.tv_sec * 1000000) + tv_now.tv_usec;
 
                 /* Passage of time: */
-                if ((now_usec - last_vsync) >= 16667) {
+#if ENABLE_AUDIO
+                int do_v_retrace = atomic_exchange(&pending_v_retrace, 0);
+#else
+                int do_v_retrace = (now_usec - last_vsync) >= 16667;
+#endif
+                if (do_v_retrace) {
                         umac_vsync_event();
-                        last_vsync = now_usec;
 
-                        /* Cheapo framerate limiting: */
                         copy_fb(framebuffer, ram_get_base() + umac_get_fb_offset());
                         SDL_UpdateTexture(texture, NULL, framebuffer,
                                           DISP_WIDTH * sizeof (Uint32));