
 Introduction

 Genesis Plus has reasonable speed, and very fast rendering. Here are some
 details about porting it to another platform

 Porting rules

 - This program is released under the GPL, so please release the source
   code for your derivative ports. Let me know if you need an alternative
   to this.

 - Do not change the name of the emulator. Something like 'Genesis Plus / MacOS'
   is fine as it keeps the original name.

 - Configure the CPU emulators for little or big-endian CPUs, and use the
   ALIGN_LONG option to handle unaligned memory accesses if needed. E.g. if
   the emulator crashes when a game scrolls horizontally, you need to enable
   it.

 Requirements

 - At this time the 16-bit rendering code has been well tested and should
   be used. There is code to support 8 through 32-bit displays, but I
   haven't checked if it still works.

 - Audio output, if enabled, uses stereo 16-bit samples.

 Functions to use:

 int load_rom(char *filename);

 Loads a game which can be in BIN, SMD or ZIP formats. It returns zero if
 an error has occured.

 void system_init(void);

 Initializes the virtual Genesis console. Call this once during startup.

 void audio_init(int rate);

 Initialize the sound emulation part of the emulator. Pass zero or don't
 call the function to disable sound. Call this after running system_init().

 void system_reset(void);

 Resets the virtual Genesis console. Call this once during setup, and later
 as needed.

 int system_frame(int skip);

 Updates the emulation for a frame. Pass 1 to prevent rendering (which can
 be used for frame skipping) and 0 to render the current display.

 This function returns 0 if the virtual Genesis console has locked up.
 You can do what you'd like with this information, such as notify the user,
 reset the machine, etc.

 If audio is enabled, the snd.buffer[] array will hold the current audio
 data for this frame.

 void system_shutdown(void);

 Shuts down the emulator.

 Variables:

 The 'bitmap' structure needs to be filled out prior to calling system_init().
 This provides the rendering code with information about the type of bitmap
 it is rendering to. I would advise sticking with a 1024x512 16-bit bitmap:

    memset(&bitmap, 0, sizeof(t_bitmap));
    bitmap.width  = 1024;
    bitmap.height = 512;
    bitmap.depth  = 16;
    bitmap.granularity = 2; /* Bytes per pixel */
    bitmap.pitch = (bitmap.width * bitmap.granularity);
    bitmap.data   = (unsigned char *)bmp->pixels; /* Pointer to your bitmap */
    bitmap.viewport.w = 256; /* Initial size of display on power-up (do not change) */
    bitmap.viewport.h = 224;
    bitmap.viewport.x = 0x20; /* Offset used for rendering (do not change) */
    bitmap.viewport.y = 0x00;
    bitmap.remap = 1;

 The variable 'input.pad[0]' holds the button states for the first player
 gamepad. Update this prior to calling system_frame(), and use the bitmasks
 defined in system.h such as 'INPUT_UP', etc.

 See the Windows/SDL and DOS drivers for examples.
