Added support for Tandy 1000 HX and SL/2. As part of this commit :

- FDC now selects drive number from DOR (0x3f2)
- FDC now fails seeks on drives > 1
- XT systems no longer have BIOS mapping at E0000-EFFFF
- SN76489 now distinguishes between SN76496 (PCjr and early Tandy 1000), NCR8496 (later Tandy 1000 - different noise handling) and PSSJ
- Removed redundant limits on memory configuration - allows selection of 768k for Tandy 1000 SL/2

src/sound_sn76489.h

@@ -1,3 +1,31 @@
+enum
+{
+        SN76496,
+        NCR8496,
+        PSSJ
+};
+
 extern device_t sn76489_device;
+extern device_t ncr8496_device;
 
 extern int sn76489_mute;
+
+typedef struct sn76489_t
+{
+        int stat[4];
+        int latch[4], count[4];
+        int freqlo[4], freqhi[4];
+        int vol[4];
+        uint32_t shift;
+        uint8_t noise;
+        int lasttone;
+        uint8_t firstdat;
+        int type;
+        int extra_divide;
+        
+        int16_t buffer[SOUNDBUFLEN];
+        int pos;
+} sn76489_t;
+
+void sn76489_init(sn76489_t *sn76489, uint16_t base, uint16_t size, int type);
+void sn74689_set_extra_divide(sn76489_t *sn76489, int enable);