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/video.c

@@ -31,6 +31,7 @@
 #include "vid_s3.h"
 #include "vid_s3_virge.h"
 #include "vid_tandy.h"
+#include "vid_tandysl.h"
 #include "vid_tgui9440.h"
 #include "vid_tvga.h"
 #include "vid_vga.h"
@@ -223,9 +224,14 @@ void video_init()
                 return;
                 
                 case ROM_TANDY:
+                case ROM_TANDY1000HX:
                 device_add(&tandy_device);
                 return;
 
+                case ROM_TANDY1000SL2:
+                device_add(&tandysl_device);
+                return;
+
                 case ROM_PC1512:
                 device_add(&pc1512_device);
                 return;