fix modplay.asm and re-enable bg music. it actually works now!

i didn't recognize _getds from anywhere so assumed it was something
commented out for a reason, but turns out that it was part of the
original modplayer code (which was originally written for watcom
anyway). _getds is an internal function from the watcom standard
libraries.

fdemo/FDEMO.C

@@ -1587,7 +1587,7 @@ StrAmount = MAX_STR_AMOUNT;
 AckSetupKeyboard();
 AckSetupTimer();
 
-//StartBGmusic();
+StartBGmusic();
 
 // Switch to mode 13
 AckSetVGAmode();
@@ -2063,7 +2063,7 @@ while (!done)
     }
     }
 
-//EndBGmusic();
+EndBGmusic();
 ShutDownFlag = 1;
 AckWrapUp(ae);
 AckSetTextmode();

fdemo/MODPLAY.ASM

@@ -21,6 +21,7 @@ global  MODPlayVoice:near
 global  MODStopVoice:near
 global  MODSetPeriod:near
 global  MODSetVolume:near
+global  _GETDS:near
 
 ;±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±
 ; EQUATES
@@ -1132,6 +1133,10 @@ IRQSetVect:
 ;°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°
 IRQHandler:
     pushad              ; pushes all the registers
+    push    ds
+    mov ax,DGROUP       ; load the DS selector
+    mov ds,ax
+    call    _GETDS
     mov dx,[IOAddr]     ; send ack to the SB DSP chip
         add     dx,0Eh
         in      al,dx
@@ -1150,6 +1155,7 @@ IRQHandler:
         out     0A0h,al
 IRQAckPIC:
         out     20h,al
+        pop     ds
         popad                           ; restores all the registers
         iretd