Fixed OPL chip detection.

- Increment timer counter before checking for overflow.
- Implemented ISA bus delay in the pc_system code and use it for the SB16.
  This makes OPL chip detection work at ips values > 4 Mips.
TODO: OPL chip should use wave output instead of MIDI (maybe port from DOSBox).
TODO: Use isa_bus_delay() for other legacy devices if required.

bochs/iodev/sound/sb16.cc

@@ -2428,7 +2428,7 @@ void bx_sb16_c::opl_timerevent()
       } else {
         mask = 0x3ff;
       }
-      if (((OPL.timer[i]++) & mask) == 0) { // overflow occured, set flags accordingly
+      if (((++OPL.timer[i]) & mask) == 0) { // overflow occured, set flags accordingly
         OPL.timer[i] = OPL.timerinit[i];      // reset the counter
         if ((OPL.tmask[i/2] >> (6 - (i % 2))) == 0) { // set flags only if unmasked
           writelog(MIDILOG(5), "OPL Timer Interrupt: Chip %d, Timer %d", i/2, 1 << (i % 2));
@@ -3372,8 +3372,9 @@ Bit32u bx_sb16_c::read(Bit32u address, unsigned io_len)
   UNUSED(this_ptr);
 #endif  // !BX_USE_SB16_SMF
 
-  switch (address)
-  {
+  bx_pc_system.isa_bus_delay();
+
+  switch (address) {
     // 2x0: FM Music Status Port
     // 2x8 and 388 are aliases
     case BX_SB16_IO + 0x00:
@@ -3488,8 +3489,9 @@ void bx_sb16_c::write(Bit32u address, Bit32u value, unsigned io_len)
   UNUSED(this_ptr);
 #endif  // !BX_USE_SB16_SMF
 
-  switch (address)
-  {
+  bx_pc_system.isa_bus_delay();
+
+  switch (address) {
     // 2x0: FM Music Register Port
     // 2x8 and 388 are aliases
     case BX_SB16_IO + 0x00:

bochs/pc_system.cc

@@ -582,3 +582,11 @@ void bx_pc_system_c::setTimerParam(unsigned timerIndex, Bit32u param)
 #endif
   timer[timerIndex].param = param;
 }
+
+void bx_pc_system_c::isa_bus_delay(void)
+{
+  // Emulate 8 MHz ISA bus speed
+  if (m_ips > 4.0) {
+    tickn((Bit32u)(m_ips * 2.0));
+  }
+}

bochs/pc_system.h

@@ -140,6 +140,7 @@ public:
 #if BX_ENABLE_STATISTICS
   static void dumpStatsTimer(void* this_ptr);
 #endif
+  void isa_bus_delay(void);
 
   // ===========================
   // Non-timer oriented features