Speed up readbyte()

Check the address first and only test for the peripherals if the address
is in the relevant range.

ChangeLog

@@ -2,6 +2,9 @@
 
         * Fuse 1.?.? released.
 
+        * Emulation core improvements:
+          * Speed up reading emulated memory locations (Jindřich Makovička).
+
         * UI improvements:
          * Timex: don't abort if the Timex dock cartridge image is not
            found (Alberto Garcia).

memory_pages.c

@@ -385,19 +385,21 @@ readbyte( libspectrum_word address )
   if( mapping->contended ) tstates += ula_contention[ tstates ];
   tstates += 3;
 
-  if( opus_active && address >= 0x2800 && address < 0x3800 )
-    return opus_read( address );
+  if( address < 0x4000 ) {
+    if( opus_active && address >= 0x2800 && address < 0x3800 )
+      return opus_read( address );
+
+    if( spectranet_paged ) {
+      if( spectranet_w5100_paged_a && address >= 0x1000 && address < 0x2000 )
+        return spectranet_w5100_read( mapping, address );
+      if( spectranet_w5100_paged_b && address >= 0x2000 && address < 0x3000 )
+        return spectranet_w5100_read( mapping, address );
+    }
 
-  if( spectranet_paged ) {
-    if( spectranet_w5100_paged_a && address >= 0x1000 && address < 0x2000 )
-      return spectranet_w5100_read( mapping, address );
-    if( spectranet_w5100_paged_b && address >= 0x2000 && address < 0x3000 )
-      return spectranet_w5100_read( mapping, address );
-  }
-
-  if( ttx2000s_paged ) {
-    if( address >= 0x2000 && address < 0x4000 )
-      return ttx2000s_sram_read( address );
+    if( ttx2000s_paged ) {
+      if( address >= 0x2000 && address < 0x4000 )
+        return ttx2000s_sram_read( address );
+    }
   }
 
   return mapping->page[ address & MEMORY_PAGE_SIZE_MASK ];