PIT: add alternative faster PIT

This is enabled by default on 486+ CPUs and can be forced disabled/enabled with pit_mode=0/1

src/include/86box/pit_fast.h

@@ -0,0 +1,72 @@
+/*
+ * 86Box	A hypervisor and IBM PC system emulator that specializes in
+ *		running old operating systems and software designed for IBM
+ *		PC systems and compatibles from 1981 through fairly recent
+ *		system designs based on the PCI bus.
+ *
+ *		This file is part of the 86Box distribution.
+ *
+ *		Header of the implementation of the Intel 8253/8254
+ *		Programmable Interval Timer.
+ *
+ *
+ *
+ * Author:	Miran Grca, <mgrca8@gmail.com>
+ *		Copyright 2019,2020 Miran Grca.
+ */
+
+#ifndef EMU_PIT_FAST_H
+#define EMU_PIT_FAST_H
+
+typedef struct {
+    uint8_t m, ctrl,
+        read_status, latch, bcd;
+
+    uint16_t rl;
+
+    int rm, wm, gate, out,
+        newcount, clock, using_timer, latched,
+        do_read_status;
+    int enabled;
+    int disabled;
+    int initial;
+    int thit;
+    int running;
+    int rereadlatch;
+
+    union {
+        int count;
+        struct {
+            int units     : 4;
+            int tens      : 4;
+            int hundreds  : 4;
+            int thousands : 4;
+            int myriads   : 4;
+        };
+    };
+
+    uint32_t   l;
+    pc_timer_t timer;
+
+    void (*load_func)(uint8_t new_m, int new_count);
+    void (*out_func)(int new_out, int old_out);
+} ctrf_t;
+
+typedef struct {
+    int    flags;
+    ctrf_t counters[3];
+
+    uint8_t ctrl;
+} pitf_t;
+
+extern const pit_intf_t pit_fast_intf;
+
+#ifdef EMU_DEVICE_H
+extern const device_t i8253_fast_device;
+extern const device_t i8254_fast_device;
+extern const device_t i8254_sec_fast_device;
+extern const device_t i8254_ext_io_fast_device;
+extern const device_t i8254_ps2_fast_device;
+#endif
+
+#endif	/*EMU_PIT_FAST_H*/