stos/amd64: isa: Add legacy i8254 driver

Signed-off-by: Chloe M. <chloe@mensia.org>

paw/stos/arch/amd64/platform/i8254.c

@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 2026, Chloe M.
+ * Provided under the BSD-3 clause.
+ *
+ * Description:  i8254 timer driver
+ * Author:       Chloe M.
+ */
+
+#include <machine/i8254.h>
+#include <machine/pio.h>
+
+USHORT
+MdPitGetCount(VOID)
+{
+    UCHAR Low, High;
+
+    MdOutb(I8254_COMMAND, 0x00);
+    Low = MdInb(0x40);
+    High = MdInb(0x40);
+    return (High << 8) | Low;
+}
+
+VOID
+MdPitSetCount(USHORT Value)
+{
+    MdOutb(I8254_COMMAND, 0x34);
+    MdOutb(0x40, Value & 0xFF);
+    MdOutb(0x40, (Value >> 8) & 0xFF);
+}
+
+VOID
+MdPitSetFrequency(UQUAD Frequency)
+{
+    UQUAD Divisor;
+
+    Divisor = I8254_DIVIDEND / Frequency;
+    if ((I8254_DIVIDEND % Frequency) > (Frequency / 2)) {
+        ++Divisor;
+    }
+
+    MdPitSetCount(Divisor);
+}

paw/stos/head/arch/amd64/i8254.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2026, Chloe M.
+ * Provided under the BSD-3 clause.
+ *
+ * Description:  i8254 timer driver
+ * Author:       Chloe M.
+ */
+
+#ifndef _MACHINE_I8254_H_
+#define _MACHINE_I8254_H_ 1
+
+#include <stdef.h>
+
+#define I8254_COMMAND     0x43
+#define I8254_CHANNEL_0   0x40
+#define I8254_CHANNEL_2   0x42
+#define I8254_DIVIDEND    1193182ULL
+
+/*
+ * Obtain the current counter value
+ */
+USHORT MdPitGetCount(VOID);
+
+/*
+ * Set a new counter value
+ */
+VOID MdPitSetCount(USHORT Value);
+
+/*
+ * Set the PIT counter to match a frequency in Hz
+ *
+ * @Frequency: Frequency in HZ to set
+ */
+VOID MdPitSetFrequency(UQUAD Frequency);
+
+#endif  /* !_MACHINE_I8254_H_ */