Add initial struct and drive code

include/apple2.disk_drive.h

@@ -0,0 +1,69 @@
+#ifndef _APPLE2_DISK_DRIVE_H
+#define _APPLE2_DISK_DRIVE_H
+
+/*
+ * These are the possible modes a drive can be in.
+ */
+enum apple2_disk_drive_mode {
+    DD_READ,
+    DD_WRITE,
+};
+
+#define MAX_DRIVE_STEPS 70
+
+typedef struct {
+    /*
+     * Disk II drives allow the stepper to move in half-tracks, so we
+     * track (pun intended) the position of the head in those
+     * half-tracks rather than in full tracks.
+     *
+     * For example, if track_pos is 4, then the effective track is 2. If
+     * the track_pos is 9, then the effective track is 4, except that
+     * the head is on the half-track position.
+     *
+     * There are, at most, 35 tracks in a conventional disk, so there
+     * would be at most 70 track positions that we can iterate to.
+     */
+    int track_pos;
+
+    /*
+     * The data field is where the actual byte data for the image is
+     * kept.
+     */
+    vm_segment *data;
+
+    /*
+     * A disk drive may be "off" or "on", regardless of whether it's
+     * been selected by the peripheral interface.
+     */
+    bool online;
+
+    /*
+     * This is one of DD_READ or DD_WRITE (defined in the enum above).
+     * The drive can only read or write at once, and the mode of
+     * operation must be made explicit through this mechanism.
+     */
+    int mode;
+
+    /*
+     * Write protection is an attribute of the disk. Back in the day, a
+     * disk would have a small segment cut out of the disk on the side;
+     * this would make it writeable. A disk without that would be
+     * write-protected. You could take a writeable disk and make it
+     * write-protected simply by putting some solid-colored tape over
+     * the cut-out.
+     *
+     * For our purposes, write protection is a simply boolean attribute
+     * that you can enable or disable on the drive.
+     */
+    bool write_protect;
+} apple2_disk_drive;
+
+extern apple2_disk_drive *apple2_disk_drive_create();
+extern void apple2_disk_drive_free(apple2_disk_drive *);
+extern void apple2_disk_drive_step(apple2_disk_drive *, int);
+extern void apple2_disk_drive_set_mode(apple2_disk_drive *, int);
+extern void apple2_disk_drive_turn_on(apple2_disk_drive *, bool);
+extern void apple2_disk_drive_write_protect(apple2_disk_drive *, bool);
+
+#endif

src/apple2.disk_drive.c

@@ -0,0 +1,73 @@
+/*
+ * apple2.disk_drive.c
+ */
+
+#include "apple2.disk_drive.h"
+
+apple2_disk_drive *
+apple2_disk_drive_create()
+{
+    apple2_disk_drive *drive;
+
+    drive = malloc(sizeof(apple2_disk_drive));
+    if (drive == NULL) {
+        log_critical("Could not malloc space for apple2 disk drive");
+        return NULL;
+    }
+
+    // To begin with, we have no segment for data; that's something that
+    // will depend on the disk you insert. For example, a DOS 3.3 or
+    // ProDOS disk will have 140k, but a NIB file would have more.
+    drive->data = NULL;
+
+    drive->track_pos = 0;
+    drive->online = false;
+    drive->write_protect = true;
+    drive->mode = DD_READ;
+
+    return drive;
+}
+
+void
+apple2_disk_drive_free(apple2_disk_drive *drive)
+{
+    if (drive->data) {
+        vm_segment_free(drive->data);
+    }
+
+    free(drive);
+}
+
+void
+apple2_disk_drive_step(apple2_disk_drive *drive, int steps)
+{
+    drive->track_pos += steps;
+
+    if (drive->track_pos > MAX_DRIVE_STEPS) {
+        drive->track_pos = MAX_DRIVE_STEPS;
+    } else if (drive->track_pos < -MAX_DRIVE_STEPS) {
+        drive->track_pos = -MAX_DRIVE_STEPS;
+    }
+}
+
+void
+apple2_disk_drive_set_mode(apple2_disk_drive *drive, int mode)
+{
+    if (mode != DD_READ && mode != DD_WRITE) {
+        return;
+    }
+
+    drive->mode = mode;
+}
+
+void
+apple2_disk_drive_turn_on(apple2_disk_drive *drive, bool online)
+{
+    drive->online = online;
+}
+
+void
+apple2_disk_drive_write_protect(apple2_disk_drive *drive, bool protect)
+{
+    drive->write_protect = protect;
+}