diff --git a/src/apple2.disk_drive.c b/src/apple2.disk_drive.c
index ebd53d8..16c3348 100644
--- a/src/apple2.disk_drive.c
+++ b/src/apple2.disk_drive.c
@@ -4,12 +4,32 @@
 
 #include "apple2.disk_drive.h"
 
-apple2_disk_drive *
-apple2_disk_drive_create()
-{
-    apple2_disk_drive *drive;
+/*
+ * This is the length of a typical disk that is formatted in either DOS
+ * 3.3 or ProDOS.
+ */
+#define _140K_ 143360
 
-    drive = malloc(sizeof(apple2_disk_drive));
+/*
+ * And this is the length of a disk that has been formatted as a nibble
+ * file (*.NIB). This is not an Apple thing, exactly; it's more of an
+ * emulator thing, that emulators had used to try and get around copy
+ * protection in emulation. It does complicate disk drive operation!
+ */
+#define _240K_ 245760
+
+/*
+ * This is the last _accessible_ sector position within a track (you can
+ * have 0 - 4095).
+ */
+#define MAX_SECTOR_POS 4095
+
+apple2dd *
+apple2dd_create()
+{
+    apple2dd *drive;
+
+    drive = malloc(sizeof(apple2dd));
     if (drive == NULL) {
         log_critical("Could not malloc space for apple2 disk drive");
         return NULL;
@@ -21,6 +41,7 @@ apple2_disk_drive_create()
     drive->data = NULL;
 
     drive->track_pos = 0;
+    drive->sector_pos = 0;
     drive->online = false;
     drive->write_protect = true;
     drive->mode = DD_READ;
@@ -29,7 +50,7 @@ apple2_disk_drive_create()
 }
 
 void
-apple2_disk_drive_free(apple2_disk_drive *drive)
+apple2dd_free(apple2dd *drive)
 {
     if (drive->data) {
         vm_segment_free(drive->data);
@@ -39,7 +60,7 @@ apple2_disk_drive_free(apple2_disk_drive *drive)
 }
 
 void
-apple2_disk_drive_step(apple2_disk_drive *drive, int steps)
+apple2dd_step(apple2dd *drive, int steps)
 {
     drive->track_pos += steps;
 
@@ -51,7 +72,7 @@ apple2_disk_drive_step(apple2_disk_drive *drive, int steps)
 }
 
 void
-apple2_disk_drive_set_mode(apple2_disk_drive *drive, int mode)
+apple2dd_set_mode(apple2dd *drive, int mode)
 {
     if (mode != DD_READ && mode != DD_WRITE) {
         return;
@@ -61,13 +82,54 @@ apple2_disk_drive_set_mode(apple2_disk_drive *drive, int mode)
 }
 
 void
-apple2_disk_drive_turn_on(apple2_disk_drive *drive, bool online)
+apple2dd_turn_on(apple2dd *drive, bool online)
 {
     drive->online = online;
 }
 
 void
-apple2_disk_drive_write_protect(apple2_disk_drive *drive, bool protect)
+apple2dd_write_protect(apple2dd *drive, bool protect)
 {
     drive->write_protect = protect;
 }
+
+static int
+position(apple2dd *drive)
+{
+    if (drive->data->size == _140K_) {
+        int track_offset;
+
+        track_offset = (drive->track_pos % 2) * 4096;
+        return track_offset + drive->sector_pos;
+    }
+
+    return 0;
+}
+
+vm_8bit
+apple2dd_read_byte(apple2dd *drive)
+{
+    vm_8bit byte = vm_segment_get(drive->data, position(drive));
+
+    // We may have read the very last byte in a sector; if so let's
+    // adjust the track_pos by two half tracks and reset the sector pos.
+    drive->sector_pos++;
+    if (drive->sector_pos > MAX_SECTOR_POS) {
+        drive->track_pos += 2;
+        drive->sector_pos = 0;
+    }
+
+    return byte;
+}
+
+void
+apple2dd_write(apple2dd *drive, vm_8bit byte)
+{
+    vm_segment_set(drive->data, position(drive), byte);
+
+    drive->sector_pos++;
+    if (drive->sector_pos > MAX_SECTOR_POS) {
+        drive->track_pos += 2;
+        drive->sector_pos = 0;
+    }
+}