diff --git a/Makefile b/Makefile
index 2ba9a15..0d3f313 100644
--- a/Makefile
+++ b/Makefile
@@ -1,12 +1,15 @@
 CFLAGS ?= -Iinclude
 
-all: build build/test-get-set-cluster build/test-mkfs-simple
+all: build build/test-get-set-cluster build/test-mkfs-simple build/test-fragment-file
 
 build:
 	mkdir build
 
-build/test-get-set-cluster: tests/get-set-cluster.c src/fat12.c include/fat12.h
-	$(CC) $(CFLAGS) tests/get-set-cluster.c src/fat12.c -o build/test-get-set-cluster
+build/test-get-set-cluster: tests/get-set-cluster.c src/fat12.c include/fat12.h src/io.c include/io.h
+	$(CC) $(CFLAGS) tests/get-set-cluster.c src/fat12.c src/io.c -o build/test-get-set-cluster
 
-build/test-mkfs-simple: tests/mkfs-simple.c src/fat12.c include/fat12.h src/mkfat.c src/io.c
-	$(CC) $(CFLAGS) tests/mkfs-simple.c src/mkfat.c src/fat12.c src/io.c -o $@
\ No newline at end of file
+build/test-mkfs-simple: tests/mkfs-simple.c src/fat12.c include/fat12.h src/mkfat.c src/io.c include/io.h
+	$(CC) $(CFLAGS) tests/mkfs-simple.c src/mkfat.c src/fat12.c src/io.c -o $@
+
+build/test-fragment-file: tests/fragment-file.c src/fat12.c include/fat12.h src/mkfat.c src/io.c include/io.h
+	$(CC) $(CFLAGS) tests/fragment-file.c src/mkfat.c src/fat12.c src/io.c -o $@
\ No newline at end of file
diff --git a/include/fat12-internals.h b/include/fat12-internals.h
new file mode 100644
index 0000000..a937150
--- /dev/null
+++ b/include/fat12-internals.h
@@ -0,0 +1,20 @@
+#ifndef FAT12_INTERNALS_H__
+#define FAT12_INTERNALS_H__
+
+struct fat_directory_entry {
+    uint8_t file_name[8];
+    uint8_t extension[3];
+    uint8_t file_attributes;
+    uint8_t reserved1;
+    uint8_t ctime_ms;
+    uint8_t ctime_hms[2];
+    uint8_t ctime_date[2];
+    uint8_t atime_date[2];
+    uint8_t ea_index[2];
+    uint8_t mtime_hms[2];
+    uint8_t mtime_date[2];
+    uint8_t first_cluster[2];
+    uint8_t file_size[4];
+};
+
+#endif /* FAT12_INTERNALS_H__ */
\ No newline at end of file
diff --git a/include/fat12.h b/include/fat12.h
index 4ca9527..353f124 100644
--- a/include/fat12.h
+++ b/include/fat12.h
@@ -3,6 +3,13 @@
 
 #include <stdint.h>
 
+struct fat12_partition;
+
+struct fat12_partition * fat12_alloc(void);
+int fat12_open(struct fat12_partition *part, const char *filename);
+int fat12_close(struct fat12_partition *part);
+void fat12_free(struct fat12_partition **part);
+
 void fat12_write_u8(void *ptr, uint8_t val);
 uint8_t fat12_read_u8(void *ptr);
 void fat12_write_u16(void *ptr, uint16_t val);
@@ -13,20 +20,4 @@ uint32_t fat12_get_cluster(void *fat, uint32_t cluster);
 int fat12_defrag(int fd);
 int fat12_mkfs(int fd, uint32_t bytes);
 
-struct fat_directory_entry {
-    uint8_t file_name[8];
-    uint8_t extension[3];
-    uint8_t file_attributes;
-    uint8_t reserved1;
-    uint8_t ctime_ms;
-    uint8_t ctime_hms[2];
-    uint8_t ctime_date[2];
-    uint8_t atime_date[2];
-    uint8_t ea_index[2];
-    uint8_t mtime_hms[2];
-    uint8_t mtime_date[2];
-    uint8_t first_cluster[2];
-    uint8_t file_size[4];
-};
-
 #endif /* PANG_O_LIN_FAT12_H__ */
\ No newline at end of file
diff --git a/include/io.h b/include/io.h
index 9d08cc3..53423ff 100644
--- a/include/io.h
+++ b/include/io.h
@@ -1,14 +1,20 @@
 #ifndef PANG_O_LIN_IO_H__
 #define PANG_O_LIN_IO_H__
 
-#include <sys/types.h>
-
 #ifdef unix
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/types.h>
 #include <unistd.h>
 #endif
 
 ssize_t pang_write(int fd, const void *buf, size_t count);
 ssize_t pang_read(int fd, void *buf, size_t count);
 off_t pang_seek(int fd, off_t offset, int whence);
+int pang_open(const char *pathname, int flags);
+int pang_close(int fd);
 
 #endif /* PANG_O_LIN_IO_H__ */
\ No newline at end of file
diff --git a/src/defrag.c b/src/defrag.c
index d9ddc34..c98c39e 100644
--- a/src/defrag.c
+++ b/src/defrag.c
@@ -1,4 +1,8 @@
 
+#include <stdint.h>
+#include <sys/types.h>
+#include <unistd.h>
+
 #include "io.h"
 
 #define try(x) do { int i = (x); if (i == -1) { return -1 } } while (0)
@@ -34,12 +38,12 @@ int fat12_defrag(int fd) {
     }
 
     // 32 root entries keeps it padded correctly.
-    if (fat12_read_u16(mbr + 0x11, 32) != 32) {
+    if (fat12_read_u16(buf + 0x11, 32) != 32) {
         return -2;
     }
 
     // Total number of 512-byte sectors
-    fat12_write_u16(mbr + 0x13, bytes / sector_size);
+    uint32_t clufat12_read_u16(buf + 0x13, bytes / sector_size);
 
     return 0;
 }
\ No newline at end of file
diff --git a/src/fat12.c b/src/fat12.c
index 6ee9bcb..d65458f 100644
--- a/src/fat12.c
+++ b/src/fat12.c
@@ -1,5 +1,32 @@
 #include "fat12.h"
+#include "fat12-internals.h"
+#include "io.h"
 
+#include <stdio.h>
+
+struct fat12_partition {
+    int fd;
+
+    // Number of bytes in a sector (512)
+    uint32_t sector_size;
+    uint8_t sector_buffer[512];
+
+    // These units are "sectors"
+    uint32_t fat_offset;
+    uint32_t fat_size;
+    uint32_t root_offset;
+    uint32_t first_offset;
+    uint32_t cluster_size;
+
+    // These are in single-unit quantities
+    uint32_t root_entries;
+    uint32_t cluster_count;
+    uint32_t cluster_bytes;
+    uint32_t sector_count;
+
+    // Used for traversing directories
+    uint32_t current_cluster;
+};
 
 void fat12_write_u8(void *ptr, uint8_t val) {
     ((uint8_t *)ptr)[0] = (val >> 0) & 0xff;
@@ -49,4 +76,93 @@ uint32_t fat12_get_cluster(void *fat, uint32_t cluster) {
     }
 
     return value;
+}
+
+struct fat12_partition *fat12_alloc(void) {
+    struct fat12_partition *part = malloc(sizeof(struct fat12_partition));
+    memset(part, 0, sizeof(*part));
+    return part;
+}
+
+int fat12_open(struct fat12_partition *part, const char *filename) {
+
+    if (!part)
+        return -1;
+
+    if (part->fd > 0)
+        return -1;
+
+    part->fd = pang_open(filename, O_RDWR);
+    if (part->fd == -1) {
+        part->fd = 0;
+        return -1;
+    }
+
+    if (pang_read(part->fd, part->sector_buffer, sizeof(part->sector_buffer)) != sizeof(part->sector_buffer)) {
+        return -1;
+    }
+
+    part->sector_size = fat12_read_u16(part->sector_buffer + 0x0b);
+
+    // These are in single-unit quantities
+    part->root_entries = fat12_read_u16(part->sector_buffer + 0x11);
+    part->sector_count = fat12_read_u16(part->sector_buffer + 0x13);
+
+    // These units are "sectors"
+    part->fat_offset = fat12_read_u16(part->sector_buffer + 0x0e);
+    part->fat_size = fat12_read_u16(part->sector_buffer + 0x16);
+    part->root_offset = part->fat_offset + part->fat_size;
+    part->first_offset = part->root_offset + (part->root_entries * sizeof(struct fat_directory_entry)) / part->sector_size;
+    part->cluster_size = fat12_read_u8(part->sector_buffer + 0x0d);
+
+    uint32_t bytes = (part->sector_count - part->first_offset) * part->sector_size;
+    part->cluster_count = bytes / part->sector_size / part->cluster_size;
+    part->cluster_bytes = (part->cluster_count * 3) / 2;
+
+///
+    fprintf(stderr, "part->sector_size: %d\n", part->sector_size);
+
+    // These are in single-unit quantities
+    fprintf(stderr, "part->root_entries: %d\n", part->root_entries);
+    fprintf(stderr, "part->sector_count: %d\n", part->sector_count);
+
+    // These units are "sectors"
+    fprintf(stderr, "part->fat_offset: %d\n", part->fat_offset);
+    fprintf(stderr, "part->fat_size: %d\n", part->fat_size);
+    fprintf(stderr, "part->root_offset: %d\n", part->root_offset);
+    fprintf(stderr, "part->first_offset: %d\n", part->first_offset);
+    fprintf(stderr, "part->cluster_size: %d\n", part->cluster_size);
+
+    fprintf(stderr, "part->cluster_count: %d\n", part->cluster_count);
+    fprintf(stderr, "part->cluster_bytes: %d\n", part->cluster_bytes);
+    fprintf(stderr, "data_bytes: %d\n", bytes);
+///
+
+    return 0;
+}
+
+int fat12_close(struct fat12_partition *part) {
+    if (!part)
+        return -1;
+
+    if (part->fd <= 0)
+        return -1;
+
+    if (-1 == pang_close(part->fd)) {
+        return -1;
+    }
+
+    part->fd = 0;
+    return 0;
+}
+
+void fat12_free(struct fat12_partition **part) {
+    if (!part)
+        return;
+
+    if (!*part)
+        return;
+
+    free(*part);
+    *part = NULL;
 }
\ No newline at end of file
diff --git a/src/io.c b/src/io.c
index 223377b..c061c00 100644
--- a/src/io.c
+++ b/src/io.c
@@ -1,3 +1,6 @@
+#include <sys/types.h>
+#include <unistd.h>
+
 #include "io.h"
 
 #ifdef unix
@@ -12,4 +15,13 @@ ssize_t pang_read(int fd, void *buf, size_t count) {
 off_t pang_seek(int fd, off_t offset, int whence) {
     return lseek(fd, offset, whence);
 }
+
+int pang_open(const char *pathname, int flags) {
+    return open(pathname, flags);
+}
+
+int pang_close(int fd) {
+    return close(fd);
+}
+
 #endif
\ No newline at end of file
diff --git a/src/mkfat.c b/src/mkfat.c
index 0fe12aa..350a937 100644
--- a/src/mkfat.c
+++ b/src/mkfat.c
@@ -3,8 +3,11 @@
 #include <string.h>
 
 #include "fat12.h"
+#include "fat12-internals.h"
 #include "io.h"
 
+#include <stdio.h>
+
 // Create a FAT12 filesystem using the given file descriptor,
 // with length _bytes_.
 // It will define a root directory size and padding size so that the file sectors
@@ -27,15 +30,15 @@ int fat12_mkfs(int fd, uint32_t bytes) {
     uint32_t root_directory_sector_count = cluster_size - (fat_size_sectors + reserved_sectors);
     uint32_t root_directory_entry_count = (sector_size / sizeof(struct fat_directory_entry)) * root_directory_sector_count;
 
-    // fprintf(stderr, "bytes: %d\n", bytes);
-    // fprintf(stderr, "sector_size: %d\n", sector_size);
-    // fprintf(stderr, "cluster_size: %d\n", cluster_size);
-    // fprintf(stderr, "reserved_sectors: %d\n", reserved_sectors);
-    // fprintf(stderr, "fat_size: %d\n", fat_size);
-    // fprintf(stderr, "fat_size_sectors: %d\n", fat_size_sectors);
-    // fprintf(stderr, "root_directory_sector_count: %d\n", root_directory_sector_count);
-    // fprintf(stderr, "root_directory_entry_count: %d\n", root_directory_entry_count);
-    // fprintf(stderr, "\n");
+    fprintf(stderr, "bytes: %d\n", bytes);
+    fprintf(stderr, "sector_size: %d\n", sector_size);
+    fprintf(stderr, "cluster_size: %d\n", cluster_size);
+    fprintf(stderr, "reserved_sectors: %d\n", reserved_sectors);
+    fprintf(stderr, "fat_size: %d\n", fat_size);
+    fprintf(stderr, "fat_size_sectors: %d\n", fat_size_sectors);
+    fprintf(stderr, "root_directory_sector_count: %d\n", root_directory_sector_count);
+    fprintf(stderr, "root_directory_entry_count: %d\n", root_directory_entry_count);
+    fprintf(stderr, "\n");
 
     memset(mbr, 0, sizeof(mbr));
 
diff --git a/tests/fragment-file.c b/tests/fragment-file.c
new file mode 100644
index 0000000..12733ad
--- /dev/null
+++ b/tests/fragment-file.c
@@ -0,0 +1,48 @@
+#include <unistd.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <fcntl.h>
+#include <errno.h>
+#include <string.h>
+
+#include "fat12.h"
+
+static int make_image(const char *name, uint32_t size) {
+
+    int fd = open(name, O_WRONLY | O_CREAT, 0777);
+    if (-1 == fd) {
+        fprintf(stderr, "couldn't open %s: %s\n", name, strerror(errno));
+        return 1;
+    }
+
+    if (-1 == fat12_mkfs(fd, size)) {
+        fprintf(stderr, "couldn't make fat12 on %s: %s\n", name, strerror(errno));
+        close(fd);
+        return 1;
+    }
+    close(fd);
+    return 0;
+}
+
+int main(int argc, char **argv) {
+    const char *img_name = "build/fat12-1800k.img";
+    int ret = 0;
+    int fd;
+    struct fat12_partition *part = fat12_alloc();
+    
+    ret = make_image(img_name, 1800 * 1024);
+    if (ret == -1) {
+        return 1;
+    }
+
+    if (-1 == fat12_open(part, img_name)) {
+        fprintf(stderr, "couldn't open fat12 on %s: %s\n", img_name, strerror(errno));
+        return 1;
+    }
+
+    fat12_close(part);
+    fat12_free(&part);
+
+    return 0;
+}
\ No newline at end of file