Embedded Systems Interfacing
Overview
DOS Data Structure
File I/O Sectors and Clusters
Master Boot Records/Boot Record
File Allocation Table
Embedded Systems Interfacing Root Directory
Interfacing
Subdirectory File
File IO Module
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Sectors and Clusters SD Card Map Structure
Sectors are hardware units with 512 bytes Partition Table (divides media into
per sector drives)
Cluster is an file system unit with 2nx512 Boot Record (divides drive into data
bytes per cluster structures)
Each cluster numbers start at a value of 2 at 16-Bit File Allocation Table 1 (FAT1)
start of the data space (see future slide)
16-Bit File Allocation Table 2 (FAT2)
Each cluster is an allocation unit in the File
Root Directory (fixed number of files)
Allocation Table or FAT (see future slide)
Data Area (files and subdirectories)
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File System Layout 512 MB SD Card Map
Master Boot Record
(Partition Table)
• Cluster #1 used for Address Sector Cluster Description
Boot Record Disk Type 0:0000 0 N.A. Master Boot Record which contains the Partition table
1:DA00 0ED N.A Boot Record
FAT #1
•MBR, BR, FATs and 1:DC00 0EE N.A File Allocation Table 1 with 16-bit FAT entries
FAT #2 Root not covered by 2:CE00 167 N.A. File Allocation Table 2 with 16-bit FAT entries
3:C000 1E0 N.A. Root Directory with VFAT and directory entries
Root Directory FAT Cluster Numbers
4:0000 200 2 Bill of Right.txt
Cluster #2 •FAT-16 uses two 4:4000 220 3 MyFile.txt
bytes per cluster
Cluster #3
512 Bytes = 1 Sector1
Cluster #nnnn 1 Hardware Term
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Copyright James Grover, 2008 1
Embedded Systems Interfacing
Partition Table Image SD Card Partition Table
Offset Size Data Description
Contains 0x1BE 1 Byte 0x00 Not bootable media
Code for
0x01BF 1 Byte 0x03 Starting Head (Boot Record)
Bootable 0x1C0 6 Bits 0B110001 Starting Sector (Boot Record)
Media 0x1C0 10 Bits 0B000000000000 Staring Cylinder (Boot Record)
0x1C2 1 Byte 0x06 System ID: BIGDOS FAT16 partition
0x1C3 1 Byte 0x0F Ending Head
0x1C4 6 bits 0B111111 Ending Sector
0x1C4 10 bits 0B1101011011 Ending Cylinder
0x1C6 4 Bytes 0x000000ED Relative Sector
Read/Executed By B.I.O.S. 0x1CA 4 Bytes 0X000F1DA3 Total Sectors
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Go to LBA 0x000000ED for Boot Record 8
Boot Record Image SD Card Boot Record
Offset Size Data Description
0x0B 2 Bytes 0x200 Bytes per sector (512)
0x0D 1 Byte 0x20 Sectors per cluster (32)
Contains 0x0E 2 Bytes 0x0001 Reserved sectors
0x10 1 Byte 0x02 Number of File Allocation Tables
Code for 0x11 2 Bytes 0x0200 Root entries (512)
Bootable 0x13 2 Bytes 0x0000 Small sectors
Media
Standard Information in BIOS Parameter Block
• 512 Bytes/Sector
Read/Executed By B.I.O.S. • 32 Sectors/Cluster
• 1 Reserved Sector
• 2 FATs
• 200 Directory Entries (x 32 Bytes/Entry)
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SD Card Boot Record FAT Entry Reference
Offset Size Data Description
0x15 1 Byte 0xF8 Media type Entry Description
0x16 2 Bytes 0x0079 Sectors per File Allocation Table (121) 0x0000 Free cluster
0x18 2 Bytes 0x003F Sectors per track (63) 0x0001 Reserved cluster
0x0002 - 0xFFEF Used cluster; value points to next cluster
0x1A 2 Bytes 0x0010 Number of heads (16)
0xFFF0 - 0xFFF6 Reserved values
0x1C 4 Bytes 0x000000ED Hidden sectors (237)
0xFFF7 Bad cluster
0x20 4 Bytes 0x000F1DA3 Large sectors (990627)
0xFFF8 - 0xFFFF Last cluster in file
0x24 1 Bytes 0x80 Physical drive number
0x25 1 Byte 0x00 Current head
0x26 1 Byte 0x29 Signature Maximum of 65520 Clusters in FAT-16
0x27 4 bytes 0x63343362 Volume serial number
0x2B 11 Bytes "NO NAME " Volumn label
Cluster is Software (OS) Term
0x36 8 Bytes "FAT16 " System ID
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Embedded Systems Interfacing
FAT1 Image FAT2 Image
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Root Directory Image Bill of Rights Directory
Offset Size Data Description
•VFAT entries 0x00
0x08
8 Bytes
3 Bytes
"BILLOF~1
"TXT
8.3 Filename.
8.3 File extension
•Directory entries 0x0B 1 Byte 0x20 Attribute
0x0C 1 Byte 0x00 Reserved
0x0D 1 Byte 0x68 Creation time milliseconds
0x0E 2 Bytes 0x4209 Creation time in hours, minutes and seconds
0x10 2 Bytes 0x3574 Creation date in years since 1980, months, and day.
0x12 2 Bytes 0x3574 Last accessed date
0x14 2 Bytes 0x0000 EA-index
0x16 2 Bytes 0x42EF Last modified time
0x18 2 Bytes 0x3571 Last modified date
0x1A 2 Bytes 0x0002 First cluster
0x1C 4 Bytes 0x00000F23 File size in bytes.
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First (and only) Sector of
Long File Reference (VFAT) Bill of Rights
Offset Size Data Description
0x00 1 Byte 01 Sequence number
0x01 10 Bytes "Bill " Name characters as five UTF-16 characters
0x0B 1 Byte 0x0F Attribute
0x0C 1 Byte 0x00 Reserved
0x0D 1 Byte 0x2A Checksum of DOS file name
0x0E 12 Bytes "of Right" Name characters as six UTF-16 characters
0x1A 2 Bytes First cluster which is always 0x0000
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Copyright James Grover, 2008 3
Embedded Systems Interfacing
File Allocation Table FAT and Directory Interaction
FAT12 used for floppy disk
Dreamed up by Bill Gates over a weekend
Very cryptic and atypical of other OS allocation
methods
FF 00 10 0FF 100
FAT16 used for small hard drives and SD
Cards
FAT32 used for large hard drives
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Get Out Calculator Get Out Calculator
Find Partition Table 0? Find Root Directory?
Sector 0, Offset 1BE 0x176 + Sector/FATBR = 0x00000001E0
Find Boot Record? Data Area?
0 + (1C6)DWORD = 0x000000ED 0x1E0 + Root EntriesBR*(Bytes/Entry)std/(Bytes/Sec)BR
= 0x0000000200
Find FAT 1?
0x000000ED + ReservedBR = 0x000000EE Cluster n
Find FAT 2? 0x200 + (n-2)FAT *Sector/ClusterBR
0xEE + Sec/FATBR = 0x00000176 0x200 + (3-2)FAT *0x20 = 0x220
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Get Out Calculator Structure Review
First Cluster of File? Template of memory
struct structTag {
0x200+(File EntryDirectory Entry-2)* Sector/ClusterBR dataType variable name;
Next Cluster of File?
●●●
};
0x200 + (n)FAT* Sector/ClusterBR Example
struct myStructTag{
int myNum;
char myLetter[4];
int * myPointer;
}
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Copyright James Grover, 2008 4
Embedded Systems Interfacing
Structure Review Structure Review
Access Array Element
Declaration
ch=myStruct.myLetter[3];
struct structTag structureName; ch=myStructPtr->myLetter[3];
Access Scalar
Examples num=myStruct.myNum;
num=myStructPtr->myNumber;
struct myStructTag myStruct = Access Pointer
{34512,’A’,’Y’,’8’,9,&Buffer[0]}; ptr=myStruct.myPointer;
ptr=myStructPtr->myPointer;
struct myStructTag * myStructPtr; Access Data Pointer To
ch=*(myStruct.(myPointer+5));
ch=*(myStruct->(myPointer+5));
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Data types Partition Information
struct PARTITIONINFO {
BYTE bootid; /* bootable? 0=no, 128=yes */
typedef unsigned char BYTE;
BYTE beghead; /* beginning head number */
typedef unsigned int WORD; BYTE begsect; /* beginning sector number */
typedef unsigned long DWORD; BYTE begcyl; /* 10 bit nmbr, with high 2 bits put in begsect */
BYTE systid; /* Operating System type indicator code */
enum BOOL {TRUE=0,FALSE=1}; BYTE endhead; /* ending head number */
BYTE endsect; /* ending sector number */
BYTE endcyl; /* also a 10 bit nmbr, with same high 2 bit trick */
DWORD relsect; /* first sector relative to start of disk */
DWORD numsect; /* number of sectors in partition */
};
How much space allocated?
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Master Boot Record Access MBR Data
struct MBR{ struct MBR * p=(struct MBR *)&buffer[0];
BYTE codes[446];
BYTE BOOTID,BEGHEAD,BEGSECT,SYSTID,ENDHEAD,ENDSECT;
struct PARTITIONINFO partition[4]; WORD BEGCYL,ENDCYL,SIGNATURE;
WORD mbrid; DWORD RELSECT,NUMSECT;
};
BOOTID=p->partition[0].bootid;
BEGHEAD=p->partition[0].beghead;
BEGSECT=(p->partition[0].begsect)&0x3F;
How much space allocated? BEGCYL=(((p->partition[0].begsect)&0xC0)<<8)+p->partition[0].begcyl;
SYSTID=p->partition[0].systid;
ENDHEAD=p->partition[0].endhead;
ENDSECT=(p->partition[0].endsect)&0x3F;
ENDCYL=(((p->partition[0].endsect)&0xC0)<<8)+p->partition[0].endcyl;
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Copyright James Grover, 2008 5
Embedded Systems Interfacing
Access MBR Data MEDIA Structure
typedef unsigned long LBA;
RELSECT=p->partition[0].relsect;
NUMSECT=p->partition[0].numsect; typedef struct {
SIGNATURE=p->mbrid; LBA fat;
LBA root;
LBA data;
unsigned maxroot;
unsigned maxcls;
unsigned fatsize;
unsigned char fatcopy;
unsigned char sxc;
} MEDIA;
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MFILE Structure MFILE Structure
typedef struct { unsigned time; // last update time
MEDIA * mda; // media structure pointer unsigned date; // last update date
unsigned char * buffer; // sector buffer char name[11]; // file name
unsigned cluster; // first cluster char chk; // checksum = ~( entry + name[0])
unsigned ccls; // current cluster in file unsigned entry; // entry position in cur directory
unsigned sec; // sector in current cluster char mode; // mode 'r', 'w'
unsigned pos; // position in current sector } MFILE;
unsigned top; // number of data bytes in the buffer
long seek; // position in the file
long size; // file size
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Directory Entries Directory Entries
Filename @ 0x00 for 8 bytes Reserved @ 0x0C for 1 byte
0x00 Unused
Time Created @ 0x0D for 3 bytes
0xE5 erased entry
0x2E Dot entry (. or ..) Dated Created @ 0x10 for 2 bytes
File extension @ 0x08 for 3 bytes, padded Date Last Accessed @ 0x12 for 2 bytes
with spaces Extended Attribute @ 0x14 for 2 bytes
File Attribute @ 0x0B for 1 byte
Last Modify Time @ 0x16 for 2 bytes
VFAT has read only, hidden, system and volumn
label or 0x0F attribute Last Modify Date @ 0x18 for 2 bytes
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Copyright James Grover, 2008 6
Embedded Systems Interfacing
Directory Entries FAT Tread
First Cluster in FAT-16 @ 0x1B for 2 Get current cluster, ccls
bytes Determine FAT sector p=ccls>>8
File Size in bytes @ 0x1C for 4 bytes Check if cached
Should be 0 for Volume label or Else read FAT sector
subdirectory
Get LBA of next FAT entry
Long File Name overlays directory entry
See web
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File I/O Support File I/O Support
Fileio.h contains: Function Prototypes
readDIR
Error Codes
writeDIR
Media Structure newDIR
Mfile Structure mount
File Attributes unmount
fopenM
Function Prototypes freadM
nextFAT fwriteM
newFAT fcloseM
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File I/O Support Homework
Fileio.c contains: Chapter 14 Handout
Offset values for items in: Exercise 1: LBA of Boot Record, FAT1,
Master Boot Record
FAT2, Root Directory, Data Area.
Partition Table
Directroy Entries Exercise 2: Given Directory and FAT find
Global Vasiables LBA of file sectors.
FERROR Exercise 3: Write structure for decoding
Media Structure Instance
directory entry
Code for Prototypes in fileio.h
Code for helper functions
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Copyright James Grover, 2008 7
