Serial Port Thermometer using DS1621

Hi there. In this article, I will discuss a project that I have postponed to write about for a long time. I am somehow stuck, on the article series about hard disks. I was postponing writing constantly to investigate further. At one point, I saw that I could not continue to write immediately and therefore decided to write a different topic.

I first saw this circuit, that I will discuss here, in Güçlü TUĞAY's Electronic Hobby book. In this book, he wrote that the circuit does not belong to him either and gave the resources he used. I have the sixth edition of the book dated January 2006 and unfortunately the given links are dead now. Although it is a turkish book, if you own it, it is fine because a circuit diagram (revised from the original, I suppose) exists in this book and Visual Basic (VB) codes can also be downloaded from the link above. There is still another problem for me and that is, I mainly use linux and even if I would have used Windows, it is difficult to find a VB compiler to compile the code because VB is now obsolete. About two years ago, VB 2005 Express Edition was available for download on Microsoft's website and I remember it was really hard to find.

The Internet Wayback machine has helped me greatly about the broken links. Circuit diagram, parts list, how circuit works and VB codes are still available in the archived webpage dated 09.02.2005. In this page, there is even its PCB layout, but I will set up the circuit on protoboard. Moreover, as can be seen, the man's name is "Alberto Ricci Bitti" who owns this page and when I search his name on google, I found another copy of the page at http://web.tiscali.it/riccibitti. Luckily, I found linux version of the same software written in C, in "Useful Links" section at the bottom of the page. Otherwise, porting the code from VB to C would cause a lot of trouble for me. And of course, this page is also in the internet archives: Linux thermometer project (rev v0.3, 26.11.2003).

How the Circuit Works

The thermometer block of the circuit consists of a single integrated circuit. I had previously written about DS1621 in I2C with Arduino article. Therefore, I will not go into much detail here again. The circuit diagram is given below:

 

PC Thermometer circuit diagram

I will not use IC U2 in this project because I don't need a second sensor. In the previous Arduino article, I wrote that using its 3 addressing pins, AT2432 (EEPROM IC) can be used up to 8 ICs in same I2C bus together. Same feature also exist on DS1621s (I forgot to mention in the previous article). Therefore, temperature can be measured with from up to 8 different points using different sensors. In the circuit diagram above, all addressing pins of U1 (A0, A1 and A2) are connected to ground but A0 addressing pin of U2 is connected to Vdd.

The upper half of the circuit is +5V voltage regulator. According to RS232 standard, serial port voltage range for logical 0 lays between +3V and +15V and logical 1 lays between -3V and -15V. This voltage from DTR and RTS pins comes to C1 and V_in pin of the regulator through D1 and D2, which acts as a half wave rectifier and it supplies +5V to Vdd pin of the IC through V_out and C2. Capacitors store energy, allowing temperature read cycles to be completed.

Similarly, the voltage from these pins (and from CTS pin) is reduced to +5V, which is the operating voltage of I2C, with the component pairs R2, DZ1 and R1, DZ2. +5V, which IC will be sending through SDA pin, lays within the operating voltage of serial port and can be read successfully. The serial data coming from SDA, is read by the software through serial port.

I set up the circuit on a protoboard. The circuit is not complicated, but it can be confusing since too many times, there are more then one connection from one single point (especially connected to SCL bus). Each time I set up this circuit, I succeeded at least in my second try.

Parts List:

2x 1N4148 Diodes

2x 5.1V 1/4W Zeners

2x 4.7K Resistors

1x 100nF Capacitor*

2x 47μF 16V Capacitor

1x IC DS1621

1x LM2940CT-5 Voltage Regulator (5V)

* 100nF capacitor(s) considered as bypass capacitors. Therefore, they must be placed very closely to the supply pin of each IC. On the other hand, I did not placed them in the circuit. It still works well "enough".

Reading the Temperature Values

As I mentioned previously, the circuit is connected to the computer over the serial port. It is hard to find a computer with serial port these days. Impossible on laptops and even rarely on desktop machines. Exceptionally, I have an old laptop with serial port. However, if you have an USB to serial converter, this will also work (unlike USB to parallel port converters). I had a male - female serial cable and used it as an extension cord, as can be seen on photos.

There are two years between the photo below and the next one. They look therefore quite different. In the photo on the right, I added an LED (with 870 ohm resistor) to the circuit, to be able to see how long +5V is supplied to the circuit from capacitors when a read request is sent from serial port. The voltage is supplied to the circuit for a considerably short time. In other words, the circuit is mostly off.

In the photo below, on the other hand, I placed the IC a little far from voltage regulator, to get simulate temperature changes by pressing my finger.

I downloaded the software from the link, I gave above. PollOnce() function in main.c file calls ReadTemp() function in readtemp.c file with sensor number as an argument. Before that, the COM port, to which the device is connected, is opened using Open_ComPort()function in i2cbus.c.

The ReadTemp()function is quite simple. The I2C_Start() and I2C_Stop() functions are placing start and stop bit sequences to the I2C bus as their names suggest. Arguments of I2C_Tx_byte() function are the address of DS1621 and its commands. The list of commands can be found in DS1621 datasheet. 0x90 is the base address of DS1621. When sensor address is added to this constant, I2C address of the specific chip is obtained. 0xEE command sends a temperature read request. Using 0xAA command, the temperature value is placed into the bus and these value is read from the bus from the bus using I2C_Rx_byte() function. The first byte is the integer part of the temperature and the second byte is the decimal part. Since DS1621's thermal resolution is half degree, the second byte can be either 0 or 128. At the temperatures below zero, DS1621 sets the first bit of the first byte (sign bit), so that 255 is read from the bus for -1 degree. But since our machine word is larger than 8 bit, the bytes will be read as a number greater than 128 for below zero temperatures, instead of eighth bit is considered as sign bit. For this reason, it is needed to subtract the integer part from 256 for the values greater than 128, to convert them to real temperature.

Functions in i2cbus.c file allows the bytes to be sent / received to be adapted to I2C protocol in order to allow serial port communication of the program. I2C_{T|R}x_byte() functions take a byte argument, process it bit by bit and place them as signals of zeros and ones for SDA and SCL pins in correct order.

However there is a serious problem with this code. The code does not work properly on today's fast computers. Therefore, minor changes had to be made. 

In main.c;

a) I assigned an initial value to i2wait variable by inserting "int i2wait = 0" to line 11. This will be used as a wait parameter.

b) At line 88, I inserted following lines after case 'l': block:

case 'w':
  i2wait = atoi( argv[i + 1] );
break;

Thus, wait time can be given from command line by using -w parameter. 

In i2cbus.c;

c) By adding "extern int i2wait;" to line 24, I defined the global i2wait variable here, which is originally defined in main.c.

d) I called Wait() function at the end of SCL_High(), SCL_Low(), SDA_High(), SDA_Low() functions. Lines 171, 176, 181 and 186 in the original file.

e) Finally, I inserted usleep(i2wait); to the line 203.

Latest version of the package, which contains these changes above, can be downloaded from this link. Just one make command is enough to compile. Finally, I added my user to the "dialout" group to be able to run the program under my user in Linux Mint. If the program does not run with your own user, but with sudo, the problem is with the permission to access serial port.

gettemp is not too difficult to use. First, I check with dmesg, which serial ports are available:

dmesg | grep -i "serial\|tty"
[    1.188451] Serial: 8250/16550 driver, 32 ports, IRQ sharing enabled
[    1.188486] serial 00:04: [io  0x03f8-0x03ff]
[    1.188570] serial 00:04: [irq 4]
[    1.189083] serial 00:04: activated
[    1.209764] 00:04: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A

Since I have a physical serial port, it is detected as soon as the kernel is loaded. If a USB to serial adapter is used, the command watch "dmesg | tail" can be used to find the serial port which is detected shortly. Since my serial port is ttyS0 according to the output above, I have to give "0" to -c parameter. I also set I2C address of the IC to 0, therefore I have to use -p 0 parameter, too. Debug mode can be optionally activated with -d 1 (or 2) parameter. Careful readers will notice that, although I introduced 'w' parameter, I am not using it. Because even an empty Wait() function provides enough delay for read cycle to complete.

Practical use of this circuit might not be on a computer, but rather with devices that have serial ports. I recorded daily temperature of my room minute by minute by putting gettemp in crontab and redirecting its output to a file. The ability to connect up to eight sensors from a single serial port might seem important at the first glance, however since the maximal length of I2C bus is limited to 1-2 meters, its only usage might be in places where high temperature changes are observed in a narrow space. (OK, long distance data communication is not my speciality, but according to the answer, if you are going to use it for long distances, choosing other protocols instead of I2C is suggested).

20 degrees today

What is the Disk System and What isn't? #2: MBR

Hi there. I mentioned hard disk hardware in the previous article. This article will be a bit more software oriented as much as an article about interrupts and IO ports could be. Let's start with the most fundamental data structure of hard disks.

Master Boot Record (MBR)
MBR is a structure in the first sector of each hard disk, holding partitioning scheme and containing some code. During a power-on of a computer, BIOS takes the execution and after it is done with its job, it reads the first sector from one of the accessible storage devices according to the boot sequence setting in CMOS. This process has not changed much since 1980s. If the boot medium is a floppy disk, that first sector is the boot sector, if it is a hard disk then the first sector is MBR. For BIOS, boot sector and MBR has no difference. BIOS only copies the first sector of the first available drive to memory location 0000:7C00 and makes a far jmp here to boot. Obviously, it has nothing to do with protected mode yet, since I was talking about 80s and using real mode addressing.

The code in MBR determines which partition is set for booting, reads the first sector of that partition to memory and branches there. MBR keeps disk partitions in a table. 

I used HxD disk editor throughout this article to read MBR under Windows. I ran HxD as administrator, selected "Open Disk" (Ctrl + Shift + D) and then selected one of the physical disks. I recommend to "Open as Readonly".

Linux users are lucky because following command is sufficient:

sudo dd if=/dev/sda count=1 bs=512 | hexdump -C -v

While I was trying to learn assembly, being able to view MBR after 3-4 months was a great success for me. I am mentioning a blast from the past.

The screenshot from HxD is above. I colored the image to make easy to distinguish. The red area is code, blue area is partition table and the green area is MBR signature. Even though the sector is intact, BIOS will not boot without that 0x55 0xAA signature. So why this number? Because it is (0101 0101 1010 1010)₂.

Same output under Linux is here:

Since GRUB is installed on both machines, both outputs above contain GRUB MBR code. Before examining the code, let's take a look at the partition table colored in blue. In the example I gave with debug.exe in the previous article, same code and signature can be easily seen.

Partition Table
Partition table is a 4*16 byte field starting from the offset 01BEh of MBR. There is a 16-byte record for each primary partition.If I exclude extended partitions for now, according to the previous statement, there can be 4 primary partitions at most. The table below shows the structure of the records:

Offset	Size	Description
0x00	byte	Bootable Flag
0x01	3 byte	Starting CHS address
0x04	byte	System ID
0x05	3 byte	Ending CHS address
0x08	dword	Starting LBA address
0x0C	dword	Total sectors in partiton

Source: https://wiki.osdev.org/Partition_Table

Bootable flag indicates that, it will be booted from that partition at startup. There can be one and only one partition with bootable flag set in each MBR. If this value is 00h, that means the partition is not bootable. If it is 80h, that means the partition is bootable but some MBR code checks only its seventh bit. Thus the values between [01h, 7Fh] are invalid.

I skip CHS address part shortly right now and come back later. 

Although system ID field specifies the type of filesystem used in that partition, this byte is not that important. Some OS and OS installers often do not check this field. This field has also no standard. There were some values used by Microsoft and IBM in 80s. During minix and linux were developed, the values that are not used by Microsoft or IBM at that time, were randomly chosen as their ID number and subsequently de facto standardized. Detailed information about this and the list of these IDs are available on Wikipedia: https://en.wikipedia.org/wiki/Partition_type

The starting LBA address field is the value obtained by translating starting CHS address into LBA. The purpose of the total number of sectors field can be understood from its name. Multiplying this value with 512 bytes, gives the size of the partition but this value is its raw space not the formattable space. I will explain the difference of the two in the boot sector article.

CHS addresses are stored with a complex scheme. The first byte of this three byte field is the head number. The low six bits of the second byte hold the sector number. The third byte is the low eight bits of cylinder number. And the high two bits of second byte also hold two high bits of cylinder number (which makes it 10 bits in total). I tried to visualise this below:

1. Byte								2. Byte								3. Byte
H7	H6	H5	H4	H3	H2	H1	H0	C9	C8	S5	S4	S3	S2	S1	S0	C7	C6	C5	C4	C3	C2	C1	C0

Okay, does it sound familiar? Yes, int 13h. First byte to DH, second to CL and third to CH, et voila. Fits perfectly. 

In the previous article, I wrote that CHS addressing is insufficient for hard disks over 8 GB:
With a more optimistic approach, without fuss; assuming that the MBR code does not need end CHS address because to load the boot sector of a partition only starting CHS address is sufficient, it can be concluded that bootable disk partitions can only be created within the first 8 GB of hard disks. It is impossible to boot from partitions beyond the 8 GB limit using CHS!

OK, how can a partition be created beyond 8 GB? There is a 32-bit sized starting LBA address field. If a boot loader checks this field, instead of CHS, LBA supports 2³² * 512 byte = 2 TB, it can boot from partitions in first 2 TB space. Additionally, since the total number of sectors in partition field has also 32 bits, a 2 TB partition can be created just before the end of this 2 TB limit and thus MBR can still be used on disks up to 4 TB, in theory. I am saying "in theory" becuase there is no guarantee that a 25 year old MBR code will run successfully with these extreme values.

I have added the screenshot with debug.exe from the previous article below again. Let's try to understand the meaning of the bytes here:

80 | 01 01 00 | 06 | 0F 3F FF | 00 00 00 C1 | EF 03 00 00
00 | 00 41 00 | 06 | 0F 7F FF | 00 F0 03 00 | 00 F0 03 00

There are two disk partitions above. First one is bootable. Both system IDs (06h) correspond to FAT16. CHS addresses of the first partition are (0, 1, 1) - (255, 15, 63) starting from 63rd sector and 257 985 sectors long. CHS addresses of the second partition are (256, 0, 1) - (511, 15, 63) starting from sector 258 048 and 258 048 sectors long. 

 

Why is this table so important? If I had been creating my own OS which will run in a computer alone, it has no importance at all. I can ignore all of this, say that I have designed a better table and going to use mine. But since all other OS developers have agreed on this structure and if I use my own partition table, my OS cannot be compatible with others.

Damn the Partition Table and Let's Write Code...
Well yes, I still could not start writing code but I will start giving examples in this section. First, I created two VMs for testing. In previous article, I had installed DOS6.22 on one of the VMs in VirtualBox for the first example. 32MB of RAM and a 500MB hard disk are enough for this VM. This machine comes with a network card after it is created. I set it host-only (Settings -> Network -> Enable Network Adapter, Attached to: Host-only Adapter). DOS floppy images can be found on Internet. I also used Damn Small Linux (DSL) iso. Live distros like Ubuntu, Knoppix etc. do not run on 32 MB RAM. DSL iso is just 50MB. Since it is no longer developed it can be downloaded either here or here. TinyCore Linux, which is the closest alternative to DSL, has an iso of 14MB but runs on 64 MB RAM.

After I booted the machine with the first MS-DOS installation disk, setup asks to create a partition but I quit setup and manually created a partition with fdisk:

1 - 1 - N - 25% - Esc

After doing that, the warning on the screen informs me that "No partitions are set active". This means, there is no partition whose first byte (bootable flag) is 80h. This warning will disappear after pressing 2 - 1 - Esc (setting partition active). Afterwards, when I tried to create another primary partition, I got an error. Unfortunately, fdisk cannot create more than one primary partition. An extended partition can be created but I will explain extended partitions at the end of this article. I pressed Esc to exit the main menu, removed the floppy disk and booted the machine with DSL. By the way, after disk partitions are created or deleted, DOS is unable to recognize them unless it is restarted.

I found the partition I created, by running sudo fdisk -l command in terminal and I added another partition by running following commands:

sudo fdisk /dev/hda
n - p - 2 - (Enter) - 512
t - 2 - 6
p
w

After powering my virtual machine again with DOS floppies, OS recognized and formatted C: and D:. A standard Microsoft installation is completed by pressing Enter key repeatedly, removing previous floppy disk and inserting next one during the installation. And by the way; a warning to those who forgot the last floppy disk in the drive and got "Non-system disk error" after reboot: In the past, when somebody boots a computer with a floppy disk whose boot sector is infected with a boot virus, the virus writes itself into the memory during the boot, shows this error and waits for the user to press a key to spread further. To avoid this, it would be necessary to restart the computer instead of pressing a key. In order to clean the boot viruses, the MBR code of harddisks could be rewritten with fdisk /mbr command. Nowadays, it can be used to remove GRUB installation from harddisk.

After installing DOS, I checked the MBR again using debug.exe: 

I booted the machine again with DSL. Machine IP address should appear on the right of the screen. I will use this IP to copy files from my host to VM. Since my host runs linux, I can copy files easily with scp. Windows users have to start sshd in VM by following: DSL -> System -> Daemons -> ssh -> start. Then WinSCP can be used (Cygwin works too).

The file, I am going to copy, is the disk editor in Norton Utilities package. There are two different versions of disk editor with same file name DISKEDIT.EXE. First one is from Norton Utilities v8.0 (1994) release and the other comes in Norton Utilities 2000 release. First one supports CHS and second one supports LBA. The file from Norton 2000 release is bigger in size (about 600K). It is really practical to work with a disk editor, therefore those who can find this file, should consider him-/herself lucky. (Disclaimer: If these files are still copyrighted, downloading them could violate the law).

I wrote the disk editor files to the disk of VM, restarted the computer once again and I started the editor. I listed the disks by pressing Alt+D. I selected "Physical Disks" as type and opened the harddisk. It showed the sectors like HxD. I selected "View as Partition Table" under View menu (or pressed F6), and all the information, I had to decode manually in previous section, has appeared. Disk editor will help very much for the boot sector and FAT. Therefore, it is useful to keep it in disk.

If you have iso file of Norton Utilities, you have to install CD drive in DOS. Readers who haven't used DOS before, may be surprised at the necessity of installing a CD drive but well those were the days. I will explain the installation in a separate article and give its link here.

According to the screenshot, second partition ends at the end of 511th cylinder. By pressing Alt+I and selecting "Drive Info" in this menu, the editor shows the properties of the hard disk (next figure).

Drive Number: 80 Hex (First physical disk)
Int 13x: Yes (BIOS supports LBA)
Sides: 16 (Heads)
Tracks: 1015 (Cylinders)
Sectors/track: 63

Total Sectors: 1 024 000

This information can also be retrieved using Int 13h/08 subroutine:

Number of cylinders is 3F6h, number of sectors per cylinder is 3Fh and number of heads is 0Fh. The return values in the registers can be interpreted using Ralf Brown's interrupt list (Int13h/08). As it can be seen, DL = 02 in the output above, because I had added another hard disk to the machine at that time for testing.

Now, I will write "denizyildizi" (a totally random word meaning starfish in Turkish) into a random sector of the disk. Let me write to CHS 600,0,1. This address corresponds to 604 800 in LBA (using the formula to convert from CHS to LBA in previous article). I opened the disk editor, pressed Alt+P and typed LBA address in the dialog box. In order to be able to write to the disk, I unchecked the Read-Only box under the menu Tools -> Configuration and pressed OK. I pressed Tab key and switched to the right column to insert text instead of hexadecimal numbers. And I wrote what I wanted. Then I hit Alt+X and clicked to "Write" button in the "Write changes to disk" dialog box.

To do the same with debug.exe (in absence of disk editor), I wrote a code snippet and used subroutine Int13h/43h. This subroutine with the 42h subroutine, are referred as Int13h extentions.

mov cx,000C      ; 12 characters
mov si,0120      ; From 0120h (source)
mov di,0200      ; to 0200h (dest)
repz movsb       ; copy
xor ax,ax
mov cl,FA        ; 0FAh words more
repz stosw       ; copy
inc si           ; DAP exist in next address
push ds
pop ax           ; Get DS
mov word ptr [si+06],ax ; and write it to DAP
mov ax,4300
mov dl,80
int 13           ; call int 13h
int 3
db 00
db 'denizyildizi',0
db 10 00 01 00 00 02 00 00 80 3A 09 00 00 00 00 00

The last line in the snippet contains the data structure named DAP (Disk Address Packet). First byte, 10h is the length of DAP. 00 is reserved. 00 01 is the number of sectors to read/write and (0000:0200)₁₆ is buffer address. It could be misleading that the segment address is 0000 here but, the correct segment address will be written there by the code. (09 3A 80)₁₆ value is LBA address (as qword). I ran this with g command of debug.exe and the sector is written as expected. I will read the data, I wrote, again but this time I use IO ports of disk controller with LBA addressing. I rewrote the snippet, I used in previous article with some minor differences (and added offset numbers again to ease reading):

0100    mov ax,0001
0103    mov dx,01F2
0106    out dx,al   ; Sector count = 1
0107    inc dx      ; dx = 01F3
0108    mov al,80
010A    out dx,al   ; LBA address1 = 80h
010B    inc dx      ; dx = 01F4
010C    mov al,3A
010E    out dx,al   ; LBA address2 = 03A
010F    inc dx      ; dx = 01F5
0110    mov al,09
0112    out dx,al   ; LBA address3 = 09h
0113    inc dx      ; dx = 01F6
0114    mov al,E0   ; LBA mode, master disk
0116    out dx,al   ; LBA address4 = 0h
0117    inc dx      ; dx = 1F7
0118    mov al,20
011A    out dx,al   ; 20h ATA read sector command
011B    in al,dx    ; Read status register
011C    test al,58  ; 0101 1000: Drive ready | Seek complete | Buffer ready
011E    jz 011B     ; Wait until disk is completely read
0120    mov dx,01F0
0123    mov bx,0200
0126    in ax,dx    ; Read data (word sized)
0127    mov [bx],ax ; Copy to the buffer
0129    inc bx
012A    inc bx
012B    cmp bx,0400 ; 0200h byte
012F    jnz 0126
0131    int 3       ; Breakpoint

When I checked again with d 0200 command in debug.exe, I verified the data written previously. 

Logical (Extended) Disk Partitions
Before starting this chapter, I created a second VM with 32 MB RAM and 1GB disk in VirtualBox (with a host-only network adapter like the previous one). I downloaded FreeDOS CD iso from FreeDOS.org and inserted it into the virtual drive. The contents of Legacy and CD are same but the only difference is their boot modes. I selected "Install to harddisk", then chose the language and selected "No - Return to DOS" to partition the disk manually and ran fdisk. Unlike DOS6.22's fdisk, FreeDOS' fdisk comes with FAT32 support. FAT32 appeared in 1996 and went into use with Win95 OSR2. I will go into the detail of FAT32 in next articles. Note on this part: fdisk tests the disk's LBA support while creating a partition. On disks that do not support LBA, partition type field in MBR is 06h and 0Bh for FAT16 and FAT32, respectively, while on disks that support LBA, these values are 07h and 0Ch.

The question of whether FAT32 support should be enabled, shown in the figure above, is only important for disks larger than 2GB. I pressed "N" and passed. I created a 340 MB primary partition with key combination of "1 - 1 - N - 340 - (Enter) - (Esc)" and with the option "2" in main menu, I activated the partition, created. Unlike the previous example, I created an extended (logical) partition with the key combination "1 - 2 - (Enter) - (Esc)". At this step, fdisk is asking if I want to create a logical drive in the extended partition. I typed 340 MB here and hit Enter. fdisk asked again, to create another disk in the remaining space. Here, I hit Enter again. Due to a bug at this step, fdisk thinks there is still 2MB unpartitioned space and asks the user again. If Enter is pressed, the partition table is written incorrectly. Therefore, I pressed to Esc.

 

As I selected the fourth option in fdisk, the disks were listed as above. I quit fdisk, rebooted the VM and continued the installation. I answered the "Do you want to format your drive" question with Yes, then selected "Full installation" and started the installation. When I finished, I removed the CD and restarted the machine. 

Now, I can see three hard disk drives (C:, D: and E:) in FreeDOS. However when I check with disk editor or debug.exe, there are only two partitions in MBR. So where is the third? (Btw, if you have the diskedit, copy it to this VM as well)

When I checked the disk with disk editor, there were 520 cylinders on the disk (I used the old version disk editor for this example, without any specific reason). The first partition ends at the cylinder 173. So, the primary partition is fine. Second partition is listed as EXTEND and there is another partition table at the address CHS(173,0,1). By pressing Enter while EXTEND is selected, will show that sector formatted as partition table.

This partition table looks like the previos one, at the address (0,0,1) but they are not same. In the lower right corner of the screen, I can read the address (173,1,1), so I am not reading MBR obviously. When I switch to the Hex view by pressing F2, there is no MBR code in this sector but a partition table and sector signature. The first entry of this partition table is a partition starting at (173,1,1) and ending at the cylinder 346. Second entry is another extended (EXTEND) partition right after where the previous partition ends. When you select this extended partition in editor and press Enter, another partition table without boot code can be seen at CHS(346,0,1) sector. In this partition table, there is a partition starting from (346,1,1) and ending at the end of the disk. As can be understood, the extended partition is a structure that contains the logical partitions inside it and keeps their information in a linked list (sort of) of partition tables.

As far as I know, there is no reason, in theory, for extended partitions not the be bootable but the code in MBR needs to search for the active partition among the extended partitions, if there is no active partition in the main table. This means an increase in the size of the code, which actually has to be limited to 512 bytes. I am not sure whether this is something undesirable for the standard makers or if there is some other reason. Further examples can be found in Wikipedia in Extended Boot Record article. 


How Does MBR Code Work?
To examine their MBR codes, I booted both machines that I set up so far, with DSL and saved their MBR code with the following command:

dd if=/dev/hda of=mbr.bin count=1 bs=512

The same command on a machine with a recent Linux distro would be;

 

dd if=/dev/sda of=mbr.bin count=1 bs=512

Then I disassembled these outputs with the following command;

objdump -M intel -D -b binary -m i8086 --adjust-vma=0x7C00 mbr.bin > mbr.asm

on a linux machine.

In order to keep the article short, I uploaded disassembled outputs of DOS6.22 MBR and FreeDOS MBR to my Google drive with comments, instead of examining the code line by line here. Both codes are same, in general: After setting the stack pointers, the code copies itself to the address 0:0600h in DOS and 1FE0h:7C00h in FreeDOS. Because boot sector code, which will be loaded right after must be placed in the same 7C00h address. As I previously mentioned, there is no difference between the boot sector and MBR codes from the BIOS perspective. The task of the BIOS is to read the first sector of boot media to 0:7C00h, check 55h AAh bytes at the end and jump to this address. If the boot media is floppy, the boot sector is read to this address since there is no MBR to read.

In DOS, it is checked whether there is one and only one 80h flag as bootable flag in partition table. However, in FreeDOS, the boot sector, corresponding to the first partition table entry with the highest bit of the bootable flag is 1, is read. If the boot sector could not be read, a "read error" is thrown. There is no other control than that.

Regarding to disk reading, DOS tries to read the boot sector, whose CHS address is got from the partition table entry,  five times using a counter kept in DI register. If the boot sector cannot be read at the fifth time, it gives the error "Error loading OS". There is no LBA support in DOS MBR code. On the other hand, FreeDOS checks for LBA support first. If it is supported, the code reads the boot sector using the DAP, stored in 7CCDh. If unsupported, almost the same code like DOS MBR runs. Signature check is done after sector is read (in FreeDOS, this check is on the return of the disk read function) and the program jumps to 0:7C00h.

In fact, I must discuss GRUB, GPT and UEFI as a continuation of this topic but since UEFI is a technology that I do not know very well, I will summarize them in a future article. Although what I have discussed in this article is the technology of 1980s, the code that is run today (unless UEFI is not used), is the same except minor differences of course. I have finished MBR, so far. In the next article, I am going to discuss the next step, the boot sector.