Reading Disk Properties in Assembler - INT 13H

This post is a continuation from the previous post [Writting a Bootloader] which was an attempt to build a Bootloader for the x86 architecture.

Although the executable size is limited to 512 Bytes, there are several important tasks that can be accomplished during booting process. One of them is to read the Geometry from several storage devices.

Interrupt 13h [INT 13h] provides sector-based disk access services using the CHS addressing  method (Cylinder/Head/Sectors). This values provide the physical location of the data (although this is not completely true nowadays).

The code below is an example of this interrupt. It reads the CHS from the first Hard Disk using Interrupt 13h.

Briefly, interrupt parameters are set in AX and DX registers. After interrupt is completed, its values can be read from the appropriate registers (CX, DX). The rest of the code includes helper functions to print numbers and strings to present results properly.

After booting our virtual system using the code, we get the following result:

    Hello Again

    Hard Drive Information

    Disk Geometry (C/H/S): 130/16/63

Comparing these results with fdisk on the same drive:

    Disk /dev/sdb: 67 MB, 67108864 bytes

    255 heads, 63 sectors/track, 8 cylinders, total 131072 sectors

    Units = sectors of 1 * 512 = 512 bytes

    Sector size (logical/physical): 512 bytes / 512 bytes

    I/O size (minimum/optimal): 512 bytes / 512 bytes

    Disk identifier: 0x00000000

I am still trying to figure why these values are so different. Anyway, the total size of the drive taking the total oh Cylinders/Heads/Sectors will be close to the drive size described by fdisk.

Finally, notice that 08h service is limited to drives up to 500 MB. If there's required to read properties from bigger drives, service 48h is the appropriate.

As exercise, following improvements can be done:

• Report an error if reading fails
• Read properties from all connected drives
• Determine drive size in LBA format
• Modify code to read bigger drives

1. BITS 16                 ;Tell compiler that this is 16 Bit code
2. org 07C00h              ;Tell compiler code will be loaded in this memory section
3.  
4. mov si, txtStr
5. call printTxt
6. call crlf
7. mov si, txtStr2
8. call printTxt
9. call crlf
10.  
11. mov si, txtDisk
12. call printTxt
13.  
14. ;Read HD1 parameters
15. mov AH, 08h     ;Read Drive Parameters
16. mov DL, 80h     ; First Drive
17. int 13h
18.  
19. push cx
20. mov ax, cx      ;Cylinders
21. and ax, 0C0h
22. shl ax, 2
23. and cx, 0FF00h
24. shr cx, 8
25. or cx, ax
26. mov ax, cx
27. inc ax
28. call printNum
29. mov si, txtSlash
30. call printTxt
31. pop cx
32.  
33. mov al, dh      ;Num Heads
34. mov ah, 0
35. inc ax
36. call printNum
37. mov si, txtSlash
38. call printTxt
39.  
40. and cx, 3Fh ;[5..0] Sectors Per Track
41. mov ax, cx
42. call printNum
43. call crlf
44.  
45. JMP $
46.  
47. ;********************* Utility Functions ******************
48.  
49. printNum:       ;Print a number (ax)
50.         push cx
51.         push dx
52.         mov cx,000Ah    ;divide by 10
53.         mov bx, sp
54.         getDigit:
55.                 mov dx,0        ;puting 0 in the high part of the divided number (DX:AX)
56.                 div cx          ;DX:AX/cx.  ax=dx:ax/cx  and dx=dx:ax%cx(modulus)
57.                 push dx
58.                 cmp ax,0
59.         jne getDigit
60.  
61.         printNmb:
62.                 pop ax
63.                 add al, 30h     ;adding the '0' char for printing
64.                 mov ah,0eh      ;print char interrupt
65.                 int 10h
66.                 cmp bx, sp
67.         jne printNmb
68.  
69.         pop dx
70.         pop cx
71.         RET
72.  
73. printTxt:       ;Print a String (si)
74.   lodsb
75.   cmp al, 0
76.   je exitFn
77.   mov ah, 0Eh
78.   int 10h
79.   jmp printTxt
80. exitFn:
81.   RET
82.  
83. crlf:   ;Print End of line
84.         mov ax, 0E0dh
85.         int 10h
86.         mov ax, 0E0ah
87.         int 10h
88.         RET
89.  
90. ;AppData
91. txtStr db "Hello Again", 0
92. txtStr2 db "Hard Drive Information", 0
93. txtDisk db "Disk Geometry (C/H/S): ", 0
94. txtSlash db "/", 0
95. times 510-($-$$) db 0   ;Fill the rest of the Loader with "0"
96. dw 0AA55h               ;Bootloader Signature