How are the addresses of CHS begin and CHS end in the entries for sections in the MBR? I mean, let's say, the first byte is the number of the cylinder, the second is the heads, the third is the sectors, or is it somehow different? Somewhere I saw that some bits were "borrowed" from neighboring bytes, but I was no longer able to find that site, and then I didn’t understand anything.

    1 answer 1

    The MBR partition descriptor looks like this:

      uint8 IsBootable; // 00 or 80h uint8 BeginHead; uint16 BeginSecCyl; // CX for int 13h uint8 FileSystem; uint8 EndHead; uint16 EndSecCyl; uint32 BeginAbsSect; uint32 TotalSectors;

    BeginSecCyl is designed to be suitable for use in int 13h .

      les BX, buf mov AX, 0201h ; 02 = read, 01 = 1 sector mov CX, [BeginSecCyl] ; значение из соответствующего поля mov DH, [BeginHead] mov DL, 80h ; 80h для HDD + номер диска int 13h

    It considers you the boot sector of the selected partition in buf .

    In BeginSecCyl lower 6 bits indicate the sector number, the upper 8 bits indicate the lower 8 bits of the cylinder number, and the remaining 2 bits (6–7) indicate the upper two bits of the cylinder number.

    Addressing CHS is outdated, it allows you to gain access to only 8 gigabytes of disk. Modern drives use LBA addressing. The LBA address of the first sector is in the BeginAbsSect .

    You cannot read large disks via int 31h , but the operating system allows you to open the entire disk as a file (Windows through CreateFile / ReadFile from "\\.\PhysicalDrive0" , Unix via /dev ), and read sectors at the desired offset. Newer operating systems ignore CHS values ​​from the MBR, and use LBA addresses.

    Since the BeginAbsSect field is 32 bits in size, this imposes another addressing limit: 2 terabytes. (An LBA address can take up to 48 bits.) This limit cannot be overcome while remaining in the MBR format. For larger discs, GPT is used.

    • Thank! <AntiFilterDown> - velikiyv4
    • @ velikiyv4: Please! - VladD
    • @ velikiyv4: Updated the answer. - VladD