Extended read of int 0 × 13 (Ah = 0 × 42)

Link: http://www.mouseos.com/win7/int_0x13.html#42

1. Extended read of int 0x13 (Ah = 0x42)

The 0x42 function of int 0x13 reads n sectors from the disk to the buffer.

Entry parameters: Ah = 0x42, DL = disk number (0x80 is the hard disk), DS: SI = Buffer

The read Buffer structure, which is described as follows in C: Struct buffer_packet { Short buffer_packet_size;/* struct's size (which can be 0x10 or 0x18 )*/ Short sectors;/* Number of Sectors Read */ Char * buffer;/* buffer address */ Long long start_sectors;/* start from which sector to read */ Long long * l_buffer;/* 64-bit buffer address */ } Buffer;

The buffer_packet structure can be 16 bytes or 24 bytes.

When buffer_packet_size is set to 0x10, the last l_buffer is invalid. When buffer_packet_size is set to 0x18, l_buffer needs to be provided.

Note: Buffer address in the buffer_packet structure. It is a logical address, that is, segment: offset. The offset value is placed in the lower word, and the segment value is placed in the higher word. Remember!

 

The following is a typical usage from Windows 7 MBR.

Typical int 0x13 0th x 42 function usage: /* Use the int 0x13 extended read disk */ 00000061 666800000000 push DWORD 0x0 00000067 66ff7608 push DWORD [bp + 0x8]/* Start sectors for read */ 0000006b 680000 push word 0x0 0000006e 68007c push word 0x7c00/* buffer address */ 00000071 680100 push word 0x1 00000074 681000 push word 0x10/* use the 16 bytes structure, that is, no 64-bit buffer address */ 00000077 b442 mov ah, 0x42 00000079 8a5600 mov DL, [bp + 0x0]/* hard disk = 0x80 */ 2017007c 8bf4 mov Si, SP/* buffer_packet address (DS: Si )*/ 2017007e CD13 int 0x13

 

 

2. Int 0x13 get disk parameters (Ah = 0x48)

The obtained disk parameters are stored in the provided buffer. These parameters include:

★Disk cylinders quantity
★Disk heads count
★Sectors per track
★Total number of sectors of a disk
★Number of bytes per slice

This Buffer structure is described in C as follows: Struct driver_parameters { Short buf_size;/* size of the buffer structure */ Short flags;/* flag */ Int cylinders;/* Number of cylinders */ Int heads;/* Number of heads */ Int sectors_per_track;/* number of sectors per track */ Long long sectors;/* Total number of sectors on the disk */ Short bytes_per_sector;/* number of bytes per slice */ } Buffer;

The following is a typical usage of DBR from Windows 7:

Typical int 0x13 0th x 48 function usage: 2017008d 1E PUSH DS 2017008e 83ec18 sub sp, byte + 0x18/* Reserved 0x18 space */ 00000091 681a00 push word 0x1a/* write the 1st words of the buffer, that is, buf_size = 0x1a */ 00000094 b448 mov ah, 0x48 00000096 8a160e00 mov DL, [0xe]/* drive ID, that is, 0x80 */ 2017009a 8bf4 mov Si, SP 2017009c 16 push SS 2017009d 1f pop DS 2017009e CD13 int 0x13/* Get Driver parameters */