Hi, My knowledge regarding assembly code is very limited and I wanted to ask if someone could help figure out what the following is doing? There is a routine that calls the disk (this is code loaded directly by BIOS) and I would like to know where this is and how it is executed: Code: //allow interrupts :0000 cli //ax=0 :0001 xor ax, ax //ss=ax(=0) :0003 mov ss, ax //sp=0x7c00 :0005 mov sp, 7C00h //si=sp :0008 mov si, sp //es=ax(=0) :000A push ax :000B pop es //ds=ax(=0) :000C push ax :000D pop ds :000E sti :000F cld //di=0x600 //cx=0x100 :0010 mov di, 600h :0013 mov cx, 100h :0016 rep movsw // // :0018 jmp far ptr 0:61Dh //dh=0 //cx=2 //di=5 :001D mov dh, 0 :001F mov cx, 2 :0022 mov di, 5 :0025 :0025 loc_25: //bx=0x700 //ax=0x201 :0025 mov bx, 700h :0028 mov ax, 201h //di=16 bit destination register :002B push di // AX= AH | AL=0x201 // CX=CH | CL = 2 //AL=1=> number of sector to read =1 //AH=2 => int 13h will be READ SECTORS INTO MEMORY // CH=track=0 // CL= sector = 2 //this code reads the second sector, that is to say the data from 0x200 to 0x3FE. :002C int 13h ; DISK - READ SECTORS INTO MEMORY :002C ; AL = number of sectors to read, CH = track, CL = sector :002C ; DH = head, DL = drive, ES:BX -> buffer to fill :002C ; Return: CF set on error, AH = status, AL = number of sectors read :002E pop di //will jump if CF=0 :002F jnb short loc_3D //ax=0 //CX=3 => track=0 and sector =3 :0031 xor ax, ax // AL=AH=0 //the disk is reseted :0033 int 13h ; DISK - RESET DISK SYSTEM :0033 ; DL = drive (if bit 7 is set both hard disks and floppy disks reset) //di-- //the destination counter is decremented :0035 dec di //jump if ZF is set //untill di=0 we loop :0036 jnz short loc_25 //end of loop, di=0 //si=source index //si=0x68A :0038 mov si, 68Ah //jump to next block :003B jmp short loc_78 :003D ; --------------------------------------------------------------------------- :003D :003D loc_3D: //cx=3 :003D mov cx, 3 :0040 :0040 loc_40: ; DATA XREF: :0083#r :0040 mov di, 5 :0043 :0043 loc_43: ; CODE XREF: :0059#j :0043 mov bx, 2000h :0046 push bx :0047 pop es :0048 assume es:nothing :0048 mov bx, 0 :004B mov ax, 220h ; DATA XREF: :002C#r :004B ; :0033#r ... :004E push di :004F int 13h ; DISK - READ SECTORS INTO MEMORY :004F ; AL = number of sectors to read, CH = track, CL = sector :004F ; DH = head, DL = drive, ES:BX -> buffer to fill :004F ; Return: CF set on error, AH = status, AL = number of sectors read :0051 pop di :0052 jnb short loc_60 :0054 xor ax, ax :0056 int 13h ; DISK - RESET DISK SYSTEM :0056 ; DL = drive (if bit 7 is set both hard disks and floppy disks reset) :0058 dec di :0059 jnz short loc_43 :005B mov si, 68Ah :005E jmp short loc_78 :0060 ; --------------------------------------------------------------------------- :0060 :0060 loc_60: ; CODE XREF: :0052#j :0060 mov cx, 3FFFh :0063 mov si, 800h :0066 xor di, di :0068 :0068 loc_68: ; CODE XREF: :0071#j :0068 lodsw :0069 and si, 0FFBFh :006C xor ax, es:[di] :006F stosw :0070 dec cx :0071 jnz short loc_68 :0073 jmp far ptr 2000h:0 :0078 ; --------------------------------------------------------------------------- :0078 :0078 loc_78: ; CODE XREF: :003B#j :0078 ; :005E#j ... :0078 lodsb :0079 cmp al, 0 :007B jz short loc_88 :007D push si :007E mov bx, 7 :0081 mov ah, 0Eh :0083 int 10h ; - VIDEO - WRITE CHARACTER AND ADVANCE CURSOR (TTY WRITE) :0083 ; AL = character, BH = display page (alpha modes) :0083 ; BL = foreground color (graphics modes) :0085 pop si :0086 jmp short loc_78 :0088 ; --------------------------------------------------------------------------- :0088 :0088 loc_88: ; CODE XREF: :007B#j :0088 ; :loc_88#j :0088 jmp short loc_88 :008A ; --------------------------------------------------------------------------- If more information is needed Ill try my best to provide as much detail as possible. Thanks!
