ATA read/write sectors
From OSDev Wiki
Since interrupts (like INT 13h) can't be called easily in protected mode or long mode, direct disk access through ports might be the only solution. The below code is example of subroutines to read and write disk sectors directly from the first hard disk (80h) in long mode using CHS and LBA.
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ATA read sectors
Read in CHS mode
Accessing disk using CHS (cylinder,head,sector) indexes seem to be kinda old type but is the base for LBA access. The following NASM long-mode subroutine reads CH sectors to memory address indicated by RDI register.
;============================================================================= ; ATA read sectors (CHS mode) ; Max head index is 15, giving 16 possible heads ; Max cylinder index can be a very large number (up to 65535) ; Sector is usually always 1-63, sector 0 reserved, max 255 sectors/track ; If using 63 sectors/track, max disk size = 31.5GB ; If using 255 sectors/track, max disk size = 127.5GB ; See OSDev forum links in bottom of [http://wiki.osdev.org/ATA] ; ; @param EBX The CHS values; 2 bytes, 1 byte (BH), 1 byte (BL) accordingly ; @param CH The number of sectors to read ; @param RDI The address of buffer to put data obtained from disk ; ; @return None ;============================================================================= ata_chs_read: pushfq push rax push rbx push rcx push rdx push rdi mov rdx,1f6h ;port to send drive & head numbers mov al,bh ;head index in BH and al,00001111b ;head is only 4 bits long or al,10100000b ;default 1010b in high nibble out dx,al mov rdx,1f2h ;Sector count port mov al,ch ;Read CH sectors out dx,al mov rdx,1f3h ;Sector number port mov al,bl ;BL is sector index out dx,al mov rdx,1f4h ;Cylinder low port mov eax,ebx ;byte 2 in ebx, just above BH mov cl,16 shr eax,cl ;shift down to AL out dx,al mov rdx,1f5h ;Cylinder high port mov eax,ebx ;byte 3 in ebx, just above byte 2 mov cl,24 shr eax,cl ;shift down to AL out dx,al mov rdx,1f7h ;Command port mov al,20h ;Read with retry. out dx,al .still_going: in al,dx test al,8 ;the sector buffer requires servicing. jz .still_going ;until the sector buffer is ready. mov rax,512/2 ;to read 256 words = 1 sector xor bx,bx mov bl,ch ;read CH sectors mul bx mov rcx,rax ;RCX is counter for INSW mov rdx,1f0h ;Data port, in and out rep insw ;in to [RDI] pop rdi pop rdx pop rcx pop rbx pop rax popfq ret
Read in LBA mode
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;============================================================================= ; ATA read sectors (LBA mode) ; ; @param EAX Logical Block Address of sector ; @param CL Number of sectors to read ; @param RDI The address of buffer to put data obtained from disk ; ; @return None ;============================================================================= ata_lba_read: pushfq and rax, 0x0FFFFFFF push rax push rbx push rcx push rdx push rdi mov rbx, rax ; Save LBA in RBX mov edx, 0x01F6 ; Port to send drive and bit 24 - 27 of LBA shr eax, 24 ; Get bit 24 - 27 in al or al, 11100000b ; Set bit 6 in al for LBA mode out dx, al mov edx, 0x01F2 ; Port to send number of sectors mov al, cl ; Get number of sectors from CL out dx, al mov edx, 0x1F3 ; Port to send bit 0 - 7 of LBA mov eax, ebx ; Get LBA from EBX out dx, al mov edx, 0x1F4 ; Port to send bit 8 - 15 of LBA mov eax, ebx ; Get LBA from EBX shr eax, 8 ; Get bit 8 - 15 in AL out dx, al mov edx, 0x1F5 ; Port to send bit 16 - 23 of LBA mov eax, ebx ; Get LBA from EBX shr eax, 16 ; Get bit 16 - 23 in AL out dx, al mov edx, 0x1F7 ; Command port mov al, 0x20 ; Read with retry. out dx, al .still_going: in al, dx test al, 8 ; the sector buffer requires servicing. jz .still_going ; until the sector buffer is ready. mov rax, 256 ; to read 256 words = 1 sector xor bx, bx mov bl, cl ; read CL sectors mul bx mov rcx, rax ; RCX is counter for INSW mov rdx, 0x1F0 ; Data port, in and out rep insw ; in to [RDI] pop rdi pop rdx pop rcx pop rbx pop rax popfq ret
ATA write sectors
A write is mostly equivalent to performing a read operation. The only changes needed are a change in command (0x30 for chs write), and the direction of the data, which is written (rep outsw from *SI) to the data port rather than read (rep insw to *DI).
;============================================================================= ; ATA write sectors (LBA mode) ; ; @param EAX Logical Block Address of sector ; @param CL Number of sectors to write ; @param RDI The address of data to write to the disk ; ; @return None ;============================================================================= ata_lba_write: pushfq and rax, 0x0FFFFFFF push rax push rbx push rcx push rdx push rdi mov rbx, rax ; Save LBA in RBX mov edx, 0x01F6 ; Port to send drive and bit 24 - 27 of LBA shr eax, 24 ; Get bit 24 - 27 in al or al, 11100000b ; Set bit 6 in al for LBA mode out dx, al mov edx, 0x01F2 ; Port to send number of sectors mov al, cl ; Get number of sectors from CL out dx, al mov edx, 0x1F3 ; Port to send bit 0 - 7 of LBA mov eax, ebx ; Get LBA from EBX out dx, al mov edx, 0x1F4 ; Port to send bit 8 - 15 of LBA mov eax, ebx ; Get LBA from EBX shr eax, 8 ; Get bit 8 - 15 in AL out dx, al mov edx, 0x1F5 ; Port to send bit 16 - 23 of LBA mov eax, ebx ; Get LBA from EBX shr eax, 16 ; Get bit 16 - 23 in AL out dx, al mov edx, 0x1F7 ; Command port mov al, 0x30 ; Write with retry. out dx, al .still_going: in al, dx test al, 8 ; the sector buffer requires servicing. jz .still_going ; until the sector buffer is ready. mov rax, 256 ; to read 256 words = 1 sector xor bx, bx mov bl, cl ; write CL sectors mul bx mov rcx, rax ; RCX is counter for OUTSW mov rdx, 0x1F0 ; Data port, in and out mov rsi, rdi rep outsw ; out pop rdi pop rdx pop rcx pop rbx pop rax popfq ret
