169 lines
2.8 KiB
NASM
169 lines
2.8 KiB
NASM
section .bss
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disk_buffer:
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resb 512
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disk_current:
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resb 1
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lba_supported:
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resb 1
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disk_chs: ; DL, DH, CL, CH for int 13h CHS functions
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resb 4
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disk_dap: ; disk access packet for int 13h EBIOS functions
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resb 0x10
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disk_spc: ; sectors per cylinder = heads * spt
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resw 1
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disk_spt:
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resb 1
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part_offset:
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resd 1
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part_size:
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resd 1
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; Disk access packet for EBIOS extensions
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dapps: equ 0 ; byte packet size
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dapnum: equ 2 ; word number of sectors to transfer
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dapbuf: equ 4 ; dword transfer buffer
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dapsec: equ 8 ; qword absolute sector number
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dapsiz: equ 16
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section .text
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; Convert between drive number and BIOS dl
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; Bidirectional mapping
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; 0 <-> 0, 1 <-> 1, 2 <-> 0x80, 3 <-> 0x81
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dnconv: mov cx, 7
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ror dx, 1
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rol dl, 1
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dncl: rcl dl, 1
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rcr dh, 1
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loop dncl
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xchg dh, dl
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ret
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; Select a drive for I/O
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; IN dl 0=A, 1=B, 2=C, 3=D
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select: mov byte [disk_current], 0xFF
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mov byte [lba_supported], 0
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cmp dl, 0x02
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jc select_floppy
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; detect EBIOS/LBA extensions
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call dnconv
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mov byte [disk_chs], dl
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mov ah, 0x41
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mov bx, 0x55AA
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int 0x13
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test cx, 1
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jz select_hdd
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cmp bx, 0xAA55
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jnz select_hdd
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mov byte [lba_supported], 1
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jmp select_hdd
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select_floppy:
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push dx
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call seek_zero
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call read
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; load disk geometry
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mov ax, [disk_buffer+0x0B+bpb_spt]
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mov [disk_spt], ax
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mul word [disk_buffer+0x0B+bpb_nos]
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mov [disk_spc], ax
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; set partition size
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; we dont rely on the 'total sectors' given in the BPB
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; instead, we assume maximum cylinder
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; 1023 is the maximum encodable for int 13h
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lea di, [part_offset]
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xor ax, ax
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; partition offset = 0
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stosw
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stosw
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; partition size = 0x00010000 = 32MB
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stosw
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inc ax
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stosw
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pop dx
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ret
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select_hdd:
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push dx
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; load CHS geometry for drive
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mov ah, 0x8
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int 0x13
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xor ah, ah
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mov al, cl
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mov [disk_spt], ax
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mul byte dh
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mov [disk_spc], ax
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; load offset and size of first partition
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call seek_zero
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call read
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mov bx, disk_buffer+0x1BE
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lea si, [bx+8]
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lea di, [part_offset]
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mov cx, 4
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rep movsw
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; read bpb
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mov dx, [part_offset+2]
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mov ax, [part_offset]
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call seek
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call read
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pop dx
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ret
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; Set absolute sector number
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; IN dx:ax 32-bit sector number
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seek: push ax
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or ax, dx
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pop ax
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jz seek_zero
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; dx:ax = lba
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div word [disk_spc]
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xchg ax, dx
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; dx = cylinder, ax = head * spt + sector
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div byte [disk_spt]
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; dx = cylinder, al = head, ah = sector
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xchg dl, dh
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ror dl, 1
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ror dl, 1
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or dl, ah
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inc dx
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; dh bit 0-7: cylinder 0-7
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; dl bit 0-5: sector number 0-5
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; dl bit 6-7: cylinder 8-9
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; store
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mov byte [disk_chs+1], al
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mov word [disk_chs+2], dx
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ret
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seek_zero:
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; set chs data to first sector
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mov byte [disk_chs+1], 0
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mov word [disk_chs+2], 1
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; set lba data to first sector
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lea di, [disk_dap+dapsec]
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mov cx, 4
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xor ax, ax
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rep stosw
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ret
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; Read a sector into buffer
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read: mov ax, 0x0201
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mov dx, [disk_chs]
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mov cx, [disk_chs+2]
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lea bx, [disk_buffer]
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push cs
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pop es
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int 0x13
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ret
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; Write a sector into buffer
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write: stc
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ret
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