rdos/kernel/drive.asm

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section .bss
drives: equ 16
; ptr into currently selected drive
drive_ptr: resw 1
drive_table: resb (drives * drive_size)
; DX and CX for int 13 CHS access
chs_dx: resw 1
chs_cx: resw 1
; disk buffer for I/O operations
diskbuf: resb 1024
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section .text
drives_init: ; CX = number of floppy drives in the system
int 0x11
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mov cl, 6
shr ax, cl
inc ax
and ax, 3
mov cx, ax
mov bx, drive_table
xor dx, dx
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.loop: push cx
push dx
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mov [bx+drive.biosnum], dl
mov byte [bx+drive.flag], 1 ; 1 = drive letter used
; query bios for floppy format
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mov ah, 8
push bx
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int 0x13
pop bx
; set up defaults if invalid data or fail
jc .defs
test cx, cx
jnz .load
.defs: ; use defaults: 360k, 40 cyl, 9 sects
les di, [0x1e * 4]
mov cx, 0x270
mov dh, 1
.load: ; copy dpt
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push es
push ds
pop es
pop ds
push cx
mov si, di
lea di, [bx+drive.dpt]
mov cx, 11
rep movsb
pop cx
; restore ds
push cs
pop ds
; get and save number of cylinders
mov ax, cx
xchg al, ah
rol ah, 1
rol ah, 1
and ax, 0x03FF
inc ax
mov [bx+drive.cylinders], ax
; save spt
mov ax, cx
and al, 0x3F
mov [bx+drive.dpt+dpt.lastsector], al
; multiply with heads and save sectors per cylinder
inc dh
mul dh ; ax = al * dh
mov [bx+drive.spc], ax
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; advance loop
pop dx
pop cx
inc dl
add bx, drive_size
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loop .loop
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ret
; select drive
; IN dl drive number
drive_select: xor dh, dh
mov ax, drive_size
mul dx
add ax, drive_table
mov word [drive_ptr], ax
ret
; reset currently selected drive
drive_reset: mov bx, [drive_ptr]
mov dl, [bx+drive.biosnum]
; our ptr = CX:BX
lea bx, [bx+drive.dpt]
mov cx, ds
; toggle in our ptr
mov si, [0x1E*4]
xchg bx, [si]
xchg cx, [si+2]
; do the call
xor ax, ax
int 0x13
pushf
; restore original ptr
xchg bx, [si]
xchg cx, [si+2]
; pass int 13 flags
popf
ret
; query status of selected drive
; OUT ah
drive_status: mov bx, [drive_ptr]
mov dl, [bx+drive.biosnum]
mov ah, 1
int 0x13
ret
; set sector for read/write operation
; IN dx:ax 32-bit sector number
drive_seek: mov bx, [drive_ptr]
div word [bx+drive.spc]
xchg ax, dx
; dx = cylinder, ax = head * spt + sector
div byte [bx+drive.dpt+dpt.lastsector]
; dx = cylinder, al = head, ah = sector
xchg dl, dh
ror dl, 1
ror dl, 1
or dl, ah
inc dx
; al: head number
; dh bit 0-7: cylinder bits 0-7
; dl bit 0-5: sector bits 0-5
; dl bit 6-7: cylinder bits 8-9
xchg al, ah
mov al, [bx+drive.biosnum]
; store
mov word [chs_dx], ax
mov word [chs_cx], dx
ret
drive_read: mov ax, 0x0201
mov cx, [chs_cx]
mov dx, [chs_dx]
mov bx, diskbuf
int 0x13
ret
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drive_write: mov ax, 0x0301
mov cx, [chs_cx]
mov dx, [chs_dx]
mov bx, diskbuf
int 0x13
ret