rdos/kernel/drive.asm

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

section .text

drives_init:	; CX = number of floppy drives in the system
		int 0x11
		mov cl, 6
		shr ax, cl
		inc ax
		and ax, 3
		mov cx, ax

		mov bx, drive_table
		xor dx, dx

.loop:		push cx
		push dx
		mov [bx+drive.biosnum], dl
		mov byte [bx+drive.flag], 1 ; 1 = drive letter used
		; query bios for floppy format
		mov ah, 8
		push bx
		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
		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
		; advance loop
		pop dx
		pop cx
		inc dl
		add bx, drive_size
		loop .loop
		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

drive_write:	mov ax, 0x0301
		mov cx, [chs_cx]
		mov dx, [chs_dx]
		mov bx, diskbuf
		int 0x13
		ret