Support managing floppy parameter table

boot/kernel.asm

@@ -24,6 +24,10 @@ print_banner:	mov si, banner
 		cmp al, 0x0D
 		jnz .loop
 
+		push dx
+		call dinit
+		pop dx
+
 		cmp dl, 0x80
 		jc .k
 		sub dl, (0x80-2)

kernel/drive.asm

@@ -28,6 +28,45 @@ bpb:		resb BPBSIZ4
 
 section .text
 
+		; initial setup for disk i/o
+dinit:		; copy previously set DPT to our data area
+		lds si, [4*0x1E]
+		mov di, dpt
+		mov cx, 11
+		rep movsb
+		; restore DS
+		xor ax, ax
+		mov ds, ax
+		; set vector
+		mov word [4*0x1E], dpt
+		mov word [4*0x1E+2], ds
+		ret
+
+		; restore DPT to default values if possible
+		; IN	dl	bios drive number
+rstdpt:		mov ah, 8
+		int 0x13
+		jc .ret
+		mov ax, es
+		or ax, di
+		test ax, ax
+		jz .ret
+		; DS:SI = ES:DI
+		mov ax, es
+		mov ds, ax
+		mov si, di
+		; ES:DI = dpt
+		xor ax, ax
+		mov es, ax
+		mov di, dpt
+		; do the copy
+		mov cx, 11
+		rep movsb
+.ret:		; restore DS to 0
+		xor ax, ax
+		mov ds, ax
+		ret
+
 		; log in drive
 		; currently only supports 18 sectors 2 heads floppies
 		; IN	dl	drive number
@@ -46,6 +85,8 @@ logdrv:		; dont do anything if drive already selected
 		mov [dsknum], dl
 		; save info for bios
 		mov [biosnum], dl
+		; reset dpt to defaults
+		call rstdpt
 		; set default geometry (1.44 MB floppy)
 		mov word [bpb+BPBNOS], 2
 		mov word [bpb+BPBSPT], 18
@@ -54,10 +95,14 @@ logdrv:		; dont do anything if drive already selected
 		xor dx, dx
 		call mapabs
 		; copy bios parameter block
-		lea si, [dskbuf+BPBOFF]
+		; TODO: guess from first byte of FAT if BPB invalid
+		mov si, dskbuf+BPBOFF
 		mov di, bpb
 		mov cx, BPBSIZ4
 		rep movsb
+		; copy SPT to DPT
+		mov al, [bpb+BPBSPT]
+		mov [dpt+4], al
 		; make sure partition offset is forced zero
 		xor ax, ax
 		mov [bpb+BPBHS], ax
@@ -68,7 +113,7 @@ calchs:		; put sectors per cylinder into bx
 		mov ax, [bpb+BPBSPT]
 		mul word [bpb+BPBNOS]
 		mov bx, ax
-		; but linear sector num into dx:ax
+		; put linear sector num into dx:ax
 		mov ax, [dskseek]
 		mov dx, [dskseek+2]
 		; dx:ax = linear count, bx = sectors / cylinder