Implement reading/writing a DTA full from a cluster without ptr advance

kernel/cluster.asm

@@ -1,7 +1,95 @@
 ; read/write data from FAT clusters
 ; requires defines from fcb.asm
 
-read	mov bp, sp
-	call fcbdrv
+section .rodata
 
+	; sector sizes for fast lookup
+sstab	dw	0x800,0x400,0x200,0x100,0x80
+
+section .text
+
+	; Map part of cluster into dskbuf
+	; IN	DX:AX	sector number within cluster area
+mapclus	mov bx, [cs:bpb+BPBRDE]
+	mov cl, 4
+	shr bx, cl ; 32 bytes per entry
+	add ax, bx
+	adc dx, 0
+	; counting from beginning of dir
+	; add fat table sizes
+maprde	xor ch, ch
+	mov cl, byte [cs:bpb+BPBFN]
+.loop	add ax, [cs:bpb+BPBFS]
+	adc dx, 0
+	loop .loop
+	; counting from beginning of FAT
+	; add reserved sector count
+mapfat	add ax, [cs:bpb+BPBRSC]
+	adc dx, 0
+
+	jmp maprel
+
+	; split AX at a bit boundary
+	; basically a DIV 2^CX
+	; IN	ax	divident
+	;	cx	integer log2(divisor)
+	; OUT	ax	quotient
+	;	dx	remainder (lower CX bits)
+split	mov dx, 1
+	shl dx, cl
+	dec dx
+	and dx, ax
+	shr ax, cl
+	ret
+
+filesec	call fcbdrv
+	mov bx, dx
+
+	; find sector size scale factor
+	mov ax, cs
+	mov es, ax
+	mov ax, [cs:bpb+BPBSS]
+	mov di, sstab
+	mov cx, 5
+	repne scasw
+	; CX is now sector size div 128
+	; same format as in DPT
+	mov ah, 0
+	mov al, [ds:bx+FCBRN]
+	call split
+	; ax = sector within current cluster
+	; dx = record number
+	push dx
+	mov di, ax
+
+	mov ax, [ds:bx+FCBCLUS]
+	sub ax, 2
+	mov ch, 0
+	mov cl, [cs:bpb+BPBSC]
+	mul cx
+	add ax, di
+	call mapclus
+
+	pop bx
+	mov cl, 7
+	shl bx, cl
+	add bx, dskbuf
+	mov dx, bx
+	mov ax, cs
+	mov ds, ax
+	ret
+
+read	mov bp, sp
+	call filesec
+	call getdta
+	mov cx, 0x80
+	call stofar
+	ret
+
+write	mov bp, sp
+	call filesec
+	call getdta
+	mov cx, 0x80
+	call lodfar
+	call dirty
 	ret