boot/fat: Use segmentation to allow up to 64Kb fat/kernel size

boot/fat.asm

@@ -1,7 +1,11 @@
 		; Bootsector for FAT12/FAT16 filesystems
-		; Takes defines:
-		; 	FAT12	FAT has 1.5 bytes per entry
-		; 	FAT16	FAT has WORD per entry
+		; Memory layout
+ourself:	equ 0x0800 ; 1 sector, this code
+cluslist:	equ 0x0A00 ; up-growing list of words
+stack:		equ 0x7C00 ; growing down
+bootsect:	equ 0x7C00 ; our origin before reloc
+		           ; default buffer for reads
+
 		cpu 8086
 
 		org 0x0800
@@ -19,7 +23,7 @@ params:		times bpb_size db 0
 
 init:		xor ax, ax
 		mov ss, ax
-		mov sp, ax
+		mov sp, stack
 
 		; save potential partition table entry
 		push ds
@@ -28,8 +32,8 @@ init:		xor ax, ax
 		mov ds, ax
 		mov es, ax
 
-		mov si, 0x7C00
-		mov di, 0x0800
+		mov si, bootsect
+		mov di, ourself
 		mov cx, 0x0100
 		rep movsw
 		jmp 0x0:main
@@ -106,9 +110,16 @@ offset_part:	; add partition offset
 		add dx, [bp+bpb.sectoroffset+2]
 		ret
 
-resetbuf:	xor ax, ax
-		mov dx, ax
-		mov di, buf
+resetbuf:	; DS = ES
+		push es
+		pop ds
+		; ES:DI = 07C0:0000
+		mov ax, ( bootsect >> 4 )
+		mov es, ax
+		xor di, di
+		; AX,CX = 0
+		xor ax, ax
+		xor cx, cx
 		ret
 
 main:		mov bp, params
@@ -116,13 +127,14 @@ main:		mov bp, params
 
 		; load root directory
 		call resetbuf
+		push di
 		call offset_rootdir
 		call rootdirsects
 		mov cx, bx
 		call read
 
 		; search for file
-		mov bx, buf
+		pop bx
 		mov cx, [bp+bpb.direntries]
 .loop:		push cx
 		mov si, filename
@@ -133,23 +145,28 @@ main:		mov bp, params
 		pop cx
 		loopne .loop
 		jne err_not_found
-		push word [bx-0x20+0x1A]
+		push word [es:bx-0x20+0x1A]
 
 		; load fat table
 		call resetbuf
+		push di
 		call offset_fat
 		mov cx, [bp+bpb.fatsectors]
 		call read
 
 		; fetch cluster numbers (FAT12-specific)
-		mov di, 0x0500
-		pop ax
+		call resetbuf
+		mov es, ax
+		mov di, cluslist
+		pop si ; buffer offset
+		pop ax ; clus num
+		push di ; save list for later
 readfat:	stosw
 		mov bx, ax
 		shr bx, 1
 		pushf
 		add bx, ax
-		mov ax, [buf+bx]
+		mov ax, [bx]
 		popf ; CF: 0=even, 1=odd entry
 		jc .odd
 		and ax, 0x0FFF
@@ -162,8 +179,8 @@ readfat:	stosw
 		stosw
 
 		; load clusters
-		mov si, 0x500
-		mov di, 0x7C00
+		pop si
+		call resetbuf
 loadclus:	lodsw
 		sub ax, 2
 		jc .jump
@@ -174,7 +191,8 @@ loadclus:	lodsw
 		call read
 		jmp loadclus
 
-.jump:		pop si
+.jump:		mov dl, [bp+bpb.drivenum]
+		pop si
 		pop ds
 		jmp 0x7C00