Add support for directly booting from FAT12 filesystems.

This shouldn't really be needed except that Linux wont
mount a small FAT16 filesystem - it always treats it as FAT12
even when you tell it otherwise.
There was enough free space before the main FS on the USB stick
I've been using for an extra small FAT fs to boot from.

sys/arch/i386/stand/fatboot/Makefile

@@ -1,6 +1,7 @@
-# $NetBSD: Makefile,v 1.2 2007/01/06 20:47:15 dsl Exp $
+# $NetBSD: Makefile,v 1.3 2012/03/10 23:59:36 dsl Exp $
 
-SUBDIR=	fat16
+SUBDIR=		fat16
+SUBDIR+=	fat12
 
 .include <bsd.subdir.mk>
 .include <bsd.obj.mk>

sys/arch/i386/stand/fatboot/fat12/Makefile

@@ -0,0 +1,6 @@
+# $NetBSD: Makefile,v 1.1 2012/03/10 23:59:36 dsl Exp $
+
+PROG=bootxx_fat12
+FAT_ENTRY_SIZE=12
+
+.include <../Makefile.fat>

sys/arch/i386/stand/fatboot/fatboot.S

@@ -1,4 +1,4 @@
-/*	$NetBSD: fatboot.S,v 1.3 2008/04/29 06:53:02 martin Exp $	*/
+/*	$NetBSD: fatboot.S,v 1.4 2012/03/10 23:59:36 dsl Exp $	*/
 
 /*-
  * Copyright (c) 2007 The NetBSD Foundation, Inc.
@@ -50,10 +50,12 @@
 #endif
 
 /* Support for FAT32 could be added - but hasn't been yet. */
-#if FAT_ENTRY_SIZE != 16
+#if FAT_ENTRY_SIZE != 16 && FAT_ENTRY_SIZE != 12
 #error Unsupported FAT_ENTRY_SIZE value
 #endif
 
+#define FAT_SIZE_STR (('0'+ FAT_ENTRY_SIZE / 10) | ('0' + FAT_ENTRY_SIZE % 10) << 8)
+
 #define PBR_AFTERBPB	62		/* BPB size in floppy master BR */
 
 #ifdef TERSE_ERROR
@@ -112,8 +114,13 @@ start0:
 	mov	%sp, %bp		/* to access the pbp */
 	push	%dx			/* save drive at -2(%bp) */
 
+/* We put the LBA bios command block on stack.
+ * Since we only want a 32bit sector number, stack a zero */
+	push	%cs			/* %cs is zero */
+	push	%cs			/* 64-bit for LBA read */
+
 	set_err(ERR_NOT_FAT16)
-	cmpl	$'A'|'T'<<8|'1'<<16|'6'<<24, bs_file_sys_type+1(%bp)
+	cmpl	$'A'|'T'<<8|FAT_SIZE_STR<<16, bs_file_sys_type+1(%bp)
 	jne	error
 
 /* Add 'reserved' (inside ptn) to 'hidden' (ptn offset) */
@@ -121,14 +128,21 @@ start0:
 	addl	bpb_hidden_secs(%bp), %eax
 	mov	%eax, fat_sector(%bp)	/* To get first sector of FAT */
 
+#if FAT_ENTRY_SIZE == 12
+/* Read the entire FAT */
+	push	%eax
+	push	%ds
+	push	$fat_buffer
+	push	$12			/* 12 sectors is assumed 6k */
+	call	read_lba
+#endif
+
 /* Determine base of root directory */
 	movzbw	bpb_FATs(%bp), %ax	/* Count of FATs */
 	mulw	bpb_FAT_secs(%bp)	/* FAT size in %dx:%ax */
 	shl	$16,%edx
 	xchg	%ax,%dx			/* FAT size now in %edx */
 	addl	fat_sector(%bp), %edx	/* Directory is after FATs */
-	push	%cs			/* %cs is zero */
-	push	%cs			/* 64-bit for LBA read */
 	pushl	%edx			/* Sector number of root dir */
 
 	push	$0x1000			/* Read to 0x10000:0 */
@@ -149,7 +163,7 @@ start0:
 /* Read in the entire root directory */
 	push	%ax			/* Sectors in root directory */
 	cwtl
-	addl	%eax, %edx		/* %edx now sector of first file */
+	addl	%eax, %edx		/* %edx now sector of first cluster */
 	call	read_lba		/* Read entire directory */
 
 /* Scan directory for our file */
@@ -195,16 +209,16 @@ found_boot:
 	jnz	1b
 	dec %cx
 	dec %cx
-	movw	%es:15(%di), %ax	/* Cluster number for file start */
+	movw	%es:(26-11)(%di), %ax	/* Cluster number for file start */
 	push	%es			/* We increment the 'segment' ... */
 	pop	%di			/* ... after each read, offset is 0 */
 
 read_data_block:
 	mov	%ax, %bx		/* Save cluster number */
 	shl	%cl, %eax		/* Convert to sector number */
-	add	%eax, %edx
+	jz	error			/* Sanity bail-out */
-	pushl	%edx			/* Sector to read */
+	add	%edx, %eax
-	sub	%eax, %edx		/* Recover base segment */
+	pushl	%eax			/* Sector to read */
 	push	%di			/* Target address segment! */
 	push	$0
 	push	sec_p_cl_w(%bp)
@@ -218,6 +232,20 @@ read_data_block:
 
 /* Lookup FAT slot number in FAT table */
 	mov	%bx, %ax		/* Recover cluster number */
+#if FAT_ENTRY_SIZE == 12
+	shr	$1, %ax
+	jc	1f
+	add	%ax, %bx
+	mov	fat_buffer(%bx), %ax
+	and	$0xf,%ah
+	jmp	2f
+1:	add	%ax, %bx
+	mov	fat_buffer(%bx), %ax
+	shr	$4, %ax
+2:
+	cmp	$0x0fff, %ax
+	jb	read_data_block
+#else
 	push	%dx
 	xor	%dx, %dx
 	divw	bpb_bytes_per_sec(%bp)
@@ -243,6 +271,7 @@ lookup_fat:
 	movzwl	fat_buffer(%bx), %eax	/* Next FAT slot */
 	cmp	$0xfff0, %ax
 	jb	read_data_block
+#endif
 
 /* Found end of FAT chain - must be EOF  - leap into loaded code */
 	mov	$0x1000, %ax
@@ -255,7 +284,7 @@ err1:	jmp	error
 /* Set parameters expected by /boot */
 magic_ok:
 	mov	bpb_hidden_secs(%bp), %ebx	/* ptn base sector */
-	movb	-2(%bp), %dl		/* disk number (could pop %dx) */
+	movb	-2(%bp), %dl		/* disk number */
 	mov	$boot_params + 4, %si
 	push	%es
 	push	$0
@@ -286,6 +315,7 @@ read_lba:
 #include <message.S>
 #if 0
 #include <dump_eax.S>
+dump_eax_buff = start
 #endif
 
 errtxt: .ascii	"Error "		/* runs into newline... */
@@ -296,7 +326,9 @@ newline:
 #ifndef TERSE_ERROR
 ERR_READ:		.asciz	"Disk read"
 ERR_NO_BOOT:		.asciz	"No /boot"
-ERR_NOT_FAT16:		.asciz	"Not FAT16 ptn"
+ERR_NOT_FAT16:		.ascii	"Not FAT"
+			.word	FAT_SIZE_STR
+			.asciz	" ptn"
 ERR_NO_BOOT_MAGIC_2:	.asciz	"No magic in /boot"
 #endif
 
@@ -321,3 +353,4 @@ fat_cache:	.word	0xffff		/* Sector number in buffer */
 	. = _C_LABEL(start) + 0x1fe
 	.word	0xaa55
 fat_buffer:				/* 2 sectors worth of FAT table */
+					/* 6k for FAT12 */