Question:

% disklabel sd0 | egrep '^#|^  [cdgh]:'
# /dev/rsd0d:
#        size   offset    fstype   [fsize bsize   cpg]
  c:  3686400   491520    unused        0     0       	# (Cyl.  240 - 2039)
  d:  4177920        0    unused        0     0       	# (Cyl.    0 - 2039)
  g:   245728       32     MSDOS                    	# (Cyl.    0*- 119*)
  h:   245728   245792     MSDOS                    	# (Cyl.  120*- 239*)
% 

  Where do the size/offset of g: come from?
  Where do the size/offset of h: come from?

Answer:

% dd if=/dev/sd0d of=mbr count=1
1+0 records in
1+0 records out
512 bytes transferred in 1 secs (512 bytes/sec)
% sed -n 42,65p /usr/include/machine/disklabel.h
#define	DOSBBSECTOR	0		/* DOS boot block relative sector # */
#define	DOSPARTOFF	446
#define	NDOSPART	4

struct dos_partition {
	unsigned char	dp_flag;	/* bootstrap flags */
	unsigned char	dp_shd;		/* starting head */
	unsigned char	dp_ssect;	/* starting sector */
	unsigned char	dp_scyl;	/* starting cylinder */
	unsigned char	dp_typ;		/* partition type (see below) */
	unsigned char	dp_ehd;		/* end head */
	unsigned char	dp_esect;	/* end sector */
	unsigned char	dp_ecyl;	/* end cylinder */
	unsigned long	dp_start;	/* absolute starting sector number */
	unsigned long	dp_size;	/* partition size in sectors */
} dos_partitions[NDOSPART];

/* Known DOS partition types. */
#define	DOSPTYP_386BSD	0xa5		/* 386BSD partition type */
#define DOSPTYP_NETBSD	DOSPTYP_386BSD	/* NetBSD partition type (XXX) */
#define DOSPTYP_FAT12	0x1		/* 12-bit FAT */
#define DOSPTYP_FAT16S	0x4		/* 16-bit FAT, less than 32M */
#define DOSPTYP_FAT16B	0x6		/* 16-bit FAT, more than 32M */
#define DOSPTYP_FAT16C	0xe		/* 16-bit FAT, CHS-mapped */
% hexdump -s 446 -e '"%06_ax:" 8/1 " %02x" 2/4 " %8d" "\n"' mbr
0001be: 00 01 01 00 06 3f 20 77       32   245728
0001ce: 00 00 01 78 05 3f 20 ef   245760   245760
0001de: 00 00 00 00 00 00 00 00        0        0
0001ee: 80 00 01 f0 a5 3f e0 fe   491520  1603584
0001fe: 55 aa                                    
% dd if=/dev/sd0d of=ext skip=245760 count=1
1+0 records in
1+0 records out
512 bytes transferred in 1 secs (512 bytes/sec)
% hexdump -v -s 446 -e '"%06_ax:" 8/1 " %02x" 2/4 " %8d" "\n"' ext
0001be: 00 01 01 78 06 3f 20 ef       32   245728
0001ce: 00 00 00 00 00 00 00 00        0        0
0001de: 00 00 00 00 00 00 00 00        0        0
0001ee: 00 00 00 00 00 00 00 00        0        0
0001fe: 55 aa                                    
% echo 245760+32 | bc
245792
% 

Hiroyuki Ito