this came purely by accident when i am doing some internal consistancy
check, according to the Inside Mac on the Apple WebSite.

TYPE Block0 = 
PACKED RECORD
   sbSig:         Integer;    {device signature}
   sbBlkSize:     Integer;    {block size of the device}
   sbBlkCount:    LongInt;    {number of blocks on the device}
   sbDevType:     Integer;    {reserved}
   sbDevId:       Integer;    {reserved}
   sbData:        LongInt;    {reserved}
   sbDrvrCount:   Integer;    {number of driver descriptor entries}
   ddBlock:       LongInt;    {first driver's starting block}
   ddSize:        Integer;    {size of the driver, in 512-byte blocks}
   ddType:        Integer;    {operating system type (MacOS = 1)}
   ddPad:         ARRAY [0..242] OF Integer; {additional drivers, if any}
END;

  ->  "SCSI.h" in Universal Headers 3.3 defines 

struct Block0 {
	unsigned short		sbSig;						
	unsigned short 		sbBlkSize;					
	unsigned long 		sbBlkCount;					
	unsigned short 		sbDevType;					
	unsigned short 		sbDevId;					
	unsigned long 		sbData;						
	unsigned short 		sbDrvrCount;			
	unsigned long 		ddBlock;					
	unsigned short 		ddSize;						
	unsigned short 		ddType;						
	unsigned short 		ddPad[243];
};

the correct pascal interpretation for [0..242] IS 243 units. However, that
puts the entire structure size to be 516 bytes rather than 512 bytes,
larger thank a 512 block, if someone happens to do

	unsigned char	buffer[512];
	read(fd, &buffer, sizeof(Block0));

it is going to bite, no?

-jake