• Everything's size is measured in bytes (C's sizeof)
• A Ptr is a char*
• A Handle is a char**

• two-
• four-
• eight-
• sixteen-
• even 32-byte chunks

• Your software will run slower
• Your application will lock-up
• Your operating system will crash
• Your software will silently fail, yielding incorrect results

• Read first chunk of unaligned address
• Shift out "unwanted" bytes from first chunk
• Read second chunk of unaligned address
• Shift out unwanted bytes from second chunk
• Merge together and place into register

• Two-byte granularity
• When presented with an unaligned address, it would throw an exception
• If a debugger wasn't installed, the Mac OS would throw up a bomb dialog box
  • One button: Restart
• Old Mac coders still stuff their uninitialized pointers with odd values

• Lean and Mean

• Hybrid
• Built with emulation in mind
• Offers hardware support for unaligned integer access
• G1s offered hardware support for unaligned floating point access
• Modern PowerPC processors don't
• Support for unaligned integer access could go away -- but likely won't

• Machine: PowerBook 2000 (Pismo) 400 MHz

• one-
• two-
• four-
• eight-bytes at a time

• 38% faster

• 19% speed penalty

• Speed penalty rises to 28%

• 5% faster than Quad-byte

• Two orders of magnitude slower
• 2,372% speed penalty!

• One of the pages could be nonresident

• Instead, the store simply fails
• Bad news: most atomic code treats storage failure as a preemption and retries the store
• Infinite loop: store always fails, code always retries

• Nothing to emulate
• Sixteen-byte alignment

• Will read from the wrong address, yielding incorrect results
• Will write to the wrong address, corrupting memory

• Automatic truncation saves explicit truncation instructions

typedef struct {
	char	a;
	long	b;
	char	c;
}	Struct;

typedef struct {
	char	a;	//	1 byte
	long	b;	//	4 bytes
	char	c;	//	1 byte
}	Struct;		//	6 bytes, right?

Field Type	Field Name	Field Offset	Field Size	Field End
char	a	0	1	1
padding		1	1	2
long	b	2	4	6
char	c	6	1	7
padding		7	1	8
Total Size in Bytes:				8

Field Type	Field Name	Field Offset	Field Size	Field End
char	a	0	1	1
padding		1	3	4
long	b	4	4	8
char	c	8	1	9
padding		9	3	12
Total Size in Bytes:				12

• 68040's MOVE16 Instruction

• CodeWarrior or Project Builder?
• CodeWarrior: suballocate or map one-to-one to NewPtr()?

• http://redshed.net/align