Inside Macintosh: Mac OS Runtime Architectures /

Chapter 11 - Classic 68K Runtime Conventions

Data Types

Table 11-1 lists the various binary data types and their sizes in the classic 68K runtime environment.

Table 11-1 Data types in the classic 68K runtime environment

Type	Size (bytes)	Alignment (bytes)	Range	Notes
UInt8	1	1	0 to 255
SInt8	1	1	-128 to 127
SInt16	2	2	-32,768 to 32,767
UInt16	2	2	0 to 65,535
SInt32	4	2	-2-31 to 231 -1
UInt32	4	2	0 to 232-1
Boolean	1	1		0 = false, nonzero = true
float	4	2	\xB1(2-149 to 2127)	IEEE 754 standard
double	8	2	\xB1(2-1074 to 21023)	IEEE 754 standard
Pointer	4	2	0 to FFFFFFFF
extended	10 or 12	2		SANE or MC68881 data type

All numeric and pointer data types are stored in big-endian format (that is, high bytes first, then low bytes). Signed integers use two's-complement representation.

IMPORTANT

The layout of the extended data type is either that of the SANE 80-bit data type or that of the 96-bit MC68881 data type, depending on the software development environment used.

The size of data structures and unions must be a multiple of two (2-byte alignment), and an extra byte may be added at the end to meet this requirement. Items inside a data structure (except for types UInt8 and SInt8) are placed on a 2-byte boundary with an extra padding byte inserted if necessary. Type UInt8 and type SInt8 items (single variables or arrays) are merely placed in the next available byte.

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© Apple Computer, Inc.
11 MARCH 1997

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