Inside Macintosh: Mac OS Runtime Architectures /

Chapter 5 - CFM-68K Runtime Conventions

Data Types

Table 5-1 lists the binary data types and their sizes in the CFM-68K runtime environment. These types and sizes are identical to those in the PowerPC runtime environment.

Table 5-1 Data types in the CFM-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	4	-2-31 to 231 -1
UInt32	4	4	0 to 232-1
Boolean	1	1		0 = false, nonzero = true
float	4	4	\xB1(2-149 to 2127)	IEEE 754 standard
double	8	8	\xB1(2-1074 to 21023)	IEEE 754 standard
Pointer	4	4	0 to FFFFFFFF
extended	10 or 12	4		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. Because of this variability, you should not use the extended data type for imported or exported routines or data.

The size of data structures and unions must be a multiple of two, 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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