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Mac OS X Panther for Unix Geeks, Second Edition by Ernest E. Rothman, Brian Jepson

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Architectural Issues

There are a few architectural issues to be aware of when developing or porting software on Mac OS X. In particular, pointer size, endianness, and inline assembly code tend to be the most common issues developers run in to.

On a 32-bit system, such as Mac OS X running on the G3 or G4, C pointers are 32 bits (4 bytes). On a 64-bit system, they are 64 bits (8 bytes). As long as your code does not rely on any assumptions about pointer size, it should be 64-bit clean. For example, on a 32-bit system, the following program prints “4”, and on a 64-bit system, it prints “8”:

#include <stdio.h>
int main( )
{
  printf("%d\n", sizeof(void *));
  return 0;
}

Some 64-bit operating systems, such as Solaris 8 on Ultra hardware (sun4u), have a 64-bit kernel space, but support both 32- and 64-bit mode applications, depending on how they are compiled. On a G5 system, the pointer size is 64-bits. Other data types are mapped onto the 64-bit data type. For example, single precision floats, which are 32-bit, are converted to double precision floats when they are loaded into registers. In the registers, single precision instructions operate on these single precision floats stored as doubles performing the required operations on the data. The results, however, are rounded to single precision 32-bit. Quad precision floating point numbers, defined by the IEEE as 128-bit are not directly supported on current PowerPC hardware. Apple has provided at least two technical notes containing information ...

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