Question

I initially thought that 64 bit instructions would not work on OS-X 10.5.

I wrote a little test program and compiled it with GCC -m64.
I used long long for my 64 bit integers.

The assembly instructions used look like they are 64 bit. eg. imultq and movq 8(%rbp),%rax.

I seems to work.

I am only using printf to display the 64 bit values using %lld.

1. Is this the expected behaviour?
2. Are there any gotcha's that would cause this to fail?
3. Am I allowed to ask multiple questions in a question?
4. Does this work on other OS’s?

Answer 1

Just to make this completely clear, here is the situation for 32- and 64-bit executables on OS X:

• Both 32- and 64-bit user space executables can be run on both 32- and 64-bit kernels in OS X 10.6, without emulation. On 10.4 and 10.5, both 32- and 64-bit executables can run on the 32-bit kernel. (This is not true on Windows)

• The user space system libraries and frameworks are built 32/64-bit fat on 10.5 and 10.6. You can link against them normally, whether you’re building for 32-bit, 64-bit, or both. A few libraries (basically the POSIX layer) are also built 32/64-bit fat on 10.4, but many of them are not.

• On 10.6, the build tools produce 64-bit executables by default. On 10.5 and earlier, the default is 32-bit.

• On 10.6, executables that are built fat will run the 64-bit side by default. On 10.5 and earlier, the 32-bit side is executed by default.

• You can always manually specify which slice of a fat executable to use by using the arch command. eg. arch -arch i386 someCommandToRunThatIWantToRunIn32BitMode. For application bundles, you can either launch them from the command line, or there is a preference if you “get info” on the application.

• OS X and Linux use the LP64 model for 64-bit executables. Pointers and long are 64 bits wide, int is still 32 bits, and long long is still 64 bits. (Windows uses the LLP64 model instead — long is 32 bits wide in 64 bit Windows).