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This chapter attempts to cover some of the common issues involved when writing 64-bit code, to run under Win64 or Unix. It covers how to write assembly code to interface with 64-bit C routines, and how to write position-independent code for shared libraries.
All 64-bit code uses a flat memory model, since segmentation is not
available in 64-bit mode. The one exception is the
Position independence in 64-bit mode is significantly simpler, since the
processor supports
64-bit programming is relatively similar to 32-bit programming, but of course pointers are 64 bits long; additionally, all existing platforms pass arguments in registers rather than on the stack. Furthermore, 64-bit platforms use SSE2 by default for floating point. Please see the ABI documentation for your platform.
64-bit platforms differ in the sizes of the fundamental datatypes, not
just from 32-bit platforms but from each other. If a specific size data
type is desired, it is probably best to use the types defined in the
Standard C header
On Unix, the 64-bit ABI is defined by the document:
Although written for AT&T-syntax assembly, the concepts apply equally well for NASM-style assembly. What follows is a simplified summary.
The first six integer arguments (from the left) are passed in
Integer return values are passed in
Floating point is done using SSE registers, except for
All SSE and x87 registers are destroyed by function calls.
On 64-bit Unix,
The Win64 ABI is described at:
What follows is a simplified summary.
The first four integer arguments are passwd in
Integer return values are passed in
Floating point is done using SSE registers, except for
On Win64,