# The Netwide Assembler: NASM

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## Chapter 11: Writing 64-bit Code (Unix, Win64)

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 `FS` and `GS` registers, which still add their bases.

Position independence in 64-bit mode is significantly simpler, since the processor supports `RIP`-relative addressing directly; see the `REL` keyword (section 3.3). On most 64-bit platforms, it is probably desirable to make that the default, using the directive `DEFAULT REL` (section 6.2).

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 `<inttypes.h>`.

In 64-bit mode, the default instruction size is still 32 bits. When loading a value into a 32-bit register (but not an 8- or 16-bit register), the upper 32 bits of the corresponding 64-bit register are set to zero.

### 11.1 Register Names in 64-bit Mode

NASM uses the following names for general-purpose registers in 64-bit mode, for 8-, 16-, 32- and 64-bit references, respectively:

         AL/AH, CL/CH, DL/DH, BL/BH, SPL, BPL, SIL, DIL, R8B-R15B
         AX, CX, DX, BX, SP, BP, SI, DI, R8W-R15W
         EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI, R8D-R15D
         RAX, RCX, RDX, RBX, RSP, RBP, RSI, RDI, R8-R15

This is consistent with the AMD documentation and most other assemblers. The Intel documentation, however, uses the names `R8L-R15L` for 8-bit references to the higher registers. It is possible to use those names by definiting them as macros; similarly, if one wants to use numeric names for the low 8 registers, define them as macros. The standard macro package `altreg` (see section 5.1) can be used for this purpose.

### 11.2 Immediates and Displacements in 64-bit Mode

In 64-bit mode, immediates and displacements are generally only 32 bits wide. NASM will therefore truncate most displacements and immediates to 32 bits.

The only instruction which takes a full 64-bit immediate is:

         MOV reg64,imm64

NASM will produce this instruction whenever the programmer uses `MOV` with an immediate into a 64-bit register. If this is not desirable, simply specify the equivalent 32-bit register, which will be automatically zero-extended by the processor, or specify the immediate as `DWORD`:

         mov rax,foo             ; 64-bit immediate
         mov rax,qword foo       ; (identical)
         mov eax,foo             ; 32-bit immediate, zero-extended
         mov rax,dword foo       ; 32-bit immediate, sign-extended

The length of these instructions are 10, 5 and 7 bytes, respectively.

The only instructions which take a full 64-bit displacement is loading or storing, using `MOV`, `AL`, `AX`, `EAX` or `RAX` (but no other registers) to an absolute 64-bit address. Since this is a relatively rarely used instruction (64-bit code generally uses relative addressing), the programmer has to explicitly declare the displacement size as `QWORD`:

         default abs

         mov eax,[foo]           ; 32-bit absolute disp, sign-extended
         mov eax,[a32 foo]       ; 32-bit absolute disp, zero-extended
         mov eax,[qword foo]     ; 64-bit absolute disp

         default rel

         mov eax,[foo]           ; 32-bit relative disp
         mov eax,[a32 foo]       ; d:o, address truncated to 32 bits(!)
         mov eax,[qword foo]     ; error
         mov eax,[abs qword foo] ; 64-bit absolute disp

A sign-extended absolute displacement can access from -2 GB to +2 GB; a zero-extended absolute displacement can access from 0 to 4 GB.

### 11.3 Interfacing to 64-bit C Programs (Unix)

On Unix, the 64-bit ABI is defined by the document:

`http://www.nasm.us/links/unix64abi`

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 `RDI`, `RSI`, `RDX`, `RCX`, `R8`, and `R9`, in that order. Additional integer arguments are passed on the stack. These registers, plus `RAX`, `R10` and `R11` are destroyed by function calls, and thus are available for use by the function without saving.

Integer return values are passed in `RAX` and `RDX`, in that order.

Floating point is done using SSE registers, except for `long double`. Floating-point arguments are passed in `XMM0` to `XMM7`; return is `XMM0` and `XMM1`. `long double` are passed on the stack, and returned in `ST0` and `ST1`.

All SSE and x87 registers are destroyed by function calls.

On 64-bit Unix, `long` is 64 bits.

Integer and SSE register arguments are counted separately, so for the case of

         void foo(long a, double b, int c)

`a` is passed in `RDI`, `b` in `XMM0`, and `c` in `ESI`.

### 11.4 Interfacing to 64-bit C Programs (Win64)

The Win64 ABI is described at:

`http://www.nasm.us/links/win64abi`

What follows is a simplified summary.

The first four integer arguments are passed in `RCX`, `RDX`, `R8` and `R9`, in that order. Additional integer arguments are passed on the stack. These registers, plus `RAX`, `R10` and `R11` are destroyed by function calls, and thus are available for use by the function without saving.

Integer return values are passed in `RAX` only.

Floating point is done using SSE registers, except for `long double`. Floating-point arguments are passed in `XMM0` to `XMM3`; return is `XMM0` only.

On Win64, `long` is 32 bits; `long long` or `_int64` is 64 bits.

Integer and SSE register arguments are counted together, so for the case of

         void foo(long long a, double b, int c)

`a` is passed in `RCX`, `b` in `XMM1`, and `c` in `R8D`.

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