PTX Writer's Guide to Interoperability

The guide to writing ABI-compliant PTX.

1. Introduction

This document defines the Application Binary Interface (ABI) for the CUDA® architecture when generating PTX. By following the ABI, external developers can generate compliant PTX code that can be linked with other code.

PTX is a low-level parallel-thread-execution virtual machine and ISA (Instruction Set Architecture). PTX can be output from multiple tools or written directly by developers. PTX is meant to be GPU-architecture independent, so that the same code can be reused for different GPU architectures. For more information on PTX, refer to the latest version of the PTX ISA reference document.

There are multiple CUDA architecture families, each with their own ISA; e.g. SM 2.x is the Fermi family, SM 3.x is the Kepler family. This document describes the high-level ABI for all architectures. Programs conforming to an ABI are expected to be executed on the appropriate architecture GPU, and can assume that instructions from that ISA are available.

2. Data Representation

2.1. Fundamental Types

The below table shows the native scalar PTX types that are supported. Any PTX producer must use these sizes and alignments in order for its PTX to be compatible with PTX generated by other producers. PTX also supports native vector types, which are discussed in Aggregates and Unions.

The sizes of types are defined by the host. For example, pointer size and long int size are dictated by the hosts ABI. PTX has an .address_size directive that specifies the address size used throughout the PTX code. The size of pointers is 32 bits on a 32-bit host or 64 bits on a 64-bit host. However, addresses of the local and shared memory spaces are always 32 bits in size.

During separate compilation we store info about the host platform in each object file. The linker will fail to link object files generated for incompatible host platforms.

PTX Type Size (bytes) Align (bytes) Hardware Representation
.b8 1 1 untyped byte
.b16 2 2 untyped halfword
.b32 4 4 untyped word
.b64 8 8 untyped doubleword
.s8 1 1 signed integral byte
.s16 2 2 signed integral halfword
.s32 4 4 signed integral word
.s64 8 8 signed integral doubleword
.u8 1 1 unsigned integral byte
.u16 2 2 unsigned integral halfword
.u32 4 4 unsigned integral word
.u64 8 8 unsigned integral doubleword
.f16 2 2 IEEE half precision
.f32 4 4 IEEE single precision
.f64 8 8 IEEE double precision

2.2. Aggregates and Unions

Beyond the scalar types, PTX also supports native-vector types of these scalar types, with both its vector syntax and its byte-array syntax. For scalar types with a size no greater than four bytes, vector types with 1, 2, 3, and 4 elements exist; for all other types, only 1 and 2 element vector types exist.

All aggregates and unions can be supported in PTX with its byte-array syntax.

The following are the size-and-alignment rules for all aggregates and unions.
  • For a non-native-vector type, an entire aggregate or union is aligned on the same boundary as its most strictly aligned member. This rule is not followed if the alignments are defined by the input language. For example, in OpenCL built-in vector data types have their alignment set to the size of the built-in data type in bytes.
  • For a native vector type -- discussed at the start of this section -- the alignment is defined as follows. (For the definitions below, the native vector has n elements and has an element type t.)
    • For a vector with an odd number of elements, its alignment is the same as its member: alignof(t).
    • For a vector with an even number of elements, its alignment is set to number of elements times the alignment of its member: n*alignof(t).
  • Each member is assigned to the lowest available offset with the appropriate alignment. This may require internal padding, depending on the previous member.
  • The size of an aggregate or union, if necessary, is increased to make it a multiple of the alignment of the aggregate or union. This may require tail padding, depending on the last member.

2.3. Bit Fields

C structure and union definitions may have bit fields that define integral objects with a specified number of bits.

Bit Field Type Width w Range
signed char 1 to 8 -2w-1 to 2w-1 - 1
unsigned char 1 to 8 0 to 2w - 1
signed short 1 to 16 -2w-1 to 2w-1 - 1
unsigned short 1 to 16 0 to 2w - 1
signed int 1 to 32 -2w-1 to 2w-1 - 1
unsigned int 1 to 32 0 to 2w - 1
signed long long 1 to 64 -2w-1 to 2w-1 - 1
unsigned long long 1 to 64 0 to 2w - 1

Current GPUs only support little-endian memory, so the below assumes little-endian layout.

The following are rules that apply to bit fields.

  • Plain bit fields (neither signed nor unsigned is specified) are treated as signed.
  • When no type is provided (e.g., signed : 6 is specified), the type defaults to int.

Bit fields obey the same size and alignment rules as other structure and union members, with the following modifications.

  • Bit fields are allocated in memory from right to left (least to more significant) for little endian.
  • A bit field must entirely reside in a storage unit appropriate for its declared type. A bit field should never cross its unit boundary.
  • Bit fields may share a storage unit with other structure and union members, including members that are not bit fields, as long as there is enough space within the storage unit.
  • Unnamed bit fields do not affect the alignment of a structure or union.
  • Zero-length bit fields force the alignment of the following member of a structure to the next alignment boundary corresponding to the bit-field type. An unnamed, zero-length bit field will not force the external alignment of the structure to that boundary. If an unnamed, zero-length bit field has a stricter alignment than the external alignment, there is no guarantee that the stricter alignment will be maintained when the structure or union gets allocated to memory.

The following figures contain examples of bit fields. Figure 1 shows the byte offsets (upper corners) and the bit numbers (lower corners) that are used in the examples. The remaining figures show different bit-field examples.

Figure 1. Bit Numbering

Figure 2. Bit-field Allocation

Figure 3. Boundary Alignment

Figure 4. Storage Unit Sharing

Figure 5. Union Allocation

Figure 6. Unnamed Bit Fields

2.4. Texture, Sampler, and Surface Types

Texture, sampler and surface types are used to define references to texture and surface memory. The CUDA architecture provides hardware and instructions to efficiently read data from texture or surface memory as opposed to global memory.

References to textures are bound through runtime functions to device read-only regions of memory, called a texture memory, before they can be used by a kernel. A texture reference has several attributes e.g. normalized mode, addressing mode, and texture filtering etc. A sampler reference can be used to sample a texture when read in a kernel. A surface reference is used to read or write data from and to the surface memory. It also has various attributes similar to a texture.

At the PTX level objects that access texture or surface memory are referred to as opaque objects. Textures are expressed by either a .texref or .samplerref type and surfaces are expressed by the .surfref type. The data of opaque objects can be accessed by specific instructions (TEX for .texref/.samplerref and SULD/SUST for .surfref). The attributes of opaque objects are implemented by allocating a descriptor in memory which is populated by the driver. PTX TXQ/SUQ instructions get translated into memory reads of fields of the descriptor. The internal format of the descriptor varies with each architecture and should not be relied on by the user. The data and the attributes of an opaque object may be accessed directly if the texture or surface reference is known at compile time or indirectly. If the reference is not known during compile time all information required to read data and attributes is contained in a .b64 value called the handle. The handle can be used to pass and return oqaque object references to and from functions as well as to reference external textures, samplers and surfaces.

3. Function Calling Sequence

This section describes the PTX-level function calling sequence, including register usage, stack-frame layout, and parameter passing. The PTX-level function calling sequence describes what gets represented in PTX to enable function calls. There is an abstraction at this level. Most of the details associated with the function calling sequence are handled at the SASS level.

PTX versions earlier than 2.0 do not conform to the ABI defined in this document, and cannot perform ABI compatible function calls. For the calling convention to work PTX version 2.0 or greater must be used.

3.1. Registers

At the PTX level, the registers that are specified are virtual. Register allocation occurs during PTX-to-SASS translation. The PTX-to-SASS translation also converts parameters and return values to physical registers or stack locations.

3.2. Stack Frame

The PTX level has no concept of the software stack. Manipulation of the stack is completely defined at the SASS level, and gets allocated during the PTX-to-SASS translation process.

3.3. Parameter Passing

At the PTX level, all parameters and return values present in a device function use the parameter state space (.param). The below table contains the rules for handling parameters and return values that are defined at the source level. For each source-level type, the corresponding PTX-level type that should be used is provided.

Source Type Size in Bits PTX Type
Integral types 8 to 32 (A) .u32 (if unsigned) or .s32 (if signed)
Integral types 64 .u64 (if unsigned) or .s64 (if signed)
Pointers (B) 32 .u32
Pointers (B) 64 .u64
Floating-point types (C) 32 .f32
Floating-point types (C) 64 .f64
Aggregates or unions Any size

.align align .b8 name[size]

Where align is overall aggregate-or-union alignment in bytes (D), name is variable name associated with aggregate or union, and size is the aggregate-or-union size in bytes.
Handles (E) 64 .b64 (assigned from .texref, .sampleref, .surfref)


(A) Values shorter than 32-bits are sign extended or zero extended, depending on whether they are signed or unsigned types.

(B) Unless the memory type is specified in the function declaration, all pointers passed at the PTX level must use a generic address.

(C) 16-bit floating-point types are only used for storage. Therefore, they cannot be used for parameters or return values.

(D) The alignment must be 1, 2, 4, 8, 16, 32, 64, or 128 bytes.

(E) The PTX built-in opaque types such as texture, sampler, and surface types are can be passed into functions as parameters and be returned by them through 64-bit handles. The handle contains the necessary information to access the actual data from the texture or surface memory as well as the attributes of the object stored in its type descriptor. See section Texture, Sampler, and Surface Types for more information on handles.

4. System Calls

System calls are calls into the driver operating system code. In PTX they look like regular calls, but the function definition is not given. A prototype must be provided in the PTX file, but the implementation of the function is provided by the driver.

The prototype for the vprintf system call is:

 .extern .func (.param .s32 status) vprintf (.param t1 format, .param t2 valist)
The following are the definitions for the vprintf parameters and return value.
  • status : The status value that is returned by vprintf.
  • format : A pointer to the format specifier input. For 32-bit addresses, type t1 is .b32. For 64-bit addresses, type t1 is .b64.
  • valist : A pointer to the valist input. For 32-bit addresses, type t2 is .b32. For 64-bit addresses, type t2 is .b64.

A call to vprintf using 32-bit addresses looks like:    %r2, _fmt;
    st.param.b32  [param0], %r2;
    cvta.local.b32  %r3, _valist_array;
    st.param.b32  [param1], %r3;
    call.uni (_), vprintf, (param0, param1);

For this code, _fmt is the format string in global memory, and _valist_array is the valist of arguments. Note that any pointers must be converted to generic space. The vprintf syscall is emitted as part of the printf function defined in "stdio.h".

The prototype for the malloc system call is:

    .extern .func (.param t1 ptr) malloc (.param t2 size)
The following are the definitions for the malloc parameters and return value.
  • ptr : The pointer to the memory that was allocated by malloc. For 32-bit addresses, type t1 is .b32. For 64-bit addresses, type t1 is .b64.
  • size : The size of memory needed from malloc. This size is defined by the type size_t. When size_t is 32 bits, type t2 is .b32. When size_t is 64 bits, type t2 is .b64.

The prototype for the free system call is:

     .extern .func free (.param t1 ptr)
The following is the definition for the free parameter.
  • ptr : The pointer to the memory that should be freed. For 32-bit addresses, type t1 is .b32. For 64-bit addresses, type t1 is .b64.

The malloc and free system calls are emitted as part of the malloc and free functions defined in "malloc.h".

In order to support assert, the PTX function call __assertfail is used whenever the assert expression produces a false value. The prototype for the __assertfail system call is:

        .extern .func __assertfail (.param t1 message, .param t1 file, .param .b32 line, .param t1 function, .param t2 charSize)
The following are the definitions for the __assertfail parameters.
  • message : The pointer to the string that should be output. For 32-bit addresses, type t1 is .b32. For 64-bit addresses, type t1 is .b64.
  • file : The pointer to the file name string associated with the assert. For 32-bit addresses, type t1 is .b32. For 64-bit addresses, type t1 is .b64.
  • line : The line number associated with the assert.
  • function : The pointer to the function name string associated with the assert. For 32-bit addresses, type t1 is .b32. For 64-bit addresses, type t1 is .b64.
  • charSize : The size in bytes of the characters contained in the __assertfail parameter strings. The only supported character size is 1. The character size is defined by the type size_t. When size_t is 32 bits, type t2 is .b32. When size_t is 64 bits, type t2 is .b64.

The __assertfail system call is emitted as part of the assert macro defined in "assert.h".

5. Debug Information

Debug information is encoded in DWARF (Debug With Arbitrary Record Format).

5.1. Generation of Debug Information

The responsibility for generating debug information is split between the PTX producer and the PTX-to-SASS backend. The PTX producer is responsible for emitting binary DWARF into the PTX file, using the .section and .b8-.b16-.b32-and-.b64 directives in PTX. This should contain the .debug_info and .debug_abbrev sections, and possibly optional sections .debug_pubnames and .debug_aranges. These sections are standard DWARF2 sections that refer to labels and registers in the PTX.

The PTX-to-SASS backend is responsible for generating the .debug_line section from the .file and .loc directives in the PTX file. This section maps source lines to SASS addresses. The backend also generates the .debug_frame section.

5.2. CUDA-Specific DWARF Definitions

In order to support debugging of multiple memory segments, address class codes are defined to reflect the memory space of variables. The address-class values are emitted as the DW_AT_address_class attribute for all variable and parameter Debugging Information Entries. The address class codes are defined in the below table.

Code Value Description
ADDR_code_space 1 Code storage
ADDR_reg_space 2 Register storage
ADDR_sreg_space 3 Special register storage
ADDR_const_space 4 Constant storage
ADDR_global_space 5 Global storage
ADDR_local_space 6 Local storage
ADDR_param_space 7 Parameter storage
ADDR_shared_space 8 Shared storage
ADDR_surf_space 9 Surface storage
ADDR_tex_space 10 Texture storage
ADDR_tex_sampler_space 11 Texture sampler storage
ADDR_generic_space 12 Generic-address storage

6. Example

The following is example PTX with debug information for implementing the following program that makes a call:

__device__ __noinline__ int foo (int i, int j)
  return i+j;

__global__ void test (int *p)
  *p = foo(1, 2);

The resulting PTX would be something like:

.version 4.2
.target sm_20, debug
.address_size 64

 .file   1 ""

.visible .func  (.param .b32 func_retval0) // return value
        .param .b32 _Z3fooii_param_0, // parameter "i"
        .param .b32 _Z3fooii_param_1) // parameter "j"
        .reg .s32       %r<4>;
        .loc 1 1 1      // following instructions are for line 1

        ld.param.u32    %r1, [_Z3fooii_param_0]; // load 1st param
        ld.param.u32    %r2, [_Z3fooii_param_1]; // load 2nd param
        .loc    1 3 1   // following instructions are for line 3
        add.s32         %r3, %r1, %r2;
        st.param.b32    [func_retval0+0], %r3; // store return value

.visible .entry _Z4testPi(
        .param .u64 _Z4testPi_param_0) // parameter *p
        .reg .s32       %r<4>;
        .reg .s64       %rd<2>;
        .loc 1 6 1

        ld.param.u64    %rd1, [_Z4testPi_param_0]; // load *p
        mov.u32         %r1, 1;
        mov.u32         %r2, 2;
        .loc    1 8 9
        .param .b32 param0;
        st.param.b32    [param0+0], %r1; // store 1
        .param .b32 param1;
        st.param.b32    [param1+0], %r2; // store 2
        .param .b32 retval0;
        call.uni (retval0), _Z3fooii, ( param0, param1); // call foo
        ld.param.b32    %r3, [retval0+0]; // get return value
        st.u32  [%rd1], %r3;              // *p = return value
        .loc    1 9 2

.section .debug_info {
 .b32 262
 .b8 2, 0
 .b32 .debug_abbrev
 .b8 8, 1, 108, 103, 101, 110, 102, 101, 58, 32, 69, 68, 71, 32, 52, 46, 57
 .b8 0, 4, 99, 97, 108, 108, 49, 46, 99, 117, 0
 .b64 0
 .b32 .debug_line // the .debug_line section will be created by ptxas from the .loc
 .b8 47, 104, 111, 109, 101, 47, 109, 109, 117, 114, 112, 104, 121, 47, 116
 .b8 101, 115, 116, 0, 2, 95, 90, 51, 102, 111, 111, 105, 105, 0, 95, 90
 .b8 51, 102, 111, 111, 105, 105, 0
 .b32 1, 1, 164
 .b8 1
 .b64 func_begin0 // start and end location of foo
 .b64 func_end0
 .b8 1, 156, 3, 105, 0
 .b32 1, 1, 164
 .b8 5, 144, 177, 228, 149, 1, 2, 3, 106, 0
 .b32 1, 1, 164
 .b8 5, 144, 178, 228, 149, 1, 2, 0, 4, 105, 110, 116, 0, 5
 .b32 4
 .b8 2, 95, 90, 52, 116, 101, 115, 116, 80, 105, 0, 95, 90, 52, 116, 101
 .b8 115, 116, 80, 105, 0
 .b32 1, 6, 253
 .b8 1
 .b64 func_begin1 // start and end location of test
 .b64 func_end1
 .b8 1, 156, 3, 112, 0
 .b32 1, 6, 259
 .b8 9, 3
 .b64 _Z4testPi_param_0
 .b8 7, 0, 5, 118, 111, 105, 100, 0, 6
 .b32 164
 .b8 12, 0
.section .debug_abbrev {
 .b8 1, 17, 1, 37, 8, 19, 11, 3, 8, 17, 1, 16, 6, 27, 8, 0, 0, 2, 46, 1, 135
 .b8 64, 8, 3, 8, 58, 6, 59, 6, 73, 19, 63, 12, 17, 1, 18, 1, 64, 10, 0, 0
 .b8 3, 5, 0, 3, 8, 58, 6, 59, 6, 73, 19, 2, 10, 51, 11, 0, 0, 4, 36, 0, 3
 .b8 8, 62, 11, 11, 6, 0, 0, 5, 59, 0, 3, 8, 0, 0, 6, 15, 0, 73, 19, 51, 11
 .b8 0, 0, 0
.section .debug_pubnames {
 .b32 41
 .b8 2, 0
 .b32 .debug_info
 .b32 262, 69
 .b8 95, 90, 51, 102, 111, 111, 105, 105, 0
 .b32 174
 .b8 95, 90, 52, 116, 101, 115, 116, 80, 105, 0
 .b32 0

7. C++

The C++ implementation for device functions follows the Itanium C++ ABI. However, not everything in C++ is supported. In particular, the following are not supported in device code.

  • Exceptions and try/catch blocks
  • RTTI
  • STL library
  • Global constructors and destructors
  • Virtual functions and classes across host and device (i.e., vtables cannot be used across host and device)

There are also a few C features that are not currently supported:

  • stdio other than printf



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