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The IA-64 System V Processor-Specific ABI is Intel Itanium’s processor supplement to the generic System V Application Binary Interface. It defines the processor-dependent contracts that let compilers, assemblers, linkers, loaders, operating systems, and runtimes interoperate: data representation, ELF metadata, position-independent code, dynamic linking, function descriptors, signals, unwinding, and C++ exception handling. It is not a replacement for the generic System V ABI; both documents and the related Itanium software-conventions manuals are used together.
What the IA-64 psABI covers
The generic System V ABI defines the interface expected by compiled applications. The IA-64 supplement specializes that interface for Itanium processors, whose 64-bit architecture, global-pointer model, function descriptors, and unwind format require rules that a processor-neutral ABI cannot provide.
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Its scope is an interoperability contract rather than a programming tutorial. It describes the object formats and runtime conventions that independent toolchains must agree on, while the Itanium Architecture Software Developer’s Manuals and the Itanium Software Conventions and Runtime Architecture Guide provide complementary architectural and implementation detail.
Data models and representation
LP64 is the fully specified model
The principal IA-64 programming model is LP64. In that model, C int remains 32 bits, while long and every pointer type are 64-bit objects. A long long occupies 8 bytes and is aligned to 8 bytes. These widths and alignments affect structure layout, calling interfaces, binary compatibility, and the interpretation of data exchanged between separately compiled modules.
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LP64 long double uses 16 bytes (128 bits) of storage, but its numerical representation is an 80-bit extended-double format. Code that copies, aligns, or serializes this type must therefore distinguish its storage size from its internal floating-point format.
ILP32 is only a limited consideration
The text discusses ILP32 environments, but it does not provide a comparably complete, binding construction for them. An implementation claiming ILP32 compatibility must therefore document the operating-system profile and toolchain rules it actually supports instead of assuming that every LP64 rule transfers unchanged.
| Model | int |
long |
Pointers | Specification status |
|---|---|---|---|---|
| LP64 | 32 bits | 64 bits | 64 bits | Fully specified construction in the supplement |
| ILP32 | 32 bits | 32 bits | 32 bits | Discussed, but not fully binding in the supplement |
Byte order and architecture modes
The ABI permits either big-endian or little-endian instantiations. A particular operating-system ABI profile can select one byte order and thereby determine which binaries and interpreter paths are valid on that system. IA-64 provides a 64-bit instruction set and IA-32 compatibility, but compatibility mode does not make an IA-32 binary an IA-64 psABI object.
How IA-64 ELF files differ
IA-64 uses ELF, extended with processor-specific identification, flags, sections, relocations, dynamic tags, and loading conventions. A conforming environment identifies the machine as EM_IA_64 and must apply the IA-64 supplement in addition to the generic ELF rules. Linux Standard Base IA64 requirements likewise describe ELF support based on the System V ABI and the Intel Itanium processor-specific ABI, with LP64 support and IA-64 machine identification.
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These sections carry information needed for global addressing, procedure linkage, small-data placement, and unwinding:
| Section | Purpose in the IA-64 object model |
|---|---|
.got |
Global offset table used for addressable data and dynamic references |
.IA_64.archext |
IA-64 architecture-extension metadata |
.IA_64.pltoff |
Procedure-linkage offsets used by IA-64 call and linkage mechanisms |
.IA_64.unwind |
Unwind-related section data |
.IA_64.unwind_info |
Encoded unwind information consumed by the unwind runtime |
.plt |
Procedure-linkage table entries for dynamically resolved calls |
.sbss |
Small uninitialized data |
.sdata |
Small initialized data |
.sdata1 |
An additional IA-64 small-data area |
The presence, flags, relocations, and interpretation of these sections are part of binary compatibility. A generic ELF reader may be able to parse the file container while still lacking the IA-64 rules needed to link or execute it correctly.
Position-independent code is an ABI requirement
For an ABI-conforming application, relocatable files, executable files, and shared-object files supplied as part of that application must use position-independent code as described by the Itanium software conventions. This requirement is broader than a recommendation for shared libraries: it applies to the listed classes of application files under the IA-64 system conventions.
Consequently, a compiler and linker must coordinate code generation, address materialization, global-pointer use, relocations, and procedure-linkage mechanisms. A file that merely happens to be ELF-formatted is not conforming if its generated references violate the IA-64 position-independent-code rules.
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Dynamic linking, the global pointer, and the loader
Global-pointer information
IA-64 dynamic linking gives the dynamic linker the object’s global-pointer value through the DT_PLTGOT entry. On this architecture, that entry supplies the address contained in the object’s gp, so a loader or linker must interpret it according to the IA-64 supplement rather than applying a generic architecture-neutral assumption about the PLT/GOT relationship.
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IA-64 PLT reservation
The processor-specific DT_IA_64_PLT_RESERVE dynamic tag reserves three contiguous 8-byte words for the dynamic linker. The reservation is part of the runtime layout contract; changing its size or treating the words as ordinary application data can break dynamic resolution.
Interpreter paths depend on the ABI variant
The specification lists /usr/lib/ia64l64/ld.so.1 for little-endian LP64. ILP32 and big-endian variants use distinct interpreter locations. Therefore, an executable’s interpreter field must match the operating-system profile, code model, and byte order for which it was linked.
Function descriptors change what a function pointer means
On IA-64, a function pointer points to a function descriptor rather than directly to the first instruction. The descriptor contains the function’s entry address and its global-pointer value. Calls, signal delivery, debuggers, trampolines, and foreign-function interfaces must follow this representation when converting or invoking function pointers.
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Signals and hardware-condition mapping
The ABI defines how hardware conditions are presented through the operating system’s signal interface. Conditions covered include TLB faults, access faults, privilege violations, register-NaT consumption, unaligned data, floating-point exceptions, and illegal instructions. Signal delivery must also understand the function-descriptor representation described above.
The exact signal numbers and policy are operating-system matters; the psABI establishes the IA-64 interpretation that lets the kernel, runtime, and user code agree on the faulting context.
Unwinding and C++ exceptions
The unwind library is a required runtime interface
An Itanium psABI-compliant system is expected to provide the unwind-library interface. Its context APIs expose both fixed general-register state and the stacked general-register state introduced by the IA-64 register model. Unwinding code and personality routines use that context to inspect frames, restore execution state, and continue or terminate a search.
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The C++ ABI’s exception-handling machinery is built on the IA-64 unwind-library interface. Compilers therefore need to emit compatible unwind metadata, linkers need to preserve and arrange the relevant .IA_64.unwind and .IA_64.unwind_info data, and the runtime needs personality and propagation code that understands the IA-64 register context.
Disabling or stripping unwind data can affect more than C++ throw and catch: language runtimes, debuggers, profilers, and any code that performs stack walking may depend on it.
Quick Recap
What to verify when implementing or comparing an IA-64 toolchain
- Data model: Confirm LP64 widths and alignments, and document any ILP32 support as an operating-system-specific extension rather than assuming full supplement coverage.
- ELF identity: Check
EM_IA_64, processor flags, IA-64 section types and attributes, relocation handling, and preservation of unwind sections. - Code model and byte order: Match the selected endianness, addressing model, interpreter path, and executable format.
- Position independence: Verify that compiler output and linker transformations satisfy the IA-64 PIC requirement for relocatable, executable, and shared-object files.
- Dynamic linking: Test
gphandling throughDT_PLTGOT, the three-wordDT_IA_64_PLT_RESERVEreservation, PLT/GOT references, and loader behavior. - Function pointers: Ensure calls, callbacks, signal trampolines, and foreign-function boundaries preserve the descriptor’s entry address and global-pointer value.
- Runtime behavior: Exercise signal contexts, unwind APIs, stacked and fixed register state, and C++ exception propagation with the same runtime conventions used by the compiler.
- Interoperability: Treat compiler, assembler, linker, loader, C/C++ runtime, debugger, and operating-system support as one ABI stack; agreement at only the ELF-container level is insufficient.
Common misconceptions
- IA-64 is not x86-64: Both are 64-bit families, but their instruction sets, function-pointer representation, ELF machine identity, linking rules, and unwind conventions are different.
- LP64 does not mean every type is 64 bits: IA-64 LP64 keeps
intat 32 bits; it makeslongand pointers 64-bit. - ELF compatibility is not universal ABI compatibility: A generic ELF parser can recognize a file without being able to relocate, link, load, call, or unwind it correctly.
- ILP32 is not equally specified: The supplement’s complete construction is LP64, so ILP32 claims require profile-specific documentation.
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