What's New for Applications

Most of the new features of the 64-bit environment are simply extensions of generic 32-bit interfaces, though there are several new features that are unique to SPARC V9.

The SPARC Compliance Definition, Version 2.4 contains details of the SPARC V9 ABI. It describes the 32-bit, SPARC V8 ABI and the 64-bit, SPARC V9 ABI. You can obtain this document from SPARC International at www.sparc.com.

Generic 64-bit ABI Features

64-bit applications are described using Executable and Linking Format (ELF64), which allows large applications and large address spaces to be described completely.

SPARC V9 ABI Features

Below is a list of the SPARC V9 ABI Features.

• The SPARC V9 ABI allows all 64-bit SPARC instructions and 64-bit wide registers to be used to their full effect. Many of the new relevant instructions are extensions of the existing V8 instruction set. See The SPARC Architecture Manual, Version 9.

• The basic calling convention is the same. The first six arguments of the caller are placed in the out registers %o0-%o5. The SPARC V9 ABI still uses a register window on a larger register file to make calling a function a cheap operation. Results are returned in %o0. Because all registers are now treated as 64-bit quantities, 64-bit values can now be passed in a single register, rather than a register pair.

• The layout of the stack is different. Apart from the increase in the basic cell size from 32-bit to 64-bit, various "hidden" parameter words have been removed. The return address is still %o7 + 8.

• %o6 is still referred to as the stack pointer register %sp, and %i6 is the frame pointer register %fp. However, the %sp and %fp registers are offset by a constant, known as the stack bias, from the actual memory location of the stack. The size of the stack bias is 2047 bits.

• Instruction sizes are still 32 bits. Address constant generation therefore takes more instructions. The call instruction can no longer be used to branch anywhere in the address space, since it can only reach within plus or minus 2Gbytes of %pc.

• Integer multiply and divide functions are now implemented completely in hardware.

• Structure passing and return are accomplished differently. Small data structures and some floating point arguments are now passed directly in registers.

• User traps allow certain traps from non-privileged code to be handled by a user trap handler (instead of delivering a signal).

• All data types are now aligned to their size.

• Many basic derived types are larger. Thus many system call interface data structures are now different sizes.

• Two different sets of libraries exist on the system: those for 32-bit SPARC applications and those for 64-bit SPARC applications.

Address Space Layout

For 64-bit applications, the layout of the address space is closely related to that of 32-bit applications, though the starting address and addressing limits are radically different. Like SPARC V8, the stack grows down from the top of the address space, while the heap extends the data segment from the bottom.

The diagram below shows the default address space provided to a 64-bit application. The regions of the address space marked as reserved might not be mapped by applications. These restrictions might be relaxed on future systems.

The actual addresses above describe a particular implementation on a particular machine, and are given for illustrative purposes only.

Placement of Text and Data

By default, 64-bit programs are linked with a starting address of 0x100000000. The whole program will be above 4 gigabytes, including its text, data, heap, stack, and shared libraries. This helps insure that 64-bit programs are correct by making it so the program will fault in the least significant 4 gigabytes of its address space if it truncates any of its pointers.

While 64-bit programs are linked above 4 gigabytes, it is still possible to link them below 4 gigabytes by using a linker mapfile and the -M option to the compiler or linker. A linker mapfile for linking a 64-bit SPARC program below 4 gigabytes is provided in /usr/lib/ld/sparcv9/map.below4G. See the ld(1) linker man page for more information.