Writing Device Drivers

SPARC Processor Issues

This section describes a number of SPARC processor-specific topics including data alignment, byte ordering, register windows, and availability of floating-point instructions. For information on x86 processor-specific topics, see "x86 Processor Issues".

SPARC Data Alignment

All quantities must be aligned on their natural boundaries. Using standard C data types:

SPARC Structure Member Alignment

Because of the data alignment restrictions imposed by the SPARC processor, C structures also have alignment requirements. Structure alignment requirements are imposed by the most strictly-aligned structure component. For example, a structure containing only characters has no alignment restrictions, while a structure containing a long long member must be constructed to guarantee that this member falls on a 64-bit boundary. See "Structure Padding" for more information on how this restriction relates to device drivers.

SPARC Byte Ordering

The SPARC processor uses big-endian byte ordering; in other words, the most significant byte of an integer is stored at the lowest address of the integer.


SPARC Register Windows

SPARC processors use register windows. Each register window is comprised of 8 in registers, 8 local registers, and 8 out registers (which are the in registers of the next window). There are also 8 global registers. The number of register windows ranges from 2 to 32, depending on the processor implementation.

Because drivers are normally written in C, the compiler usually hides the fact that register windows are used. However, it may be necessary to use them when debugging the driver. See "Debugging Tools" for more information on how register windows are used when debugging.

SPARC Floating-Point Operations

Drivers should not perform floating-point operations, as they are not supported in the kernel.

SPARC Multiply and Divide Instructions

The Version 7 SPARC processors do not have multiply or divide instructions. These instructions are emulated in software and should be avoided. Because a driver cannot determine whether it is running on a Version 7, Version 8, or Version 9 processor, intensive integer multiplication and division should be avoided if possible. Instead, use bitwise left and right shifts to multiply and divide by powers of two.

SPARC Architecture Manual

The SPARC Architecture Manual, Version 9, contains more specific information on the SPARC CPU.