BSS segment

In computer programming, the name .bss or bss is used by many compilers and linkers for a part of the data segment containing statically-allocated variables represented solely by zero-valued bits initially (i.e., when execution begins). It is often referred to as the "bss section" or "bss segment".

Typically only the length of the bss section, but no data, is stored in the object file. The program loader allocates and initializes memory for the bss section when it loads the program. Operating systems may use a technique called copy on write to efficiently implement the bss segment (McKusick & Karels 1986). In embedded software, the bss segment is mapped into memory that is initialized to zero by the C run-time system before main() is entered.

On some computer architectures the application binary interface also supports an sbss segment for "small data". Typically, these data items can be accessed using shorter instructions that may only be able to access a certain range of addresses.

Historically, BSS (from Block Started by Symbol) was a pseudo-operation in UA-SAP (United Aircraft Symbolic Assembly Program), the assembler developed in the mid-1950s for the IBM 704 by Roy Nutt, Walter Ramshaw, and others at United Aircraft Corporation. The BSS keyword was later incorporated into FAP (FORTRAN Assembly Program), IBM's standard assembler for its 709 and 7090/94 computers. It defined a label (i.e. symbol) and reserved a block of uninitialized space for a given number of words (Timar 1996). In this situation BSS served as a shorthand in place of individually reserving a number of separate smaller data locations. Some assemblers support a complementary or alternative directive BES, for Block Ended by Symbol, where the specified symbol corresponds to the end of the reserved block.

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