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(1) See scroll bar and thumb culture.

(2) (Thumb) An extension to the architecture of an ARM chip that provides enhanced code density. It stores a subset of 32-bit instructions as compressed 16-bit instructions and decompresses them back to 32 bits upon execution.

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1.	(processor)	Thumb - An extension to the Advanced RISC Machine architecture, announced on 06 March 1995 by Advanced RISC Machines Ltd. By identifying the critical subset of the ARM instruction set and encoding it into 16 bits, ARM has succeeded in reducing typical program size by 30-40% from ARM's already excellent code density. Since this Thumb instruction set uses less memory for program storage, cost is further reduced. All Thumb-aware processor cores combine the capability to execute both the 32-bit ARM and the 16-bit Thumb instruction sets. Careful design of the Thumb instructions allow them to be decompressed into full ARM instructions transparently during normal instruction decoding without any performance penalty. This differs from other 32-bit processors, like the Intel 486SX, with a 16-bit data bus, which require two 16-bit memory accesses to execute every 32-bit instruction and so halve performance. The patented Thumb decompressor has been carefully designed with only a small amount of circuitry additional to the existing instruction decoder, so chip size and thus cost do not significantly increase. Designers can easily interleave fast ARM instructions (for performance critical parts of a program) with compact Thumb code to save memory.
2.	(jargon)	thumb - The slider or "bubble" on a window system scrollbar. So called because moving it allows you to browse through the contents of a text window in a way analogous to thumbing through a book.