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Overview of the Assembler 
Overview of the ARM Architecture About the ARM architecture ARM, Thumb, and ThumbEE instruction sets Changing between ARM, Thumb, and ThumbEE state Processor modes, and privileged and unprivileged s Processor modes in ARMv6-M and ARMv7-M VFP coprocessor ARM registers General-purpose registers Register accesses Predeclared core register names Predeclared extension register names Predeclared coprocessor names Program Counter Application Program Status Register The Q flag Current Program Status Register Saved Program Status Registers (SPSRs) Instruction set overview Media processing instructions Access to the inline barrel shifter Structure of Assembly Language Modules Writing ARM Assembly Language Condition Codes Using the Assembler Symbols, Literals, Expressions, and Operators VFP ProgrammingARM, Thumb, and ThumbEE instruction sets
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The ARM instruction set is a set of 32-bit instructions providing a comprehensive range of operations.
ARMv4T and later define a 16-bit instruction set called the Thumb instruction set. Most of the functionality of the 32-bit ARM instruction set is available, but some operations require more instructions. The Thumb instruction set provides better code density, at the expense of performance.
ARMv6T2 introduces a major enhancement of the Thumb instruction set by providing 32-bit Thumb instructions. The 32-bit and 16-bit Thumb instructions together provide almost exactly the same functionality as the ARM instruction set. The enhanced Thumb instruction set (Thumb®-2) achieves the high performance of ARM code and better code density like 16-bit Thumb code.
ARMv7 defines the Thumb Execution Environment (ThumbEE). The ThumbEE instruction set is based on Thumb, with some changes and additions to make it a better target for dynamically generated code, that is, code compiled on the device either shortly before or during execution.
See also- Concepts
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