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Unformatted text preview: lexibly to optimize performance or code density on a routine-by-routine basis. This chapter covers the Thumb architecture and implementation, and suggests the characteristics of applications that are likely to benefit from using Thumb. In the right application, use of the Thumb instruction set can improve power-efficiency, save cost and enhance performance all at once.
188 The Thumb bit in the CPSR 189 7.1 The Thumb bit in the CPSR
ARM processors which support the Thumb instruction set can also execute the standard 32-bit ARM instruction set, and the interpretation of the instruction stream at any particular time is determined by bit 5 of the CPSR, the T bit (see Figure 2.2 on page 40). If T is set the processor interprets the instruction stream as 16-bit Thumb instructions, otherwise it interprets it as standard ARM instructions. Not all ARM processors are capable of executing Thumb instructions; those that are have a T in their name, such as the ARM7TDMI described in Section 9.1 on page 248. Thumb entry ARM cores start up, after reset, executing ARM instructions. The normal way they switch to execute Thumb instructions is by executing a Branch and Exchange instruction (BX, see Section 5.5 on page 115). This instruction sets the T bit if the bottom bit of the specified register was set, and switches the program counter to the address given in the remainder of the register. Note that since the instruction causes a branch it flushes the instruction pipeline, removing any ambiguity over the interpretation of any instructions already in the pipeline (they are simply not executed). Other instructions which change from ARM to Thumb code include exception returns, either using a special form of data processing instruction or a special form of load multiple register instruction (see 'Exception return' on page 109). Both of these instructions are generally used to return to whatever instruction stream was being executed before the exception was entered and are not intended for a deliberate switch to Thumb mode. Like BX, they also change the program co...
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This document was uploaded on 10/30/2011 for the course CSE 378 380 at SUNY Buffalo.
- Spring '09