ARM.SoC.Architecture

Where the table refers to signed or unsigned

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Unformatted text preview: sult minus or negative Arithmetic operation did not give carry-out Unsigned comparison gave lower Arithmetic operation gave carry-out Unsigned comparison gave higher or same Signed integer operation; no overflow occurred Signed integer operation; overflow occurred Signed integer comparison gave greater than Signed integer comparison gave greater or equal Signed integer comparison gave less than Signed integer comparison gave less than or equal Unsigned comparison gave higher Unsigned comparison gave lower or same Control flow instructions 65 Conditional execution An unusual feature of the ARM instruction set is that conditional execution applies not only to branches but to all ARM instructions. A branch which is used to skip a small number of following instructions may be omitted altogether by giving those instructions the opposite condition. For example, consider the following sequence: This may be replaced by: The new sequence is both smaller and faster than the old one. Whenever the conditional sequence is three instructions or fewer it is better to exploit conditional execution than to use a branch, provided that the skipped sequence is not doing anything complicated with the condition codes within itself. (The three instruction guideline is based on the fact that ARM branch instructions typically take three cycles to execute, and it is only a guideline. If the code is to be fully optimized then the decision on whether to use conditional execution or a branch must be based on measurements of the dynamic code behaviour.) Conditional execution is invoked by adding the 2-letter condition after the 3-letter opcode (and before any other instruction modifier letter such as the 's' that controls setting the condition codes in a data processing instruction or the 'B' that specifies a byte load or store). Just to emphasize the scope of this technique, note that every ARM instruction, including supervisor calls and coprocessor instructions, may have a condition appended which causes it to be skipped if the condition...
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