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atmelAVR91
Joined: 02 Aug 2006
Posts: 70
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 02 Nov 2006 12:09 Arm Cortex-M3 Core with ThumbŪ-2 ISA Instruction Set |
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What is difference between ThumbŪ-2 ISA of Cortex-M3
and ARM7TDMI ARM/THUMB mode.
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dainis
Joined: 15 May 2001
Posts: 1451
Helped: 56
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02 Nov 2006 12:31 Re: Arm Cortex-M3 Core with ThumbŪ-2 ISA Instruction Set |
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| http://www.arm.com/products/CPUs/archi-thumb2.html
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nooknikz
Joined: 22 Apr 2002
Posts: 404
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Location: thailand
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robert_from_philips
Joined: 10 Apr 2006
Posts: 35
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Location: US
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02 Nov 2006 22:27 Arm Cortex-M3 Core with ThumbŪ-2 ISA Instruction Set |
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Just a couple really significant changes:
1. While thumb2 is a superset of thumb. Thumb2 includes a mix of 16-bit and 32-bit instructions, thumb only 16-bit instructions.
2. On Cortex M3 there is no ARM mode, which makes it absolutely mandatory to change existing ARM7 code as ARM7 code can not exist without some ARM code.
3. In general, Thumb2 mode is said to have similar performance as ARM mode in a system that is not memory access constraint while it has the small code similar to thumb. You get the best of both worlds.
If you have an ARM7 and you would program the 10% most time ciritcal code in ARM mode and the remaining part of the code in Thumb mode, the difference in code size would be less than 5% and the difference in speed also less thna 5%. However, the beauty of Thumb2 is simplicity, you do not need to know which part of the code needs to be fast and which is OK to run in slower Thumb mode.
Summary: Assuming the same speed and a smart programmer, Thumb2 does not change performance and code size a lot but it is simpler and for new designs reduces time to market.
If you have existing ARM7 code that might be very different.
Robert
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