How many cycles is a DEC instruction ?

[aral]

How many cycles is a DEC instruction for the MSP430 ucontroler ? The documentation for the msp430 doesn't say .

 

[bonxer]

I am not familiar with that chip, but I'm going to assume that the DEC instruction decrements a register by 1, and that it only takes 1 cycle. I see no reason for it to take any additional cycles. It is only performing one function, there are no jumps or additional comparisons or anything else. I would treat it as any other single function instruction.

 

[aral]

It seems like DEC is an emulated instruction for the msp430 ,I read this in the msp430 documentation, and it is emulated with SUB . Problem is the documentation doesn't specific the number of cycles for DEC and neither for SUB . Also , by my calculations , after mesuring a periodic signal on the oscilloscop, it has to be somwhere between 8 and 10 cycles , I can't seem to find out exactly how many ... If someone know , please tell me !

 

[bonxer]

Here's the document that describes the instructions: **broken link removed**

PDF page 71 says that DEC is an emulation of the SUB instruction, and that it performs "destination -1 -> destination".

Page 51 says: There is no code or performance penalty for using emulated instruction.

Page 106 says that the number of instruction cycles do not depend on the instruction itself; instead it depends on the instruction format and the addressing mode.

Page 107 has a big table with lots of symbols, which I do not understand what they mean, and it has lots of different possible timings depending on exactly what you are doing. The number of cycles can vary anywhere from 1 to 6. So whatever you are doing in your code, look for a similar construction on this page.