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instruction cycle timing & dual issue in Cortex A8

Devi Prasad
Note: This was originally posted on 8th April 2011 at http://forums.arm.com

Hi, I'm trying to really understand the instruction cycle timing and dual-issue interactions in ARM Cortex-A8. Having spent quite a few hours on this, I have a feeling that this aspect is not explained in ARM technical reference manuals. So, here is my question with respect to the following snippet taken straight from the manual (section 16.3, Table 16-15 Dual-issue restrictions):


Data source hazard
----------------------------
ADD r1, r2, r3         1         -
ADD r4, r1, r6         2        Wait for r1
LDR r7, [r4]             4        Wait two cycles for r4

I think LDR must be issued in the third cycle and not in the fourth cycle because the second ADD instruction would yield R4 by the end of cycle 2.

And BTW I have tried http://www.avison.me.uk/ben/programming/cortex-a8.html before reaching here :)

thanks!
devi prasad
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