prefetch makes MOVD not work for next instruction

EricR

I find that I cannot use MOVD to modify the next instruction in code immediately.· I suspect the next instruction is being prefetched, thus delaying the affect of MOVD.· If I target MOVD to the following instruction, the MOVD works as I wanted.

code:
· looptop
············· ...
············· movd Writer, DiagCogAddr ' place the desination address that we·want in the wrlong instruction
············· nop·· ' this makes a world of difference!?· Could there be pre fetching?
· Writer··· wrlong 10, DiagHubAddress ' note that·10 is a dummy field value here, it is overwritten every loop
············· ...
············· jmp #looptop

Without the NOP instruction, the affect of changing the desitnation field only works the next time around in the loop.· With the NOP instruction, the Writer WRLONG instruction works as I planned.

Can anybody confirm this prefetch?

Eric R

JT Cook

I cannot remember where I read it, but I do know that you need an instruction between modifying code (MOVS, MOVD) and the code that is being modified. So you would need a NOP or another instruction inbetween, most like due to the fact that the Propeller is fecting the next instruction.

Mike Green

Yes, the next instruction is fetched late in the execution of an instruction, so you have to have some other instruction (doesn't have to be a NOP) between one instruction and the instruction modified by it.

EricR

Thanks Mike and TJ.· I think I may submit a suggestion about this being added to the Propeller documentation.

TreeLab

This is discussed in DeSilva's tutorial on Propeller asm coding, along with the timing diagrams for the prefetch etc. By coincidence, I found this out last night!

EricR

TreeLab,
I found (or rather Google found) the deSilva Assembly Tutorial at http://www.vaguelogic.com/propeller/Programming/DeSilvaAssemblyTutorial.pdf . I am enjoying reading this, thanks for the reference.
Eric