[f-cpu] register move in 1 x-bar cycle

[jaap stolk <jwstolk@yahoo.com>]

hi,

this could be a simple typo, but
according to the manual (chapter 6)
a register move looks like this:

Fetch > Decod         > Xbar      > Register write
       (+reg read)     (+schedule) 
       (+bypass check)

i would like to turn that into

Fetch > Decod         > Xbar    >  Xbar  > Register w.
       (+reg read)     (+schedule) 
       (+bypass check)

ther will be no extra lost cycle (the next instuction
will use a direct bypass instead of a delayed bypass)
http://f-cpu.seul.org/whygee/parinux/conf_yg.html
seems to conferm this.

i tryed doing it in one cycle in the c simulator,
but it has the folowing complications:

-its a reversed bypass (so we need extra hardware
 to bypass FROM the read bus TO the write bus)
-we can't use the schedule queue for the register
 write (schedule cycle is to late), so we have
 to control the xbar directly from the decoder.
-the x-bar needs to ignore these commands from the
 decoder in normal operation.

can we change it into two x-bar cycles ?

jaap.

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