[Gllug] Re: Dual core AMDs

[Christian Smith]

On Wed, 13 Jul 2005, John Winters wrote:

>>  - and, anyway, does not compare to the Z80s memory
>> transfer built ins - write your screen RAM in memory and - zap - load it
>> up when the *cough* monitor - oh alright then, B&W portable TV - is in
>> refresh
>
>The LDIR on the Z80 always seemed a bit messed up to me.  It didn't
>actually do it anything like as quickly as it could have done because it
>went back and re-read the instruction again on every cycle.
>
>	Program:	You see these bytes here (HL)
>	CPU:		Yup!
>	Program:	And you see that space there (DE)
>	CPU:		Yup!
>	Program:	Well take this many of the bytes (BC)
>	CPU:		Yup!
>	Program:	And copy them (LDIR)
>	CPU:		(Copies 1 byte) What was it you wanted
>			me to do again?
>	Program:	Copy them! (LDIR)
>	CPU:		(Copies 1 more byte)  Sorry, what was
>			it again?
>	Program:	Copy them!! (LDIR)
>	...repeat ad nauseam
>
>A bit like the CPU equivalent of ADD.


Ah, but it did save a branch instruction, which was at least 2 (3?) cycles
per loop. There was non of this modern pipelining technology back then,
you know(*).



>
>John
>

Christian

(*) Actually, the Z80 did make a few instructions execute 1 cycle slower,
    so that it could get the next instruction in flight, thus forming a
    primitive 2 stage pipeline under some circumstances. Can't remember
    the instructions, but they could have been some memory writes.


-- 
    /"\
    \ /    ASCII RIBBON CAMPAIGN - AGAINST HTML MAIL
     X                           - AGAINST MS ATTACHMENTS
    / \
-- 
Gllug mailing list  -  Gllug at gllug.org.uk
http://lists.gllug.org.uk/mailman/listinfo/gllug