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From: uhclem@trsvax.UUCP
Newsgroups: net.micro.trs-80
Subject: Re: model 4P slower than model4??? - (nf)
Message-ID: <70700022@trsvax.UUCP>
Date: Thu, 27-Sep-84 12:28:00 EDT
Article-I.D.: trsvax.70700022
Posted: Thu Sep 27 12:28:00 1984
Date-Received: Sun, 30-Sep-84 00:58:19 EDT
References: <1230@ucla-cs.UUCP>
Lines: 44
Nf-ID: #R:ucla-cs:-123000:trsvax:70700022:37777777600:1838
Nf-From: trsvax!uhclem    Sep 27 11:28:00 1984

#R:ucla-cs:-123000:trsvax:70700022:37777777600:1838
trsvax!uhclem    Sep 27 11:28:00 1984



The Model 4P and the Model 4 both run at 4mhz.  The secret is in a
"wonderful" invention of hardware: RAM wait states.  This allowed the
use of memory that was too slow with a processor that was too fast.

The Model 4P has never had any RAM wait states, so if the Z80 book says
an instruction will take so long, that is how long it will take.
The Model 4 has gone through three phases of existance.
The first version had 2 wait states on each fetch.  The middle version
had 1, and the version they have been making recently has none.

So if you compare a Model 4P to an older Model 4, the 4P will win every
test.  Since wait states on fetch operations penalizes short opcodes,
and most OS's are written with tight code, I have seen a 20% speed difference
between an old Model 4 and new one or a 4P.

In fact, the Model 4P and the new Model 4s run faster than a Model II
and its descendants, which had two wait states.
The Model II/12 makes up for the wait states by not having wait states
built into the video circuitry, having IM 2 interrupts, and DMA.
The Model 4/4P has video waits, no DMA and no IM 2.

The idea behind the video waits was to eliminate the screen "hashing"
that happens when the CPU references video RAM when the character
generator is trying to get a look.  The Model I/II did not have them,
the Model III had them as an option, and the Model 4/4P has them on
all of the time. 

When the Model 4/4P are running at 2mhz, they all run a 2mhz, no wait
states.


 
						
						"Thank you, Uh Clem."
						Frank Durda IV
						@