[coco] wondering about 68k machines

[jdaggett at gate.net]

Willard

A bit ackward to try and compare the 6809 and 68K execution times 
with regard to peripheral access. The 6809 and 68K use different state 
machines. The 68K state machine does more than the 6809. Mostly 
new activity is in buss arbitration. 

Yes you are correct in that the it takes the equivalent to four machine 
cyles or 8 states to clock throught the state machiine. IN byte transfers 
one is required twice through the state machine to transfer one word. 
Thus 8 machine cylces . In word wide transfers this requires only once 
through the state machine and thus two bytes are transferred in 4 
cycles. In long word transfers t he same effective time is needed as the 
state machine is used twice to effect a long word transfer. Thus 2 bytes 
every 4 machine cycles. 

The hard part of comparison is that the 68K state machine does more 
bus arbitration than that of the 6809. During state 4 of read and write 
cycles the 68K checks for cycle termination before entering the 
peripheral cycle, the second half of the state sequence. If it does not 
see the proper cycle termination signal, the state  machine inserts wait 
states. This allows better interface with slower peripherals. 

The major knock on the 68K is that it appears to slow down in 
preipheral operations. This can happen on slow peripherals. 

james


On 10 Jul 2005 at 2:33, Willard Goosey wrote:

Date sent:      	Sun, 10 Jul 2005 02:33:57 -0600
From:           	Willard Goosey <goosey at virgo.sdc.org>
To:             	coco at maltedmedia.com
Subject:        	Re: [coco]  wondering about 68k machines
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> Remember, you're comparing a 2MHz _synchronous_ cpu to an 8MHz
> _asynchronous_ cpu.  The 6809 reads|writes a byte every cycle. The 68K
> reads|writes a 16-bit word every 4(?) cycles.  Memory is about the
> same speed on both machines.  
> 
> So, the 8MHz 68K box is about twice as fast as a 2MHz 6809.  And it
> does show in things like displaying gifs.