Thread: 68k details
View Single Post
Old 26 August 2018, 14:35   #219
roondar
Registered User

 
Join Date: Jul 2015
Location: The Netherlands
Posts: 1,288
Quote:
Originally Posted by plasmab
Incorrect. The 4816AP was used in the BBC B..

Page 229

http://www.bitsavers.org/components/..._Data_Book.pdf
I did get the part number I used from your own quote on page 6
Anyway, it's not that important - if you check the datasheets they're pretty similar, except the 4816AP-3 has slower random access cycles posted.



Quote:
Originally Posted by plasmab View Post
The BBC B specifically used the 4816AP-3

http://mdfs.net/Info/Comp/BBC/Circuits/BBC/bbc.gif

The manual says..

Access Time From RAS: 100ns
Access Time From CAS: 55ns

So my assertion stands.

I accept that you would need time either side for setup etc.

Another way to put it would be that you would have been stupid to use that RAM with the 68000 as fast ram.. because you'd never max it out.

EDIT: There is another time quoted below for random access.. 235ns.. thats with a refresh i believe.
Well, I found that 235ns figure as well (and a 260ns one for the RAM in the Amiga) and my reasoning is as follows (I admit, it could be wrong):

The BBC micro runs at 2MHz, but accesses memory at both rising and falling edge of the clock due to the video chip. That is basically equivalent to a 4MHz clock. Such a clock does 1 memory cycle every 250ns (1/4.000.000). A clock that would do 1 memory cycle every 100ns would run at 10MHz.

This fits with the wikipedia article linked to earlier:
Quote:
Originally Posted by BBC Micro wiki
This gave the BBC Micro a fully unified memory address structure without speed penalties. To use the CPU at full speed (2 MHz) required the memory system to be capable of performing four million access cycles per second. Hitachi was the only company, at the time, that made a DRAM that went that fast
Note that the claim is 4 million accesses per second (of which 2 million go to the CPU as the 6502 does one access per cycle). That's still equivalent to 1/4 million=250ns.


So my question becomes: why do you need 100ns RAM to service a system that only accesses memory once every 250ns? The same goes for the Amiga. The system required 150ns RAM. So why is that? Why not just run it with 280ns RAM (which would be much cheaper) and be done with it?

Because to me that just does not add up. Either the 235ns figure is correct for memory accesses and the remaining 15ns are either used by other stuff on the board or as 'headroom', or the 100ns figure is correct and then what the hell does the system do for the remaining 150ns?!


I hope you see why I don't get this. I'm not trying to troll or be rude, but a simple division of 1/clock speed shows that the numbers you quote can't really work without something else taking up a significant portion of time. And if there's a reason for this, I'm more than happy to admit I'm wrong.
roondar is offline  
 
Page generated in 0.04191 seconds with 11 queries