Re: about the plus4

From: Gerrit Heitsch <>
Date: Fri, 20 Apr 2012 18:11:07 +0200
Message-ID: <>
On 04/19/2012 10:42 PM, Nate Lawson wrote:
> On Apr 19, 2012, at 8:54 AM, Gerrit Heitsch wrote:
>> On 04/19/2012 02:53 PM, Anders Carlsson wrote:
>>> When it comes to a replacement for the aging VIC-20, Commodore already
>>> had a 40 column chip 6562/63 in development but apparently it never saw
>>> its way into production.
>>  From what I read that chip was working, but needed rather expensive SRAM since it used the 'no badline' approach of the original VIC. That meant 2 memory cycles per 8 Bits displayed.
>> I could be mistaken, but I think the 6567 was the first VIC that used DRAM, everything before used SRAM, mostly 2114s and later 6116s. Look up their price at that time.
> Not only was it SRAM, but it had to be fast SRAM (for the time) and a lot of it (more pixels).

Yes... Due to the way the 6502 works, using 450ns 2114 was very close 
even for a 1 MHz system (it's too slow for a C64 though), so it would 
mean you'd need something in the range of 200ns to get the required 
bandwidth for a 40 character display in color. Today that's a joke, but 
look up the price for a 6116-200 (2Kx8 CMOS SRAM, 200ns) in 1980.

> By the time the 6567 was designed in 1981, DRAM prices were coming down low enough to put 64 KB in the machine and the badline design allowed it to be slower RAM.

Using the 4164 DRAM also meant that you didn't need the extra voltages. 
OK, the VIC and SID needed +12V like the 4116, but at least you didn't 
have to deal with -5V for the 4116. Ask ZX spectrum owners how much 
headaches the 4116 and their support circuits can cause.


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Received on 2012-04-20 17:00:13

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