Discussion in 'General Electronics Chat' started by programmer6502, Apr 15, 2014.

Feb 1, 2014
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Hi,

I've tried to look this up but I couldn't figure out how to put it in to keywords. So my question is, (talking old school) let's say for example you have a CPU like a MOS 6507 with a 12 bit address bus and 8-bit data bus that you would like to add some RAM too. By calculating 2^12, you see that the processor has a maximum addressing range of 4K. So you get yourself say... a 4K (512x8) RAM chip like a X20C04. The chip has 8 data lines, so you simply hook those up to the processor. (Here's what I don't understand) When you get to wiring the address bus, notice it only has 8 address lines! So how would the processor access the full 4K from the RAM chip with only 8 of the 12 needed address lines?

Note: The 6507 actually has 13 address lines from A0-A12 but pin A12 is used as a chip enable.

2. odinhg Active Member

Jul 22, 2009
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The X20C04 is 512x8bit. That is 4Kibit, not 4kilobyte.
It has 9 address lines A0..A8 which can select one of the 512 bytes that should be read from/written to on the I/O lines I/O0..I/O7.

That's how I read the datasheet at least

Feb 1, 2014
132
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Ok, I don't really get it though. Wouldn't 4k(bits) relate to speed? And 4k(bytes) would of course be memory. Wait... so is the X20C04 actually a 512 byte RAM chip? Let me get this straight, correct me if I'm wrong. So with a 4k(512x8) ram chip the 4k is the transfer rate, 512 is the memory capacity, and 8 is how many data lines, correct?

Last edited: Apr 15, 2014
4. MrChips Moderator

Oct 2, 2009
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Why are you mentioning speed or transfer rate?

X20C04 is a 4k-bit memory organized as 512 x 8 RAM or 512 bytes.

5. AnalogKid AAC Fanatic!

Aug 1, 2013
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X20C04 is only part of the part number. You didn't mention the dash and what comes after it. That part is the speed code. For example, if the full part number is X20C04-20, the read "speed" is 200 ns minimum, or 5 MHz maximum. This has nothing to do with the size or organization of the memory array.

But enough about that. What are you doing messing around with a 6507, cousin to the greatest 8-bitter of all time, the 6502? How old are you?

ak

6. MrChips Moderator

Oct 2, 2009
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MC6809 was better than the 6502.

Feb 1, 2014
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Defiantly not as old as the 6507 And yes, I do agree that the 6502 was one of the best processors of those days. Tell the truth I'm actually messing with an Atari 2600.

Anyway, I was just asking some questions guys. Until today I didn't know what some of that stuff properly stood for. I just wanted you guys to explain. Now that you guys made some stuff clear, things make much more sense! But I still don't understand what the 4kilobits is referring to in the X20C04! All I know is Kilobits are a rate of speed, not memory! But I understand everything else now.

So what is the purpose of the 4kilobits in the X20C04?? Sorry!

(Also, I'm not really using a X20C04, it was just for the example earlier.)

8. MrChips Moderator

Oct 2, 2009
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Geez. Must be a bad day today!

Kilobits is not speed nor rate nor rate of speed.
Watch your language! Rate of speed means acceleration.

Kilo in computerese means 1024.

4k bits mean 4 x 1024 = 4096 bits.

The X20C04 contains 4096 bits. It is classified as a 4k-bit memory.

Over and out.

Feb 1, 2014
132
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Chill out dude!! I'm sorry I asked!

10. AnalogKid AAC Fanatic!

Aug 1, 2013
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These days everyone talks about speed in shorthand. Cable TV ads talk about 50 megabit service, and people are losing an important distinction. It isn't 50 megabit service, it's 50 megabit *per second* service. Sounds like you've made the same mistake. 4K bits or bytes or words or double-words is quantity. 4K bits per second, or 4Kbps, is speed.

ak

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11. shteii01 AAC Fanatic!

Feb 19, 2010
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4096 bits divided by 8 bits is equal to 512 bytes.

One byte is 8 bits.

programmer6502 likes this.

Feb 1, 2014
132
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Ok thanks. I was just asking that because for example, a common old laptop stock WIFI card around here will max it 54 mega-bits of speed. So I was thinking Kilo-bits would be a step down from mega-bits. And it is, I just didn't know that with out the "per second" deal, it would be something totally different! I also didn't know that kilo(bits) existed in the memory world since just plan "bits" are as small as it can possible get! But according to MrChips, a kilobit = 1024 bits. So that of course means that a kilobit is larger then a bit.

Ok, well, thanks for helping me understand this stuff guys! Even if part of it wasn't super friendly. We just to remember that this stuff doesn't automatically come second nature to most people. That's what this forum is used for!

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13. MrChips Moderator

Oct 2, 2009
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If you care to stick around long enough you will discover that this is a super friendly site.

We can easily spot when someone is new to the field and is using the wrong terminology.

It is our role to make sure newbies get the language and jargon correct otherwise it leads to miscommunication.

It doesn't help when someone has a misconception but insists that they know what they are talking about.

14. MrChips Moderator

Oct 2, 2009
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By the way, a kilobit is not larger than a bit.

A bit is one.
A kilobit is 1024 bits, just 1024 times as many, not larger.

You don't say that ten persons are larger than one person.