How do we determine if a mcu is 8-bit; 16-bit or 32-bit

[overmind]

hi everyone, just a question.. can anybody tell me how an mcu is labelled as 8 bit or 16 bit or 32 bit? is it the data bus width? the number of registers? im confused.. it all seems so vague to me.. please help.. thanks in advance

 

[mdanh2002]

Look at the datasheet

 

[Nigel Goodwin]

Basically the internal databus and registers, the external bus could still be only 8 bits, even for 32 bit processors.

 

[3v0]

I was taught that it was based on the data bus width. At that time we did not have a lot of memory cashing and the external data bus matched the width of the internal buses and registers. It seems that is dated.

As Nigel post suggests things are not that simple anymore. Calling a processor an 8 or 16 bitter is not all that descriptive at this point. We need to look at both the internal and memory data paths etc.

In the end we often want to know how much processing the processor can do. That is why the invented benchmarks. However for micro controllers knowing the instruction width and the MIPS is often a good indicator.

 

[Nigel Goodwin]

I was taught that it was based on the data bus width. At that time we did not have a lot of memory cashing and the external data bus matched the width of the internal buses and registers. It seems that is dated.

It was dated a LONG time ago

For example the 68000 was a 32 bit processor, available in different versions with 8, 16, or 32 bit external bus connections.

 

[3v0]

Recently scientists have been talking about the non linearity of time. Not so much about how Einstein's relativity relates to time but rather our perception.

As we age each year passes more quickly. We all get old much sooner then we expected too. Don't blink.

 

[Nigel Goodwin]

Recently scientists have been talking about the non linearity of time. Not so much about how Einstein's relativity relates to time but rather our perception.

As we age each year passes more quickly. We all get old much sooner then we expected too. Don't blink.

I've always said, the older you get, the faster time seems to pass.

 

[KeepItSimpleStupid]

It's not that simple anymore and it wasn't simple with the PDP-11. Words were 16 bits, but the system was addressed as byte locations. It was considered a 16 bit processor. One way that might work is what's the largest integer that can be represented.

 

[3v0]

It's not that simple anymore and it wasn't simple with the PDP-11. Words were 16 bits, but the system was addressed as byte locations. It was considered a 16 bit processor. One way that might work is what's the largest integer that can be represented.

I am going to be contrary and point out that byte addressing in a CPU, greater then 8 bits, does not tend to make it an 8 bit machine.

The opcodes or addressing modes that allow for 8 bit addressing are used for character and other 8 bit data manipulation. Without byte addressing character manipulation becomes increasingly difficult as the word size increases.

There is a trade off between byte addressing and memory space. In 32 bit machines like the IBM360/370 and Motorola 68000 byte addressing cuts the addressable memory to 1/4. That was quite a hit to take given that no one knew know how to decode memory addresses fast enough to utilize large address spaces.

Googled to find this regarding the 360/370

Through the 1970's and 1980's, the machines got bigger and faster, and multi-processor systems became common, but the basic architecture did not change. Around 1982, addresses were extended from 24 bits to 31 bits (370-XA),

It is amazing to recall that these multi million beasties (IBM 360/40 etc) had 16MB of memory. Amazing because it was so little and amazing because it was core. That is a lot of little magnets. The memory cycle time for this machine was 2.5 ms. Register to register operations were much faster at 625ns.

 

[KeepItSimpleStupid]

I fogot to point out or it wasn't clear the PDP-11 could address 8 bits bytes and 16 bit words. A MOVB or MOV instructions were used. MOV was MOV Byte and MOV was move word.

 

[Noggin]

I've always said, the older you get, the faster time seems to pass.

That's because when you're 10, 1 year is 10% of your life. When you're 50, its only 2% of your life.