[Moved] Question on microprossor based design

[mr_vasanth]

We have a microprocessor with 16 bit address line and 32 bit data line. Assume that we have only a 16KB memory which will be accessed by this microprocessor. To access 16KB memory address line LSB 14 bits of the address line is sufficient. But what will happen if the microprocessor tries to access some location which is beyond 16KB ? How it should be handled while we do a processor based design ?

 

[bigdogguru]

Re: Question on microprossor based design

But what will happen if the microprocessor tries to access some location which is beyond 16KB ?

It may depend largely on the address decoding scheme implemented in the processor and circuit design. Typically, in real hardware it maybe undefined or simply wrap around, addressing the same implemented storage over and over.

How it should be handled while we do a processor based design ?

I'm not sure this thread belongs in the "Electronic Elementary Questions" section.

Is the processor design being done in an HDL? If so, I think we should move the thread to a more appropriate section.

BigDog

 

[mr_vasanth]

Re: Question on microprossor based design

Hi BigDog,

Do you have any idea when we will leave the address space undefined, when we will wrap around and when we will access the same implemented storage. I just want to get a glimpse of the difference in architecture terms.

 

[RetroTechie]

Re: Question on microprossor based design

With a 64K address space, 16K present and the upper address lines not decoded, the 16K would simply be mirrored 4 times. So locations 16384, 32768, 49152 would be the same as location 0. Locations 16385, 32769, 49153 the same as location 1, etc (until you hit the next mirror area).

With mirror = not a 'copy' of a location / memory area, but one and the same thing 'visible' in several different places. So write location 16384, read that value from location 0 etc.

 

[bigdogguru]

Re: Question on microprossor based design

@RetroTechie - Good explanation.

I actually have a few early development boards which exhibit this behavior.

@mr_vasanth - Obviously to avoid this wrap around/mirroring behavior all address line must be utilized in the decoding scheme.

I'm going to move this thread to a more appropriate section as I feel the topic is beyond elementary electronics.

BigDog