How to use external memory on a microcontroller

Question

In the past, I've worked a lot with 8 bit AVR's and MSP430's where both the RAM and flash were stored on the chip directly. When you compile and download your program, it sort of "just works" and you don't need to worry about where and how variables are actually stored.

Now I'm starting a project where I'd like to be able to add some external memory to a microcontroller (a TI Stellaris LM3S9D92 if that matters) but I'm not entirely sure how you get your code to use the external RAM. I can see how you configure the external bus pretty much like any other peripheral but what confuses me is how the processor keeps track of when to talk to the external memory and when to talk to the internal one.

From what I can tell, the external RAM is mapped to the same address space as the internal SRAM (internal starts at 0x20000000 and external starts at 0x60000000). Does that mean if I wrote something like this:

int* x= 0x20000000; int* y= 0x60000000; 

Would x and y would point to the first 4 bytes (assuming 32 bit ints) of internal and external RAM respectively? If so, what if I did something like this:

int x[999999999999]; //some super big array that uses all the internal ram int y[999999999999]; //this would have to be in external ram or it wouldn't fit 

I imagine that I'd need to tell something about the boundaries of where each type of memory is or do I have it all wrong and the hardware figures it out on its own? Do linker scripts deal with this? I know they have something to do with memory mapping but I don't know what exactly. After reading about how to set up an ARM cross compiler I get the feeling that something like winavr (avr-gcc) was doing a lot of stuff like this for me behind the scenes so I wouldn't have to deal with it.

Sorry for rambling a bit but I'd really appreciate it if someone could tell me if I'm on the right track with this stuff.

Update

For any future readers I found this after another few hours of googling http://www.bravegnu.org/gnu-eprog/index.html. Combined with answers here it helped me a lot.

Answer 1

Generally that is exactly how it works. You have to properly setup the hardware and/or the hardware may already have things hardcoded at fixed addresses.

You could ask the same question, how does the hardware know that when I write a byte to address 0x21000010 (I just made that up) that that is the uart transmit holding register and that write means I want to send a byte out the uart? The answer because it is hardcoded in the logic that way. Or the logic might have an offset, the uart might be able to move it might be at some other control register contents plus 0x10. change that control register (which itself has some hardcoded address) from 0x21000000, to 0x90000000 and then write to 0x90000010 and another byte goes out the uart.

I would have to look at that particular part, but if it does support external memory, then in theory that is all you have to do know what addresses in the processors address space are mapped to that external memory and reads and writes will cause external memory accesses.

Intel based computers, PC's, tend to like one big flat address space, use the lspci command on your Linux box (if you have one) or some other command if windows or a mac, and you will find that your video card has been given a chunk of address space. If you get through the protection of the cpu/operating system and were to write to an address in that space it will go right out the processor through the pcie controllers and into the video card, either causing havoc or maybe just changing the color of a pixel. You have already dealt with this with your avr and msp430s. Some addresses in the address space are flash, and some are ram, there is some logic outside the cpu core that looks at the cpu cores address bus and makes decisions on where to send that access. So far that flash bank and ram bank and logic are all self contained within the boundaries of the chip, this is not too far of a stretch beyond that the logic responds to an address, and from that creates an external memory cycle, when it is done or the result comes back on a read it completes the internal memory cycle and you go on to the next thing.

Does that make any sense or am I making it worse?