dwelch67 / Qemu_arm_samples

This is a collection of simple bare metal examples for qemu-system-arm

http://wiki.qemu.org/

QEMU supports a number of targets, this repo is specific to the arm and even more specific to one virtual machine. (one is more than enough)

Basically you can get your feet wet without it costing you anything more than some time and a little disk space. You cant brick it, you cant let any smoke out.

I run linux, you can to, you dont have to install it you can use a thumb drive, and download a live image (ubuntu, linuxmint, etc) and run that then within that live boot you can do these things. (In the linux/unix world we developers still spend a considerable amount of time using a console).

You will of course need QEMU

apt-get install qemu

QEMU supports many targets and machines, for example

qemu-system-arm -machine help Supported machines are: versatileab ARM Versatile/AB (ARM926EJ-S) versatilepb ARM Versatile/PB (ARM926EJ-S) lm3s811evb Stellaris LM3S811EVB z2 Zipit Z2 (PXA27x) ...

Some folks comment on the quality or lack there of of some of the targets or instruction set support. I have not yet formed an opinion. There are some arm based boards whose developers use QEMU to semi-natively build various things for those boards since for example those boards may have enough resources to run linux but might not have enough resources to provide a development environment. And some of those use versatilepb. One day years ago I tried that one for that reason, and that is the one I am using here. Lets just assume it somewhat works. If not we have the qemu source code to sort it out.

apt-get install git-core git clone https://github.com/qemu/qemu

qemu/hw/arm/versatilepb.c contains goodies on this system that we will need.

For example

/* Memory map for Versatile/PB:  */
...
/*  0x101f1000 UART0.  */
/*  0x101f2000 UART1.  */
/*  0x101f3000 UART2.  */
...
sysbus_create_simple("pl011", 0x101f1000, pic[12]);
sysbus_create_simple("pl011", 0x101f2000, pic[13]);
sysbus_create_simple("pl011", 0x101f3000, pic[14]);

With real hardware we would have some documentation on the board, and the chip or chips on the board. Instead we just have the source code from qemu. It may or may not be derived from some real hardware in the past, at the same time we dont know how accurate it is to that hardware if derived from it.

If you google pl011 you find a uart core from ARM. And can get the documentation. In there we find that the data register UARTDR is at offset 0x000 from the base address.

And UART0 we see from the qemu source above is at address 0x101f1000 su UARTDR for uart0 is at address 0x101f1000.

And here is where we get to cheat, initally. With a real uart we have to figure out clocks and clock divisors, we have to initialize the uart, we have to make sure the transmitter is not busy or at least has room for us to store the next byte we want to send, and so on. Maybe if we are lucky (and I already know the answer), since this is an emulator and not real hardware that has to wait for bits to shift out slowly. What if we were to just jam byte after byte into the UARTDR register without checking...or even without initializing... It happens to work.

So start with the uart01 example

Unless otherwise specified for all of the examples

Run with

qemu-system-arm -M versatilepb -m 128M -nographic -kernel notmain.bin

then ctrl-a then x to exit the qemu console.

Another way is

qemu-system-arm -M versatilepb -m 128M -kernel notmain.bin

and then ctrl-alt-3 (not F3 but 3) will switch to the serial0 console and you can see the output, and can close out of qemu.

You will need a toolchain to cross compile/assemble these programs, see the file TOOLCHAIN in this directory for more information.

I could/should re-write this to be generic or maybe re-target it for this platform, but for now you can look at the baremetal/README in my raspberrypi repo for more info on basics of bare metal development (specifically with the gnu tools, but after that other tools are more similar than different), the bootstrap, lauching the C program, etc. Likewise a little deeper is the bssdata/README which may shine a light on why I do things the way I do them.