Adding Two Numbers:
.text
start: @ Label, not really required
mov r0, #5 @ Load register r0 with the value 5
mov r1, #4 @ Load register r1 with the value 4
add r2, r1, r0 @ Add r0 and r1 and store in r2
stop: b stop @ Infinite loop to stop execution
Building the Binary:
Save the program in a file say add.s. To assemble the file, invoke the GNU Toolchain’s assembler as, as shown in the following command.
$ arm-none-eabi-as -o add.o add.s
The -o option specifies the output filename.
To generate the executable file, invoke the GNU Toolchain’s linker ld, as shown in the following command.
$ arm-none-eabi-ld -Ttext=0x0 -o add.elf add.o
Here again, the -o option specifies the output filename. The -Ttext=0x0, specifies that addresses should be assigned to the labels, such that the instructions were starting from address 0x0. To view the address assignment for various labels, the nm command can be used as shown below.
$ arm-none-eabi-nm add.elf
... clip ...
00000000 t start
0000000c t stop
Note the address assignment for the labels start and stop. The address assigned for start is 0x0. Since it is the label of the first instruction. The label stop is after 3 instructions. Each instructions is 4 bytes. Hence stop is assigned an address 12 (0xC).
Linking with a different base address for the instructions will result in a different set of addresses being assigned to the labels.
$ arm-none-eabi-ld -Ttext=0x20000000 -o add.elf add.o
$ arm-none-eabi-nm add.elf
... clip ...
20000000 t start
2000000c t stop
The output file created by ld is in a format called ELF. Various file formats are available for storing executable code. The ELF format works fine when you have an OS around, but since we are going to run the program on bare metal, we will have to convert it to a simpler file format called the binary format.
A file in binary format contains consecutive bytes from a specific memory address. No other additional information is stored in the file. This is convenient for Flash programming tools, since all that has to be done when programming is to copy each byte in the file, to consecutive address starting from a specified base address in memory.
The GNU toolchain’s objcopy command can be used to convert between different object file formats. A common usage of the command is given below.
objcopy -O
To convert add.elf to binary format the following command can be used.
$ arm-none-eabi-objcopy -O binary add.elf add.bin
Check the size of the file. The file will be exactly 16 bytes. Since there are 4 instructions and each instruction occupies 4 bytes.
Executing in Qemu
When the ARM processor is reset, it starts executing from address 0x0. On the connex board a 16MB Flash is located at address 0x0. The instructions present in the beginning of the Flash will be executed.
When qemu emulates the connex board, a file has to be specified which will be treated file as Flash memory. The Flash file format is very simple. To get the byte from address X in the Flash, qemu reads the byte from offset X in the file. In fact, this is the same as the binary file format.
To test the program, on the emulated Gumstix connex board, we first create a 16MB file representing the Flash. We use the dd command to copy 16MB of zeroes from /dev/zero to the file flash.bin. The data is copied in 4K blocks.
$ dd if=/dev/zero of=flash.bin bs=4096 count=4096
add.bin file is then copied into the beginning of the Flash, using the following command.
$ dd if=add.bin of=flash.bin bs=4096 conv=notrunc
This is the equivalent of programming the bin file on to the Flash memory.
After reset, the processor will start executing from address 0x0, and the instructions from the program will get executed. The command to invoke qemu is given below.
$ qemu-system-arm -M connex -pflash flash.bin -nographic -serial /dev/null
The -M connex option specifies that the machine connex is to be emulated. The -pflash options specifies that flash.bin file represents the Flash memory. The -nographic specifies that simulation of a graphical display is not required. The -serial /dev/null specifies that the serial port of the connex board is to be connected to /dev/null, so that the serial port data is discarded.
The system executes the instructions and after completion, keeps looping infinitely in the stop: b stop instruction. To view the contents of the registers, the monitor interface of qemu can be used. The monitor interface is a command line interface, through which the emulated system can be controlled and the status of the system can be viewed. When qemu is started with the above mentioned command, the monitor interface is provided in the standard I/O of qemu.
To view the contents of the registers the info registers monitor command can be used.
(qemu) info registers
R00=00000005 R01=00000004 R02=00000009 R03=00000000
R04=00000000 R05=00000000 R06=00000000 R07=00000000
R08=00000000 R09=00000000 R10=00000000 R11=00000000
R12=00000000 R13=00000000 R14=00000000 R15=0000000c
PSR=400001d3 -Z-- A svc32
Note the value in register R02. The register contains the result of the addition and should match with the expected value of 9.
