qemu-arm with Cortex-M4 on Linux - c

Question

I am using qemu-arm and the ARM Workbench IDE to run/profile an ARM binary which was built with armcc/armlink (an .axf-File, program written in C). This works fine with Cortex-A9 and ARM926/ARM5TE. However, whatever I tried, it doesnt work when the binary is built for Cortex-M4. Both the simulator and qemu-arm hang when M4 is selected as CPU.
I know that this processor requires some additional startup code, but I could find any comprehensive tutorial on how to get it running. Does anyone know how to do this? I have a quite big project with one main function, but it would already help if a "hello world" or some simple program which takes arguments would run.
Here is the command line I am using with Cortex-A9:
qemu-system-arm -machine versatileab -cpu cortex-a9 -nographic -monitor null -semihosting -append 'some program arguments' -kernel program.axf

Answer 1

I do not know how to do it with the versatilepb, it did not "just work", but this does work:
flash.s
.thumb
.thumb_func
.global _start
_start:
stacktop: .word 0x20001000
.word reset
.word hang
.thumb_func
reset:
bl notmain
b hang
.thumb_func
hang: b .
.thumb_func
.globl PUT32
PUT32:
str r1,[r0]
bx lr
notmain.c
void PUT32 ( unsigned int, unsigned int );
#define UART0BASE 0x4000C000
int notmain ( void )
{
unsigned int rx;
for(rx=0;rx<8;rx++)
{
PUT32(UART0BASE+0x00,0x30+(rx&7));
}
return(0);
}
flash.ld
ENTRY(_start)
MEMORY
{
rom : ORIGIN = 0x00000000, LENGTH = 0x1000
ram : ORIGIN = 0x20000000, LENGTH = 0x1000
}
SECTIONS
{
.text : { *(.text*) } > rom
.rodata : { *(.rodata*) } > rom
.bss : { *(.bss*) } > ram
}
(I am told the entry point being a thumb function address is critical YMMV)
arm-none-eabi-as --warn --fatal-warnings -mcpu=cortex-m3 flash.s -o flash.o
arm-none-eabi-gcc -Wall -O2 -ffreestanding -mcpu=cortex-m3 -mthumb -c notmain.c -o notmain.o
arm-none-eabi-ld -nostdlib -nostartfiles -T flash.ld flash.o notmain.o -o notmain.elf
arm-none-eabi-objdump -D notmain.elf > notmain.list
arm-none-eabi-objcopy -O binary notmain.elf notmain.bin
check the vector table, etc.
00000000 <_start>:
0: 20001000
4: 0000000d
8: 00000013
0000000c <reset>:
c: f000 f804 bl 18 <notmain>
10: e7ff b.n 12 <hang>
00000012 <hang>:
12: e7fe b.n 12 <hang>
Looks good.
And run it
qemu-system-arm -M lm3s811evb -m 8K -nographic -kernel notmain.bin
01234567
Then ctrl-a then x to exit
QEMU: Terminated
-cpu cortex-m4 works as well as one would expect. Would have to try to find things different between the m3 and m4 that might show up in a sim like this and go from there.
After Luminary Micro (acquired by ti a while ago now) I do not think anyone else put the effort in for a machine. But as already discussed in at least one question at this site, you can run the cores (an exercise for the reader).
For versatilepb
int notmain ( void )
{
unsigned int ra;
for(ra=0;;ra++)
{
ra&=7;
PUT32(0x101f1000,0x30+ra);
}
return(0);
}
qemu-system-arm -machine versatileab -cpu cortex-m4 -nographic -monitor null -kernel notmain.elf
qemu-system-arm: This board cannot be used with Cortex-M CPUs

Answer 2

You can't arbitrarily plug different CPU types into an Arm board model. If you try it then the resulting system may work by luck, or may crash, or have odd behaviour; in some cases the -cpu option will just be ignored. This is because the CPU integration with the board matters: things like interrupt controllers are part of the board, not the CPU, but not all CPUs will work with all interrupt controllers. Often QEMU is not as good as it could be about detecting and reporting errors for user options that aren't valid.
In this case you're probably using an older QEMU: newer ones will correctly report:
qemu-system-arm: This board cannot be used with Cortex-M CPUs
if you try to use '-machine versatilepb' with '-cpu cortex-m4'. Older ones would either crash or just misbehave.
Generally the best thing is to use the CPU type that the board has by default (ie don't specify a -cpu option), for every board type except the "virt" board. If you want to write code for a Cortex-M4, you should look for a board type that has a Cortex-M4. The mps2-an386 is probably a good option. (If your QEMU doesn't have that board type, upgrade to a newer one: there have been a lot of M-profile emulation bug fixes anyway that you'll want to have.)