How Can I put ARM processor in different modes using C program?

Question

I am going through different mode of ARM processor. I want to check the processor state(ex: register values) while it is in different mode.

So can someone help me to

Answer 1

You'll need inline assembly for this since C has no built in way to achieve what you want. This isn't exactly the most efficient way to do this (my GCC still generates a useless mov r3, #0 instruction).

You're probably better off implementing the function in bare assembly.

#define MODE_USR        0x10
#define MODE_FIQ        0x11
#define MODE_IRQ        0x12
#define MODE_SVC        0x13
#define MODE_ABT        0x17
#define MODE_UND        0x1B
#define MODE_SYS        0x1F

#define MODE_MASK       0x1F

void switch_mode(int mode) {
        register unsigned long tmp = 0;

        mode &= ~MODE_MASK;

        asm volatile(
                "mrs %[tmp], cpsr_all \n"
                "bic %[tmp], %[tmp], %[mask] \n"
                "orr %[tmp], %[tmp], %[mode] \n"
                "msr cpsr_all, %[tmp] \n"
                : : [mode] "r" (mode), [mask] "I" (MODE_MASK), [tmp] "r" (tmp)
        );
}

The function just sets the mode bits in the program status register. You can find the constants in the ARM reference manual for your exact architecture.

Here is the code generated by my compiler:

00000000 :
   0:   e3c0001f    bic r0, r0, #31   ; sane-ify the input
   4:   e3a03000    mov r3, #0        ; useless instruction generated by gcc
   8:   e10f3000    mrs r3, CPSR      ; saves the current psr
   c:   e3c3301f    bic r3, r3, #31   ; clear mode bits
  10:   e1833000    orr r3, r3, r0    ; set mode bits to the inputted bits
  14:   e129f003    msr CPSR_fc, r3   ; set current psr to the modified one
  18:   e12fff1e    bx  lr