How to create inline assembly functions in C?

Question

I am using a MSP 432 and have to create an assembly functions in C. I tried creating the functions using __asm void PendSV_Handler{}. But that does not work and says Expected an Identifier.

Also, I am trying to run this assembler command cpsid i but it says CPSID is undefined but CPSIE i works. I am a bit confused at this point. I am fairly new to this and I am still learning.

Below is the code I am trying to make assembly. I try making the function assembly by doing __asm void PendSV_handler.

I was not sure if it would be easier to just create a asm. file with these instructions.

OSThread *volatile OS_curr;
OSThread *volatile OS_next;

void PendSV_Handler(void){


__asm__("cpsid      i"

//if(OS_curr != (OSThread *)0)
        "ldr        r0, =OS_curr"
        "ldr        r0, [r0]"
        "cbz        r0, PendSV_restore");

// Push r4  - r11
__asm__("PUSH {r4-r11}"

        "ldr        r1, =OS_curr"
        "ldr        r1, [r1]"

// OS_curr -> sp = sp;
        "str         sp, [r1]");

PendSV_restore

// sp=OS_next -> sp;
        __asm__("ldr r0, =OS_next;"
                "ldr        r0, [r0]"
                "ldr        r0, [r0]"
                "str        sp, [r13]");

// OS_curr = OS_next;
        __asm__("ldr        r0, =OS_next"
                "ldr        r1, [pc, 0xc]"
                "ldr        r0, =OS_curr"
                "str        r0, [r1]"

//Pop  r4-r11
                "POP        {r4-r11}"

//            __enable_interrupts();
                "cpsie      i"

//return to next thread
                "bx         r14");

}

What makes you think `CPSIE i` works? Is it more likely that it simply stops parsing after the first error? Your `PendSV_restore` label is missing its `:`, so the parser has clearly given up, or this is not the code you are building. — Clifford, Jan 15 '21 at 20:33
This is my first time actually delving into assembly. I had gotten this from the disassembler but it was riddled with errors when I brought it into the c file. — Mperez, Jan 15 '21 at 22:37

Clifford · Answer 1 · 2021-01-15T22:53:44.323

Inline assembler syntax is not redefined in the C programming language, and its support and syntax is compiler specific. In GCC:

void PendSV_Handler(void) 
{
    __asm__("cpsid      i");

//if(OS_curr != (OSThread *)0)
    __asm__("ldr        r0, =OS_curr");
    __asm__("ldr        r0, [r0]");
    __asm__("cbz        r0, PendSV_restore");

// Push r4  - r11
    __asm__("PUSH {r4-r11}");

    __asm__("ldr        r1, =OS_curr");
    __asm__("ldr        r1, [r1]");

// OS_curr -> sp = sp;
    __asm__("str         sp, [r1]");

PendSV_restore:

// sp=OS_next -> sp;
    __asm__("ldr r0, =OS_next;");
    __asm__("ldr        r0, [r0]");
    __asm__("ldr        r0, [r0]");
    __asm__("str        sp, [r13]");

// OS_curr = OS_next;
    __asm__("ldr        r0, =OS_next");
    __asm__("ldr        r1, [pc, 0xc]");
    __asm__("ldr        r0, =OS_curr");
    __asm__("str        r0, [r1]");

//Pop  r4-r11
    __asm__("POP        {r4-r11}");

//            __enable_interrupts();
    __asm__("cpsie      i");

//return to next thread
    __asm__("bx         r14");
}

I was able to fix the CPSID issue. I am facing another issue with the OS_curr and OS_next. I would like to use these and load them in the register but I get a ton of errors when doing this. The errors: Bad term in expression, Illegal symbol, Unexpected trailing operand(s). I have been looking around but I haven't found anything that can remedy this. — Mperez, Jan 15 '21 at 22:35
@Mperez Did you fix it; what did you do? If you did what I said, then I think I fixed it. If you have a different problem, you need to post another question. I am no expert, I Googled it. t.b.h through looks like an X-Y problem. — Clifford, Jan 15 '21 at 23:01

score 0 · Answer 2 · answered Jan 18 '21 at 19:56

Referencing

Inline assembler syntax is not redefined in the C programming language

I did in the past for a university project

inline void function(param1,param2)
{
   asm volatile ("param1");
   asm volatile ("param2");
}

But, if u are working on ARM, look at the instruction set to see whitch commands are possible

As an example: If you want to write some timing critical stuff, that needs to be in the written order you could do something like this

inline void COLOUR_GLCD_write_address_data(uint8_t address, uint16_t data)
{
  asm volatile ("DMB");
  *(volatile uint16_t*)LCD_CMD_ADDRESS = address; 
  asm volatile ("DMB");
  *(volatile uint16_t*)LCD_DATA_ADDRESS = data; 
}

This was for sending data to an external lcd via the BUS Interface on an Atmel SAME70. Hope that helped =)

How to create inline assembly functions in C?

2 Answers2