Volatile within a struct

Projects Programming
1

Hi,
I have a structure of which one member only can be modified by an Interrupt service routine.

Will
typedef struct
{
byte a;
volatile byte b;
} type_t;

type_t x;

Can I actually make only b volatile as above or do I need to make the whole instantiation of the type volatile.
as in

typedef struct
{
byte a;
byte b;
} type_t;
volatile type_t x;

Asking as it would be difficult to test.

2

You can make just the one byte be volatile...

3

Thank You.

4

it would be difficult to test.

you COULD confirm proper behavior by inspecting the assembly language produced. If the "volatile" is working, then a code segment like:

  x.b = 1;
  if (x.b == 0)
    Serial.print("ta da");

should produce something like:

   ldi r24, 1  ;; get a 1
   sts r24, 0x1234  ;; store it in the memory location for x.b
   lds t24, 0x1234 ;; get the value in the memory location for x.b
   cpi r24, 1 ;; compare

But without "volatile" (or if it doesn't work), the "lds" instruction would be omitted since the compiler clearly knows that r24 still has that variable's value. (probably the whole if statement would get omitted in this case, since it's clearly false...)

5

Could examine the assembly but is does not correspond to GTL!
There are 14 registers, R0-R13, all are created equal, but convention dictates how some are used!

6

Ah. ARM, then? Compiled for Due, from:

struct {
  int a;
  volatile int b;
} x;

void setup() {
  // put your setup code here, to run once:
  x.a = 1;
  if (x.a == 0) {
    Serial.print("x.a magically zero.");
  }
  x.b = 1;
  if (x.b == 0) {
    Serial.print("x.b magically zero.");
  }
}

I get

   80128:       4b05            ldr     r3, [pc, #20]   ;  get data area pointer.
   8012a:       2201            movs    r2, #1  ; get constant 1
   8012c:       605a            str     r2, [r3, #4]   ;store 1 in x.b
   8012e:       601a            str     r2, [r3, #0]   ; and also in x.a
          ; here we'd compare x.a for zero, but the compile knows it's not
          ; so it just omitted all that code.
   80130:       685b            ldr     r3, [r3, #4]  ;  read x.b again!  Because it's volatile!
   80132:       b91b            cbnz    r3, 8013c <setup+0x14>  ; test and jump from 'if'
   80134:       4803            ldr     r0, [pc, #12]   ; get string
   80136:       4904            ldr     r1, [pc, #16]   ; Another arg for "print" ?
   80138:       f000 b89b       b.w     80272 <_ZN5Print5printEPKc>  ; print it
   8013c:       4770            bx      lr  ; "return" instruction at end of setup()
   8013e:       bf00            nop  ; branch delay slot.
7

Not ARM. Gods True Language has always been IBM 370 Assembler!!