Example code to demostrate use and need of volatile
LEVEL 0 Optimization
10 uint32_t value = 0;
080001ee: 00 23 movs r3, #0
080001f0: 7b 60 str r3, [r7, #4]
11 uint32_t *p = (uint32_t *)SRAM_ADDRESS1;
080001f2: 04 4b ldr r3, [pc, #16] ; (0x8000204 <main+28>)
080001f4: 3b 60 str r3, [r7, #0]
15 value = *p;
080001f6: 3b 68 ldr r3, [r7, #0]
080001f8: 1b 68 ldr r3, [r3, #0]
080001fa: 7b 60 str r3, [r7, #4]
16 if(value) break;
080001fc: 7b 68 ldr r3, [r7, #4]
080001fe: 00 2b cmp r3, #0
08000200: f9 d0 beq.n 0x80001f6 <main+14>
20 while(1);================================================================================================ LEVEL 02 Optimization without volatile keyword
080001e6: 00 08 lsrs r0, r0, #32
15 value = *p;
main:
080001e8: 4f f0 00 53 mov.w r3, #536870912 ; 0x20000000
080001ec: 5b 68 ldr r3, [r3, #4]
16 if(value) break;
; compiler optimizes away reads
080001ee: 03 b9 cbnz r3, 0x80001f2 <main+10>
080001f0: fe e7 b.n 0x80001f0 <main+8>
20 while(1);================================================================================ Level 02 Optimization with volatile keyword Note the generated assembly code now branches on comparison With volatile keyword
080001e6: 00 08 lsrs r0, r0, #32
16 value = *p;
main:
080001e8: 4f f0 00 52 mov.w r2, #536870912 ; 0x20000000
080001ec: 53 68 ldr r3, [r2, #4]
17 if(value) break;
; compiler jumps back to read every loop
080001ee: 00 2b cmp r3, #0
080001f0: fc d0 beq.n 0x80001ec <main+4>
21 while(1);
080001f2: fe e7 b.n 0x80001f2 <main+10>