\nstr sp, [r0], #4
\nstr lr, [r0], #4<\/td>\n
\nldr sp, [r0], #4
\nldr lr, [r0], #4<\/td>\n
\nmcr p15, 0, r1, c7, c14, 0<\/td>\n
\nmsr cpsr_c, r1<\/td>\n
<\/br>Whenever you are trying to use co processor 15 you may get the warning as to use ISB, DSB or DMB instruction. These instructions are called memory barrier instructions.
\n<\/br>In some circumstances, processor optimizations such as speculative reads or out-of-order execution (as in the example above), are undesirable and can lead to unintended program behaviour. In such situations it is necessary to insert barrier instructions into code where there is a requirement for stricter, ‘Classic ARM processor-like’ behaviour. There are three types of barrier instructions. For simplicity, note that the descriptions below are for a uni-processor environment:<\/p>\n
- \n
- A Data Synchronization Barrier (DSB)<\/strong> completes when all instructions before this instruction complete.<\/li>\n
- A Data Memory Barrier (DMB)<\/strong> ensures that all explicit memory accesses before the DMB instruction complete before any explicit memory accesses after the DMB instruction start.<\/li>\n
- An Instruction Synchronization Barrier (ISB)<\/strong> flushes the pipeline in the processor, so that all instructions following the ISB are fetched from cache or memory, after the ISB has been completed.<\/li>\n<\/ul>\n
Note that the CP15 equivalent barrier instructions available in ARMv6 are deprecated in ARMv7. Therefore, if possible, it is recommended that any code that uses these instructions is migrated to use the new barrier instructions described above instead.<\/p>\n
- A Data Memory Barrier (DMB)<\/strong> ensures that all explicit memory accesses before the DMB instruction complete before any explicit memory accesses after the DMB instruction start.<\/li>\n
![\"WEC](\"https:\/\/www.e-consystems.com\/blog\/system-on-module-som\/wp-content\/uploads\/2014\/05\/WEC2013-THUMB2-ARM-UAL-syntax.jpg\" "\\"WEC2013-THUMB2-ARM-UAL-syntax\\"")
<\/a>\n