I read article about GCC Inline Assembler (http://www.ethernut.de/en/documents/arm-inline-asm.html). In this article, "memory" Clobber forces the compiler to store all cached values before and reload them after executing the assembler instructions. And it must retain the sequence. this is the example. The following code intends to multiply c with b, of which one or both may be modified by an interrupt routine. Disabling interrupts before accessing the variables and re-enable them afterwards looks like a good idea. This may fail. Because the optimizer may decide to do the multiplication first and then execute both inline assembler instructions or vice versa. : asm volatile("mrs r12, cpsr\n\t" "orr r12, r12, #0xC0\n\t" "msr cpsr_c, r12\n\t" ::: "r12", "cc"); c *= b; /* This may fail. */ asm volatile("mrs r12, cpsr\n" "bic r12, r12, #0xC0\n" "msr cpsr_c, r12" ::: "r12", "cc"); This is safe by adding "memory" Clobber . asm volatile("mrs r12, cpsr\n\t" "orr r12, r12, #0xC0\n\t" "msr cpsr_c, r12\n\t" :: : "r12", "cc", "memory"); c *= b; /* This is safe. */ asm volatile("mrs r12, cpsr\n" "bic r12, r12, #0xC0\n" "msr cpsr_c, r12" ::: "r12", "cc", "memory"); But I disassemble code by objdump -d . "memory" Clobber don't works, the code is to do execute both inline assembler instructions, and then do the multiplication. mrs ip, CPSR orr ip, ip, #192 ; 0xc0 msr CPSR_c, ip mrs ip, CPSR bic ip, ip, #192 ; 0xc0 msr CPSR_c, ip mul r0, r1, r0 mov pc, lr Can anyone help me？ Remarks: @ubuntu:~$ arm-linux-gcc -v Using built-in specs. Target: arm-unknown-linux-gnu Configured with: /home/lihua/toolchain/cross/bin/targets/src/gcc-4.3.4/configure --build=i386-build_redhat-linux-gnu --host=i386-build_redhat-linux-gnu --target=arm-unknown-linux-gnu --prefix=/opt/compiler/glibc-oabi-toolchain-arm-generic --with-sysroot=/opt/compiler/glibc-oabi-toolchain-arm-generic/arm-unknown-linux-gnu//sys-root --enable-languages=c,c++,java,objc,obj-c++ --disable-multilib --with-float=soft --with-pkgversion='<lihua_338@xxxxxxx>' --enable-__cxa_atexit --with-gmp=/opt/compiler/glibc-oabi-toolchain-arm-generic --with-mpfr=/opt/compiler/glibc-oabi-toolchain-arm-generic --with-ppl=/opt/compiler/glibc-oabi-toolchain-arm-generic --with-cloog=/opt/compiler/glibc-oabi-toolchain-arm-generic --with-mpc=/opt/compiler/glibc-oabi-toolchain-arm-generic --with-local-prefix=/opt/compiler/glibc-oabi-toolchain-arm-generic/arm-unknown-linux-gnu//sys-root --disable-nls --enable-threads=posix --enable-symvers=gnu --enable-c99 --enable-long-long --enable-target-optspace Thread model: posix
