查看ARM inline Assembler的源代码 ARM inline Assembler 0

asm(code : output operand list : input operand list : clobber list);

example: Rotating bits example
int x;
int y;
y= (x>>1);

<source lang="c">
asm("mov %[result], %[value], ror #1" : [result] "=r" (y) : [value] "r" (x));
</source>

register value (r1) = x
register result (r2) = y
result = (value >> 1)

example: sets the current program status register of the ARM CPU.
<source lang="c">
asm("msr cpsr,%[ps]" : : [ps]"r"(status));
</source>
int status;
ps (r2) = status
cpsr = ps

For ARM processors, GCC 4 provides the following constraints. 
{| class="wikitable" style="width:75%"
! Constraint      !! Usage in ARM state           !! Usage in Thumb state 
|- 
|| f     || Floating point registers f0 .. f7     || Not available   
|-  
|| h|| Not available|| Registers r8..r15
|-
||G ||Immediate floating point constant ||Not available
|-
||H ||Same a G, but negated ||Not available
|-
||I ||Immediate value in data processing instructions e.g. ORR R0, R0, #operand ||Constant in the range 0 .. 255 e.g. SWI operand 
|-
||J ||Indexing constants -4095 .. 4095 e.g. LDR R1, [PC, #operand] ||Constant in the range -255 .. -1 e.g. SUB R0, R0, #operand 
|-
||K ||Same as I, but inverted ||Same as I, but shifted 
|-
||L ||Same as I, but negated ||Constant in the range -7 .. 7 e.g. SUB R0, R1, #operand 
|-
||l ||Same as r ||Registers r0..r7 e.g. PUSH operand  
|-
||M ||Constant in the range of 0 .. 32 or a power of 2 e.g. MOV R2, R1, ROR #operand  ||Constant that is a multiple of 4 in the range of 0 .. 1020 e.g. ADD R0, SP, #operand  
|-
||m ||Any valid memory address 
|-
||N ||Not available ||Constant in the range of 0 .. 31 e.g. LSL R0, R1, #operand 
|-
||O ||Not available ||Constant that is a multiple of 4 in the range of -508 .. 508 e.g. ADD SP, #operand 
|-
||r ||General register r0 .. r15 e.g. SUB operand1, operand2, operand3 ||Not available
|-
||w ||Vector floating point registers s0 .. s31 ||Not available
|-
||X ||Any operand ||
|}


Constraint characters may be prepended by a single constraint modifier. Constraints without a modifier specify read-only operands. Modifiers are: 
{| class="wikitable" style="width:75%"
! Modifier      !! Specifies
|- 
||=  ||Write-only operand, usually used for all output operands 
|-
||+ ||Read-write operand, must be listed as an output operand 
|-
||& ||A register that should be used for output only 
|}

Examples:
* y=(y<<1);
<source lang="c">
asm("mov %[value], %[value], ror #1" : [value] "+r" (y));
</source>
use '+' for read-write able. Or,
<source lang="c">
asm("mov %0, %0, ror #1" : "=r" (value) : "0" (value));
</source>
Constraint "0" input reg = output reg

* rdv = table[0]; table[4]=wdr;
<source lang="c">
asm volatile("ldr %0, [%1]"     "\n\t"
             "str %2, [%1, #4]" "\n\t"
             : "=&r" (rdv) /* &  rdv 只能写  不可共用  
             : "r" (&table), "r" (wdv)
             : "memory");
</source>

Referernce: http://www.ethernut.de/en/documents/arm-inline-asm.html