Asm [volatile ] ( AssemblerTemplate: OutputOperands [: InputOperands [: Clobbers ] ]) asm [volatile ] goto ( AssemblerTemplate:: InputOperands: Clobbers: GotoLabels) The asm keyword is a GNU extension. When writing code that can be compiled with -ansi and the various -std options, use __asm__ instead of asm (see ). Qualifiers volatile The typical use of extended asm statements is to manipulate input values to produce output values. However, your asm statements may also produce side effects. If so, you may need to use the volatile qualifier to disable certain optimizations. Goto This qualifier informs the compiler that the asm statement may perform a jump to one of the labels listed in the GotoLabels. Parameters AssemblerTemplate This is a literal string that is the template for the assembler code.

It is a combination of fixed text and tokens that refer to the input, output, and goto parameters. OutputOperands A comma-separated list of the C variables modified by the instructions in the AssemblerTemplate. An empty list is permitted.

InputOperands A comma-separated list of C expressions read by the instructions in the AssemblerTemplate. An empty list is permitted. Clobbers A comma-separated list of registers or other values changed by the AssemblerTemplate, beyond those listed as outputs. An empty list is permitted. GotoLabels When you are using the goto form of asm, this section contains the list of all C labels to which the code in the AssemblerTemplate may jump. Asm statements may not perform jumps into other asm statements, only to the listed GotoLabels.

GCC’s optimizers do not know about other jumps; therefore they cannot take account of them when deciding how to optimize. The total number of input + output + goto operands is limited to 30. Remarks The asm statement allows you to include assembly instructions directly within C code. This may help you to maximize performance in time-sensitive code or to access assembly instructions that are not readily available to C programs. Note that extended asm statements must be inside a function. Only basic asm may be outside functions (see ). Functions declared with the naked attribute also require basic asm (see ).

While the uses of asm are many and varied, it may help to think of an asm statement as a series of low-level instructions that convert input parameters to output parameters. So a simple (if not particularly useful) example for i386 using asm might look like this. Int src = 1; int dst; asm ('mov%1,%0 n t' 'add $1,%0': '=r' (dst): 'r' (src)); printf('%d n', dst); This code copies src to dst and add 1 to dst.

6.45.2.1 Volatile GCC’s optimizers sometimes discard asm statements if they determine there is no need for the output variables. Also, the optimizers may move code out of loops if they believe that the code will always return the same result (i.e. None of its input values change between calls).

Using the volatile qualifier disables these optimizations. Asm statements that have no output operands, including asm goto statements, are implicitly volatile. This i386 code demonstrates a case that does not use (or require) the volatile qualifier. If it is performing assertion checking, this code uses asm to perform the validation. Otherwise, dwRes is unreferenced by any code. As a result, the optimizers can discard the asm statement, which in turn removes the need for the entire DoCheck routine. By omitting the volatile qualifier when it isn’t needed you allow the optimizers to produce the most efficient code possible.

Uint64_t msr; asm volatile ( 'rdtsc n t' // Returns the time in EDX:EAX. 'shl $32,%%rdx n t' // Shift the upper bits left. 'or%%rdx,%0' // 'Or' in the lower bits.: '=a' (msr):: 'rdx'); printf('msr:%llx n', msr); // Do other work. // Reprint the timestamp asm volatile ( 'rdtsc n t' // Returns the time in EDX:EAX. 'shl $32,%%rdx n t' // Shift the upper bits left.

'or%%rdx,%0' // 'Or' in the lower bits.: '=a' (msr):: 'rdx'); printf('msr:%llx n', msr); GCC’s optimizers do not treat this code like the non-volatile code in the earlier examples. They do not move it out of loops or omit it on the assumption that the result from a previous call is still valid.

I would like to use printf to diplay text on a serial port of an ARM microcontroller. Using Printf to display on serial. Example links for AVR micro with GCC.

Note that the compiler can move even volatile asm instructions relative to other code, including across jump instructions. For example, on many targets there is a system register that controls the rounding mode of floating-point operations. Setting it with a volatile asm, as in the following PowerPC example, does not work reliably. [ [ asmSymbolicName] ] constraint ( cvariablename) asmSymbolicName Specifies a symbolic name for the operand.

Reference the name in the assembler template by enclosing it in square brackets (i.e. The scope of the name is the asm statement that contains the definition. Any valid C variable name is acceptable, including names already defined in the surrounding code.

No two operands within the same asm statement can use the same symbolic name. When not using an asmSymbolicName, use the (zero-based) position of the operand in the list of operands in the assembler template. For example if there are three output operands, use ‘%0’ in the template to refer to the first, ‘%1’ for the second, and ‘%2’ for the third.

Constraint A string constant specifying constraints on the placement of the operand; See, for details. Output constraints must begin with either ‘ =’ (a variable overwriting an existing value) or ‘ +’ (when reading and writing).

When using ‘ =’, do not assume the location contains the existing value on entry to the asm, except when the operand is tied to an input; see. After the prefix, there must be one or more additional constraints (see ) that describe where the value resides.

Common constraints include ‘ r’ for register and ‘ m’ for memory. When you list more than one possible location (for example, '=rm'), the compiler chooses the most efficient one based on the current context.

If you list as many alternates as the asm statement allows, you permit the optimizers to produce the best possible code. If you must use a specific register, but your Machine Constraints do not provide sufficient control to select the specific register you want, local register variables may provide a solution (see ).

Cvariablename Specifies a C lvalue expression to hold the output, typically a variable name. The enclosing parentheses are a required part of the syntax.

When the compiler selects the registers to use to represent the output operands, it does not use any of the clobbered registers (see ). Output operand expressions must be lvalues. The compiler cannot check whether the operands have data types that are reasonable for the instruction being executed. For output expressions that are not directly addressable (for example a bit-field), the constraint must allow a register.

In that case, GCC uses the register as the output of the asm, and then stores that register into the output. Operands using the ‘ +’ constraint modifier count as two operands (that is, both as input and output) towards the total maximum of 30 operands per asm statement. Use the ‘ &’ constraint modifier (see ) on all output operands that must not overlap an input.

Otherwise, GCC may allocate the output operand in the same register as an unrelated input operand, on the assumption that the assembler code consumes its inputs before producing outputs. This assumption may be false if the assembler code actually consists of more than one instruction. The same problem can occur if one output parameter ( a) allows a register constraint and another output parameter ( b) allows a memory constraint.

The code generated by GCC to access the memory address in b can contain registers which might be shared by a, and GCC considers those registers to be inputs to the asm. As above, GCC assumes that such input registers are consumed before any outputs are written. This assumption may result in incorrect behavior if the asm writes to a before using b. Combining the ‘ &’ modifier with the register constraint on a ensures that modifying a does not affect the address referenced by b. Otherwise, the location of b is undefined if a is modified before using b. Asm supports operand modifiers on operands (for example ‘%k2’ instead of simply ‘%2’).

Typically these qualifiers are hardware dependent. The list of supported modifiers for x86 is found. If the C code that follows the asm makes no use of any of the output operands, use volatile for the asm statement to prevent the optimizers from discarding the asm statement as unneeded (see ). This code makes no use of the optional asmSymbolicName. Therefore it references the first output operand as%0 (were there a second, it would be%1, etc). The number of the first input operand is one greater than that of the last output operand. In this i386 example, that makes Mask referenced as%1.

Uint32_t Mask = 1234; uint32_t Index; asm ('bsfl%1,%0': '=r' (Index): 'r' (Mask): 'cc'); That code overwrites the variable Index (‘ =’), placing the value in a register (‘ r’). Using the generic ‘ r’ constraint instead of a constraint for a specific register allows the compiler to pick the register to use, which can result in more efficient code.

This may not be possible if an assembler instruction requires a specific register. The following i386 example uses the asmSymbolicName syntax. It produces the same result as the code above, but some may consider it more readable or more maintainable since reordering index numbers is not necessary when adding or removing operands.

Fgtech V54 Keygen Software on this page. The names aIndex and aMask are only used in this example to emphasize which names get used where. It is acceptable to reuse the names Index and Mask.

Uint32_t c = 1; uint32_t d; uint32_t *e = &c; asm ('mov%[e],%[d]': [d] '=rm' (d): [e] 'rm' (*e)); Here, d may either be in a register or in memory. Bootstrap Installation Procedure Of The Bladder there. Since the compiler might already have the current value of the uint32_t location pointed to by e in a register, you can enable it to choose the best location for d by specifying both constraints. 6.45.2.4 Flag Output Operands Some targets have a special register that holds the “flags” for the result of an operation or comparison. Normally, the contents of that register are either unmodifed by the asm, or the asm is considered to clobber the contents. On some targets, a special form of output operand exists by which conditions in the flags register may be outputs of the asm. The set of conditions supported are target specific, but the general rule is that the output variable must be a scalar integer, and the value is boolean. When supported, the target defines the preprocessor symbol __GCC_ASM_FLAG_OUTPUTS__.

Because of the special nature of the flag output operands, the constraint may not include alternatives. Most often, the target has only one flags register, and thus is an implied operand of many instructions. In this case, the operand should not be referenced within the assembler template via%0 etc, as there’s no corresponding text in the assembly language. X86 family The flag output constraints for the x86 family are of the form ‘ =@cc cond’ where cond is one of the standard conditions defined in the ISA manual for j cc or set cc. A “above” or unsigned greater than ae “above or equal” or unsigned greater than or equal b “below” or unsigned less than be “below or equal” or unsigned less than or equal c carry flag set e z “equal” or zero flag set g signed greater than ge signed greater than or equal l signed less than le signed less than or equal o overflow flag set p parity flag set s sign flag set na nae nb nbe nc ne ng nge nl nle no np ns nz “not” flag, or inverted versions of those above 6.45.2.5 Input Operands Input operands make values from C variables and expressions available to the assembly code.

Operands are separated by commas. Each operand has this format. [ [ asmSymbolicName] ] constraint ( cexpression) asmSymbolicName Specifies a symbolic name for the operand. Reference the name in the assembler template by enclosing it in square brackets (i.e. The scope of the name is the asm statement that contains the definition. Any valid C variable name is acceptable, including names already defined in the surrounding code.

No two operands within the same asm statement can use the same symbolic name. When not using an asmSymbolicName, use the (zero-based) position of the operand in the list of operands in the assembler template. For example if there are two output operands and three inputs, use ‘%2’ in the template to refer to the first input operand, ‘%3’ for the second, and ‘%4’ for the third. Constraint A string constant specifying constraints on the placement of the operand; See, for details. Input constraint strings may not begin with either ‘ =’ or ‘ +’. When you list more than one possible location (for example, ‘ 'irm'’), the compiler chooses the most efficient one based on the current context.

If you must use a specific register, but your Machine Constraints do not provide sufficient control to select the specific register you want, local register variables may provide a solution (see ). Input constraints can also be digits (for example, '0'). This indicates that the specified input must be in the same place as the output constraint at the (zero-based) index in the output constraint list. When using asmSymbolicName syntax for the output operands, you may use these names (enclosed in brackets ‘ []’) instead of digits. Cexpression This is the C variable or expression being passed to the asm statement as input. The enclosing parentheses are a required part of the syntax.

When the compiler selects the registers to use to represent the input operands, it does not use any of the clobbered registers (see ). If there are no output operands but there are input operands, place two consecutive colons where the output operands would go. __asm__ ('some instructions': /* No outputs. */: 'r' (Offset / 8)); Warning: Do not modify the contents of input-only operands (except for inputs tied to outputs). The compiler assumes that on exit from the asm statement these operands contain the same values as they had before executing the statement.

It is not possible to use clobbers to inform the compiler that the values in these inputs are changing. One common work-around is to tie the changing input variable to an output variable that never gets used. Note, however, that if the code that follows the asm statement makes no use of any of the output operands, the GCC optimizers may discard the asm statement as unneeded (see ).

Asm supports operand modifiers on operands (for example ‘%k2’ instead of simply ‘%2’). Typically these qualifiers are hardware dependent. The list of supported modifiers for x86 is found. In this example using the fictitious combine instruction, the constraint '0' for input operand 1 says that it must occupy the same location as output operand 0. Only input operands may use numbers in constraints, and they must each refer to an output operand. Only a number (or the symbolic assembler name) in the constraint can guarantee that one operand is in the same place as another. The mere fact that foo is the value of both operands is not enough to guarantee that they are in the same place in the generated assembler code.

Asm ('cmoveq%1,%2,%[result]': [result] '=r'(result): 'r' (test), 'r' (new), '[result]' (old)); 6.45.2.6 Clobbers and Scratch Registers While the compiler is aware of changes to entries listed in the output operands, the inline asm code may modify more than just the outputs. For example, calculations may require additional registers, or the processor may overwrite a register as a side effect of a particular assembler instruction. In order to inform the compiler of these changes, list them in the clobber list. Clobber list items are either register names or the special clobbers (listed below). Each clobber list item is a string constant enclosed in double quotes and separated by commas. Clobber descriptions may not in any way overlap with an input or output operand.

For example, you may not have an operand describing a register class with one member when listing that register in the clobber list. Variables declared to live in specific registers (see ) and used as asm input or output operands must have no part mentioned in the clobber description. In particular, there is no way to specify that input operands get modified without also specifying them as output operands. When the compiler selects which registers to use to represent input and output operands, it does not use any of the clobbered registers. As a result, clobbered registers are available for any use in the assembler code.

Here is a realistic example for the VAX showing the use of clobbered registers. Asm volatile ('movc3%0,%1,%2': /* No outputs. */: 'g' (from), 'g' (to), 'g' (count): 'r0', 'r1', 'r2', 'r3', 'r4', 'r5', 'memory'); Also, there are two special clobber arguments: 'cc' The 'cc' clobber indicates that the assembler code modifies the flags register.

On some machines, GCC represents the condition codes as a specific hardware register; 'cc' serves to name this register. On other machines, condition code handling is different, and specifying 'cc' has no effect. But it is valid no matter what the target. 'memory' The 'memory' clobber tells the compiler that the assembly code performs memory reads or writes to items other than those listed in the input and output operands (for example, accessing the memory pointed to by one of the input parameters). To ensure memory contains correct values, GCC may need to flush specific register values to memory before executing the asm. Further, the compiler does not assume that any values read from memory before an asm remain unchanged after that asm; it reloads them as needed. Using the 'memory' clobber effectively forms a read/write memory barrier for the compiler.

Note that this clobber does not prevent the processor from doing speculative reads past the asm statement. To prevent that, you need processor-specific fence instructions. Flushing registers to memory has performance implications and may be an issue for time-sensitive code.

You can provide better information to GCC to avoid this, as shown in the following examples. At a minimum, aliasing rules allow GCC to know what memory doesn’t need to be flushed. Here is a fictitious sum of squares instruction, that takes two pointers to floating point values in memory and produces a floating point register output.

Notice that x, and y both appear twice in the asm parameters, once to specify memory accessed, and once to specify a base register used by the asm. You won’t normally be wasting a register by doing this as GCC can use the same register for both purposes. However, it would be foolish to use both%1 and%3 for x in this asm and expect them to be the same. In fact,%3 may well not be a register. It might be a symbolic memory reference to the object pointed to by x.

Asm ('foo': '=t' (a): 'f' (b)); This code says that input b is not popped by the asm, and that the asm pushes a result onto the reg-stack, i.e., the stack is one deeper after the asm than it was before. But, it is possible that reload may think that it can use the same register for both the input and the output. To prevent this from happening, if any input operand uses the ‘ f’ constraint, all output register constraints must use the ‘ &’ early-clobber modifier.

The example above is correctly written as. Asm ('foo': '=&t' (a): 'f' (b)); • Some operands need to be in particular places on the stack. All output operands fall in this category—GCC has no other way to know which registers the outputs appear in unless you indicate this in the constraints. Output operands must specifically indicate which register an output appears in after an asm. ‘ =f’ is not allowed: the operand constraints must select a class with a single register. • Output operands may not be “inserted” between existing stack registers. Since no 387 opcode uses a read/write operand, all output operands are dead before the asm, and are pushed by the asm.

It makes no sense to push anywhere but the top of the reg-stack. Output operands must start at the top of the reg-stack: output operands may not “skip” a register.

• Some asm statements may need extra stack space for internal calculations. This can be guaranteed by clobbering stack registers unrelated to the inputs and outputs. This asm takes one input, which is internally popped, and produces two outputs.