If you need to write low-level software that interacts directly
with the hardware, Ada provides two ways to incorporate assembly
language code into your program. First, you can import and invoke
external routines written in assembly language, an Ada feature fully
supported by GNAT. However, for small sections of code it may be simpler
or more efficient to include assembly language statements directly
in your Ada source program, using the facilities of the implementation-defined
System.Machine_Code, which incorporates the gcc
Inline Assembler. The Inline Assembler approach offers a number of advantages,
including the following:
No need to use non-Ada tools
Consistent interface over different targets
Automatic usage of the proper calling conventions
Access to Ada constants and variables
Definition of intrinsic routines
Possibility of inlining a subprogram comprising assembler code
Code optimizer can take Inline Assembler code into account
This appendix presents a series of examples to show you how to use the Inline Assembler. Although it focuses on the Intel x86, the general approach applies also to other processors. It is assumed that you are familiar with Ada and with assembly language programming.
Basic Assembler Syntax¶
The assembler used by GNAT and gcc is based not on the Intel assembly
language, but rather on a language that descends from the AT&T Unix
as (and which is often referred to as ‘AT&T syntax’).
The following table summarizes the main features of
and points out the differences from the Intel conventions.
See the gcc
pre-processor) documentation for further information.
as: Prefix with ‘%’; for example
as: Prefix with ‘$’; for example
as: Prefix with ‘$’; for example
as: No extra punctuation; for example
as: Parentheses; for example
as: Leading ‘0x’ (C language syntax); for example
as: Explicit in op code; for example
movwto move a 16-bit word
as: Split into two lines; for example
as: Source first; for example
movw $4, %eax
mov eax, 4
A Simple Example of Inline Assembler¶
The following example will generate a single assembly language statement,
nop, which does nothing. Despite its lack of run-time effect,
the example will be useful in illustrating the basics of
the Inline Assembler facility.
with System.Machine_Code; use System.Machine_Code; procedure Nothing is begin Asm ("nop"); end Nothing;
Asm is a procedure declared in package
here it takes one parameter, a template string that must be a static
expression and that will form the generated instruction.
Asm may be regarded as a compile-time procedure that parses
the template string and additional parameters (none here),
from which it generates a sequence of assembly language instructions.
The examples in this chapter will illustrate several of the forms
Asm; a complete specification of the syntax
is found in the
Machine_Code_Insertions section of the
GNAT Reference Manual.
Under the standard GNAT conventions, the
should be in a file named
You can build the executable in the usual way:
$ gnatmake nothing
However, the interesting aspect of this example is not its run-time behavior but rather the generated assembly code. To see this output, invoke the compiler as follows:
$ gcc -c -S -fomit-frame-pointer -gnatp nothing.adb
where the options are:
compile only (no bind or link)
generate assembler listing
do not set up separate stack frames
do not add runtime checks
This gives a human-readable assembler version of the code. The resulting
file will have the same name as the Ada source file, but with a
extension. In our example, the file
nothing.s has the following
.file "nothing.adb" gcc2_compiled.: ___gnu_compiled_ada: .text .align 4 .globl __ada_nothing __ada_nothing: #APP nop #NO_APP jmp L1 .align 2,0x90 L1: ret
The assembly code you included is clearly indicated by
the compiler, between the
delimiters. The character before the ‘APP’ and ‘NOAPP’
can differ on different targets. For example, GNU/Linux uses ‘#APP’ while
on NT you will see ‘/APP’.
If you make a mistake in your assembler code (such as using the
wrong size modifier, or using a wrong operand for the instruction) GNAT
will report this error in a temporary file, which will be deleted when
the compilation is finished. Generating an assembler file will help
in such cases, since you can assemble this file separately using the
as assembler that comes with gcc.
Assembling the file using the command
$ as nothing.s
will give you error messages whose lines correspond to the assembler
input file, so you can easily find and correct any mistakes you made.
If there are no errors,
as will generate an object file
Output Variables in Inline Assembler¶
The examples in this section, showing how to access the processor flags, illustrate how to specify the destination operands for assembly language statements.
with Interfaces; use Interfaces; with Ada.Text_IO; use Ada.Text_IO; with System.Machine_Code; use System.Machine_Code; procedure Get_Flags is Flags : Unsigned_32; use ASCII; begin Asm ("pushfl" & LF & HT & -- push flags on stack "popl %%eax" & LF & HT & -- load eax with flags "movl %%eax, %0", -- store flags in variable Outputs => Unsigned_32'Asm_Output ("=g", Flags)); Put_Line ("Flags register:" & Flags'Img); end Get_Flags;
In order to have a nicely aligned assembly listing, we have separated multiple assembler statements in the Asm template string with linefeed (ASCII.LF) and horizontal tab (ASCII.HT) characters. The resulting section of the assembly output file is:
#APP pushfl popl %eax movl %eax, -40(%ebp) #NO_APP
It would have been legal to write the Asm invocation as:
Asm ("pushfl popl %%eax movl %%eax, %0")
but in the generated assembler file, this would come out as:
#APP pushfl popl %eax movl %eax, -40(%ebp) #NO_APP
which is not so convenient for the human reader.
We use Ada comments at the end of each line to explain what the assembler instructions actually do. This is a useful convention.
When writing Inline Assembler instructions, you need to precede each register
and variable name with a percent sign. Since the assembler already requires
a percent sign at the beginning of a register name, you need two consecutive
percent signs for such names in the Asm template string, thus
In the generated assembly code, one of the percent signs will be stripped off.
Names such as
%2, etc., denote input or output
variables: operands you later define using
An output variable is illustrated in
the third statement in the Asm template string:
movl %%eax, %0
The intent is to store the contents of the eax register in a variable that can
be accessed in Ada. Simply writing
movl %%eax, Flags would not
necessarily work, since the compiler might optimize by using a register
to hold Flags, and the expansion of the
movl instruction would not be
aware of this optimization. The solution is not to store the result directly
but rather to advise the compiler to choose the correct operand form;
that is the purpose of the
%0 output variable.
Information about the output variable is supplied in the
Outputs => Unsigned_32'Asm_Output ("=g", Flags));
The output is defined by the
Asm_Output attribute of the target type;
the general format is
Type'Asm_Output (constraint_string, variable_name)
The constraint string directs the compiler how to store/access the associated variable. In the example
Unsigned_32'Asm_Output ("=m", Flags);
"m" (memory) constraint tells the compiler that the variable
Flags should be stored in a memory variable, thus preventing
the optimizer from keeping it in a register. In contrast,
Unsigned_32'Asm_Output ("=r", Flags);
"r" (register) constraint, telling the compiler to
store the variable in a register.
If the constraint is preceded by the equal character ‘=’, it tells the compiler that the variable will be used to store data into it.
Get_Flags example, we used the
"g" (global) constraint,
allowing the optimizer to choose whatever it deems best.
There are a fairly large number of constraints, but the ones that are most useful (for the Intel x86 processor) are the following:
global (i.e., can be stored anywhere)
use one of eax, ebx, ecx or edx
use one of eax, ebx, ecx, edx, esi or edi
The full set of constraints is described in the gcc and
documentation; note that it is possible to combine certain constraints
in one constraint string.
You specify the association of an output variable with an assembler operand
%n notation, where n is a non-negative
integer. Thus in
Asm ("pushfl" & LF & HT & -- push flags on stack "popl %%eax" & LF & HT & -- load eax with flags "movl %%eax, %0", -- store flags in variable Outputs => Unsigned_32'Asm_Output ("=g", Flags));
%0 will be replaced in the expanded code by the appropriate operand,
the compiler decided for the
In general, you may have any number of output variables:
Count the operands starting at 0; thus
Outputsparameter as a parenthesized comma-separated list of
Asm ("movl %%eax, %0" & LF & HT & "movl %%ebx, %1" & LF & HT & "movl %%ecx, %2", Outputs => (Unsigned_32'Asm_Output ("=g", Var_A), -- %0 = Var_A Unsigned_32'Asm_Output ("=g", Var_B), -- %1 = Var_B Unsigned_32'Asm_Output ("=g", Var_C))); -- %2 = Var_C
Var_C are variables
in the Ada program.
As a variation on the
Get_Flags example, we can use the constraints
string to direct the compiler to store the eax register into the
variable, instead of including the store instruction explicitly in the
Asm template string:
with Interfaces; use Interfaces; with Ada.Text_IO; use Ada.Text_IO; with System.Machine_Code; use System.Machine_Code; procedure Get_Flags_2 is Flags : Unsigned_32; use ASCII; begin Asm ("pushfl" & LF & HT & -- push flags on stack "popl %%eax", -- save flags in eax Outputs => Unsigned_32'Asm_Output ("=a", Flags)); Put_Line ("Flags register:" & Flags'Img); end Get_Flags_2;
"a" constraint tells the compiler that the
variable will come from the eax register. Here is the resulting code:
#APP pushfl popl %eax #NO_APP movl %eax,-40(%ebp)
The compiler generated the store of eax into Flags after expanding the assembler code.
Actually, there was no need to pop the flags into the eax register; more simply, we could just pop the flags directly into the program variable:
with Interfaces; use Interfaces; with Ada.Text_IO; use Ada.Text_IO; with System.Machine_Code; use System.Machine_Code; procedure Get_Flags_3 is Flags : Unsigned_32; use ASCII; begin Asm ("pushfl" & LF & HT & -- push flags on stack "pop %0", -- save flags in Flags Outputs => Unsigned_32'Asm_Output ("=g", Flags)); Put_Line ("Flags register:" & Flags'Img); end Get_Flags_3;
Input Variables in Inline Assembler¶
The example in this section illustrates how to specify the source operands for assembly language statements. The program simply increments its input value by 1:
with Interfaces; use Interfaces; with Ada.Text_IO; use Ada.Text_IO; with System.Machine_Code; use System.Machine_Code; procedure Increment is function Incr (Value : Unsigned_32) return Unsigned_32 is Result : Unsigned_32; begin Asm ("incl %0", Outputs => Unsigned_32'Asm_Output ("=a", Result), Inputs => Unsigned_32'Asm_Input ("a", Value)); return Result; end Incr; Value : Unsigned_32; begin Value := 5; Put_Line ("Value before is" & Value'Img); Value := Incr (Value); Put_Line ("Value after is" & Value'Img); end Increment;
Outputs parameter to
that the result will be in the eax register and that it is to be stored
Inputs parameter looks much like the
but with an
"=" constraint, indicating an output value, is not present.
You can have multiple input variables, in the same way that you can have more than one output variable.
The parameter count (%0, %1) etc, still starts at the first output statement, and continues with the input statements.
Just as the
Outputs parameter causes the register to be stored into the
target variable after execution of the assembler statements, so does the
Inputs parameter cause its variable to be loaded into the register
before execution of the assembler statements.
Thus the effect of the
Asm invocation is:
load the 32-bit value of
store the contents of eax into the
The resulting assembler file (with
-O2 optimization) contains:
_increment__incr.1: subl $4,%esp movl 8(%esp),%eax #APP incl %eax #NO_APP movl %eax,%edx movl %ecx,(%esp) addl $4,%esp ret
Inlining Inline Assembler Code¶
For a short subprogram such as the
Incr function in the previous
section, the overhead of the call and return (creating / deleting the stack
frame) can be significant, compared to the amount of code in the subprogram
body. A solution is to apply Ada’s
Inline pragma to the subprogram,
which directs the compiler to expand invocations of the subprogram at the
point(s) of call, instead of setting up a stack frame for out-of-line calls.
Here is the resulting program:
with Interfaces; use Interfaces; with Ada.Text_IO; use Ada.Text_IO; with System.Machine_Code; use System.Machine_Code; procedure Increment_2 is function Incr (Value : Unsigned_32) return Unsigned_32 is Result : Unsigned_32; begin Asm ("incl %0", Outputs => Unsigned_32'Asm_Output ("=a", Result), Inputs => Unsigned_32'Asm_Input ("a", Value)); return Result; end Incr; pragma Inline (Increment); Value : Unsigned_32; begin Value := 5; Put_Line ("Value before is" & Value'Img); Value := Increment (Value); Put_Line ("Value after is" & Value'Img); end Increment_2;
Compile the program with both optimization (
-O2) and inlining
Incr function is still compiled as usual, but at the
Increment where our function used to be called:
pushl %edi call _increment__incr.1
the code for the function body directly appears:
movl %esi,%eax #APP incl %eax #NO_APP movl %eax,%edx
thus saving the overhead of stack frame setup and an out-of-line call.
This section describes two important parameters to the
Clobber, which identifies register usage;
Volatile, which inhibits unwanted optimizations.
One of the dangers of intermixing assembly language and a compiled language
such as Ada is that the compiler needs to be aware of which registers are
being used by the assembly code. In some cases, such as the earlier examples,
the constraint string is sufficient to indicate register usage (e.g.,
the eax register). But more generally, the compiler needs an explicit
identification of the registers that are used by the Inline Assembly
Using a register that the compiler doesn’t know about
could be a side effect of an instruction (like
storing its result in both eax and edx).
It can also arise from explicit register usage in your
assembly code; for example:
Asm ("movl %0, %%ebx" & LF & HT & "movl %%ebx, %1", Outputs => Unsigned_32'Asm_Output ("=g", Var_Out), Inputs => Unsigned_32'Asm_Input ("g", Var_In));
where the compiler (since it does not analyze the
Asm template string)
does not know you are using the ebx register.
In such cases you need to supply the
Clobber parameter to
to identify the registers that will be used by your assembly code:
Asm ("movl %0, %%ebx" & LF & HT & "movl %%ebx, %1", Outputs => Unsigned_32'Asm_Output ("=g", Var_Out), Inputs => Unsigned_32'Asm_Input ("g", Var_In), Clobber => "ebx");
The Clobber parameter is a static string expression specifying the
register(s) you are using. Note that register names are not prefixed
by a percent sign. Also, if more than one register is used then their names
are separated by commas; e.g.,
Clobber parameter has several additional uses:
Use ‘register’ name
ccto indicate that flags might have changed
Use ‘register’ name
memoryif you changed a memory location
Compiler optimizations in the presence of Inline Assembler may sometimes have
unwanted effects. For example, when an
Asm invocation with an input
variable is inside a loop, the compiler might move the loading of the input
variable outside the loop, regarding it as a one-time initialization.
If this effect is not desired, you can disable such optimizations by setting
Volatile parameter to
True; for example:
Asm ("movl %0, %%ebx" & LF & HT & "movl %%ebx, %1", Outputs => Unsigned_32'Asm_Output ("=g", Var_Out), Inputs => Unsigned_32'Asm_Input ("g", Var_In), Clobber => "ebx", Volatile => True);
Volatile is set to
False unless there is no
True prevents unwanted
optimizations, it will also disable other optimizations that might be
important for efficiency. In general, you should set
True only if the compiler’s optimizations have created