zig

Assembly Syntax Explained.zig (3286B) - Raw

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 pub fn syscall1(number: usize, arg1: usize) usize {
     // Inline assembly is an expression which returns a value.
     // the `asm` keyword begins the expression.
     return asm
     // `volatile` is an optional modifier that tells Zig this
     // inline assembly expression has side-effects. Without
     // `volatile`, Zig is allowed to delete the inline assembly
     // code if the result is unused.
     volatile (
     // Next is a comptime string which is the assembly code.
     // Inside this string one may use `%[ret]`, `%[number]`,
     // or `%[arg1]` where a register is expected, to specify
     // the register that Zig uses for the argument or return value,
     // if the register constraint strings are used. However in
     // the below code, this is not used. A literal `%` can be
     // obtained by escaping it with a double percent: `%%`.
     // Often multiline string syntax comes in handy here.
         \\syscall
         // Next is the output. It is possible in the future Zig will
         // support multiple outputs, depending on how
         // https://github.com/ziglang/zig/issues/215 is resolved.
         // It is allowed for there to be no outputs, in which case
         // this colon would be directly followed by the colon for the inputs.
         :
         // This specifies the name to be used in `%[ret]` syntax in
         // the above assembly string. This example does not use it,
         // but the syntax is mandatory.
           [ret]
           // Next is the output constraint string. This feature is still
           // considered unstable in Zig, and so LLVM/GCC documentation
           // must be used to understand the semantics.
           // http://releases.llvm.org/10.0.0/docs/LangRef.html#inline-asm-constraint-string
           // https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html
           // In this example, the constraint string means "the result value of
           // this inline assembly instruction is whatever is in $rax".
           "={rax}"
           // Next is either a value binding, or `->` and then a type. The
           // type is the result type of the inline assembly expression.
           // If it is a value binding, then `%[ret]` syntax would be used
           // to refer to the register bound to the value.
           (-> usize),
           // Next is the list of inputs.
           // The constraint for these inputs means, "when the assembly code is
           // executed, $rax shall have the value of `number` and $rdi shall have
           // the value of `arg1`". Any number of input parameters is allowed,
           // including none.
         : [number] "{rax}" (number),
           [arg1] "{rdi}" (arg1),
           // Next is the list of clobbers. These declare a set of registers whose
           // values will not be preserved by the execution of this assembly code.
           // These do not include output or input registers. The special clobber
           // value of "memory" means that the assembly writes to arbitrary undeclared
           // memory locations - not only the memory pointed to by a declared indirect
           // output. In this example we list $rcx and $r11 because it is known the
           // kernel syscall does not preserve these registers.
         : .{ .rcx = true, .r11 = true });
 }
 
 // syntax