C Inline Assembly

Clang supports most of the standard GCC inline assembly syntax. There are a few notable exceptions:

Unsupported Constraints: o, V, <, >, and g
Goto Labels are unsupported.

"General purpose registers" for the purposes of inline assembly are taken to be 8-bit (__rcxx) or 16-bit (__rsxx) imaginary registers, depending on the size of the operand.

The compiler also supports a few MOS-specific constraints:

a: The A register.
x: The X register.
y: The Y register.
R: One of A, X, or Y.
d: Either X or Y.
c: The C flag.
v: The V flag.

These modifiers are case-sensitive, and violating constraint use in inline assembly can crash the compiler, so use with a high degree of caution.

The unique opcode syntax of the 6502 allows using these operands in unorthodox ways. For example, to issue a copy from a d constrained operand to A, you can use the following which will issue either txa or tya, depending on where the compiler places c:

unsigned char c = 1;
__attribute__((leaf)) __asm__ volatile ("t%0a" : /* no output */ : /* input */ "d"(c) : /* clobbers */ "a");

Using GCC-style inline assembly has a lot of sharp edges and footguns, so beware. For a detailed guide on how to use this feature well, see: Frequently asked questions#Why is inline assembly behaving so strangely?

One more thing to note: the compiler tracks the contents of C and V (but not N and Z). Accordingly, if the inline assembly fragment can clobber C or V, they must be added to the clobber or output register list with e.g. "c", "v", or "p".

If you're writing a lot of assembly, note that mos-clang works well as a frontend to the MOS assembler. You can put your labels and assembly code in its own .s file, and compile it with something like:

mos-clang -c my-assembly-file.s