Data directives and layout expressions

Data directives emit values directly, without an instruction opcode:

All four accept comma-separated expressions terminated by a semicolon. Multi-byte values use little-endian byte order, with no automatic alignment or padding.

db 60+5,66,0;       // 41 42 00
byte 0xAA,0x55;    // AA 55
dw 4660;           // 34 12
word 0x55AA;       // AA 55
dd 0x12345678;     // 78 56 34 12
dword 40+2;        // 2A 00 00 00
dq 42;            // 2A 00 00 00 00 00 00 00
qword 0;          // 00 00 00 00 00 00 00 00

Each word list element occupies two bytes: word 0xAA,0x55; emits AA 00 55 00. Empty lists, trailing commas, missing commas, negative results, and results above the directive’s range are rejected. The existing signed 32-bit expression rules still apply to intermediate calculations. DD and DQ store four and eight bytes respectively, but full unsigned 32-bit and 64-bit expression values are not yet supported. Even though their current range diagnostics name wider storage limits, the expression evaluator rejects results above 2147483647 first.

Prefix a data directive with times <count> or fill <count> to repeat its complete comma-separated list. The count is an expression, must be nonnegative, and must fit size_t.

times 3 db 170;          // AA AA AA
fill 2 dw 4660,0;        // 34 12 00 00 34 12 00 00
times 2 dd 42;           // 2A 00 00 00 2A 00 00 00

Labels include the size of preceding data. Executable examples must jump over embedded data, as in examples/data_all_jump.asm, which mixes all four widths. Data-only examples are encoding fixtures, not functions to execute.

The CLI writes source-named .bin and .h files for images containing data, but prints Data emitted; instruction-only decoding skipped. instead of trying to disassemble the image. Instruction-only images retain their decoded listing. TIMES/FILL repetition is supported for data directives. Strings, alignment, and symbol-valued data remain future work.

Generated images are capped at 16 MiB (16,777,216 bytes) by KASM_IMAGE_LIMIT. This output limit includes instructions and repeated data; it is separate from dynamically allocated source-file storage.

Location-aware padding

The assembler also supports $ and $$ inside repeat-count expressions:

mov eax, 42;
times 510-($-$$) db 0;
dw 0xAA55;

$ is the current image offset and $$ is the image start, which is offset zero for the current single-image layout. The count is resolved during layout, after earlier statements have known sizes. In this example, mov eax, 42; is five bytes, so the padding count is 510 - (5 - 0) = 505, followed by the little-endian signature bytes 55 AA.

This produces a 512-byte data image. It is a tested layout fixture, not yet a bootable sector: a signature alone does not define an entry convention, segment state, stack, BIOS services, or an emulator workflow. Negative padding is rejected before the repeat count is converted to size_t.

Permanent tests cover all eight spellings, expression values, range boundaries, exact bytes, mixed-width layout, jump targets, constant and location-aware TIMES/FILL repetition, and invalid lists and ranges. The Windows Debug build validates the data-directive suite, including encode.times_fill, encode.boot_pad, and invalid-input cases within semantic.data_invalid. After configuring and building, run them with:

ctest --test-dir build/windows-debug -C Debug -R "(encode.data_|encode.times_fill|encode.boot_pad|semantic.data_invalid)" --output-on-failure

Symbol-valued data (label expressions inside directives) resolves after layout as of 0.33.0; constant-only expressions retain the checked arithmetic described in Control instructions and overflow safety.