x86-64 directives

Opens a section: .text for code, .data for initialised data, .bss for space that starts as zeroes, .rodata for constants. Everything after the line belongs to that section until the next one. segment is the same directive under its other name.

Makes a label visible outside the file. The program has to declare _start, because that is the label the linker makes the entry point.

Declares that a name is defined in another file, so this one may refer to it.

Assembles for 16, 32 or 64 bit mode. Programs here are 64 bit and the object format already says so, so a program needs this only to say something unusual.

Chooses whether a bare memory reference is rel (an offset from rip) or abs (an absolute address). Long mode code is usually written default rel.

Refuses to assemble instructions newer than the processor named, which is a way to keep a program inside the instruction set a course is teaching.

Starts a block of labels that describe a layout without emitting anything, the way a structure declaration does.

Emit bytes, words (2 bytes), doublewords (4) and quadwords (8), in order, where the line is. They take numbers, characters, strings and label addresses: msg: db "hi", 10.

Emit 10, 16, 32 and 64 byte values, for the x87 and vector types.

Reserve room without writing anything: buffer: resb 64. They belong in .bss, which costs nothing in the file because it is all zeroes.

Gives a name to a value, evaluated once where it is written: LEN equ 64. Nothing is emitted and nothing can change it later.

Copies a file into the output at this point, byte for byte.

Repeats the rest of the line: times 64 db 0 emits 64 zero bytes, and times 8 - ($ - start) db 0 pads to a fixed size.

A macro expanded wherever its name appears, with arguments if it was given any. Expanded late, so it sees the value a name has when it is used.

A single line macro whose value is evaluated at once, and may be reassigned.

A multi line macro, ended by %endmacro. The number after the name says how many arguments it takes, and %1, %2 stand for them.

Assembles another file here, the way a header is included.

Assemble a block only under a condition, ending at %endif, with %else and %elif in between.

Repeats a block a fixed number of times, ending at %endrep.

Makes a read-modify-write instruction atomic against other processors. Only a handful of instructions accept it, and only when the destination is memory.

Repeats a string instruction rcx times, counting down. On movs and stos this is the whole loop, written in one instruction.

Repeat while the comparison keeps saying equal, or keeps saying not equal, and stop early when it changes. They are for cmps and scas, which set the flags on every step.

Force the operand or address size of one instruction, overriding what the mode implies. NASM chooses these on its own; writing one by hand is for the rare case where the choice matters.

Say how wide a memory operand is, which the instruction cannot always work out on its own: mov qword [rsp], 0 writes eight bytes, mov byte [rsp], 0 writes one.

Choose the reach of a jump or call. NASM picks the shortest form that works, so these are for forcing a longer one.

Say whether a memory reference is an offset from rip or an absolute address, for one operand rather than for the file the way default does.