Structures in Assembly

A structure in assembly is a fixed layout of named byte offsets within a contiguous block of memory, grouping fields of possibly different sizes under one label the way a C struct does, except the assembler has no built-in concept of 'struct' — the programmer (or the assembler's macro facility) defines the offsets by hand. NASM's struc/endstruc directives let you declare field names that expand to constant offsets, so mov eax, [rsi + Point.y] reads a field by name instead of a hardcoded magic number, which is purely a readability aid; the generated machine code is identical to using the raw offset.

The struc directive computes each field's offset automatically based on the cumulative size of the fields declared before it, so reordering fields changes every subsequent offset — this is why struct layout changes are binary-incompatible across separately assembled modules unless everything is rebuilt together. Padding and alignment are not automatic either; if a struct mixes byte and quadword fields, the programmer (or an alignment-aware macro) must insert explicit padding to keep multi-byte fields on aligned addresses, since misaligned accesses on x86-64 are legal but slower, and mandatory on some architectures.

An array of structures is addressed the same way as an array of primitives, except the 'element size' is the struct's total size (available as StructName_size after struc/endstruc), so element i starts at base + i*StructName_size, and a field within that element is reached by adding the field's own offset on top: base + i*StructName_size + StructName.field. Nested structures (a struct containing another struct as a field, as with Player.pos above) simply chain offsets further — the total offset is the sum of every level's offset, computed once at assembly time and baked into the instruction as a single constant displacement.