Assembly Language Quick Reference

x86-64 provides 16 general-purpose 64-bit registers, each accessible at multiple widths: rax/eax/ax/al covers 64/32/16/8 bits respectively of the same physical register, and this sub-register aliasing applies uniformly to rbx, rcx, rdx and, with slightly different byte-naming, to rsi/rdi/rbp/rsp and r8-r15 (e.g. r8, r8d, r8w, r8b). Writing to a 32-bit sub-register like eax automatically zero-extends into the full 64-bit rax on x86-64, a quirk with no equivalent when writing to the 16-bit or 8-bit sub-registers, which merge instead of zero-extending.

Core arithmetic and data-movement instructions form the bulk of everyday assembly: mov dst, src copies data, add/sub perform arithmetic and update flags, cmp subtracts without storing the result purely to set flags for a following conditional jump, and lea dst, [expr] computes an address expression without dereferencing it, often repurposed for fast arithmetic. Addressing syntax in Intel-style assembly wraps a memory reference in brackets: [base + index*scale + displacement], where scale must be 1, 2, 4, or 8 — for example [rbp - 0x8] accesses a local variable and [rax + rbx*4] accesses the element at index rbx in a 4-byte-element array pointed to by rax.

The EFLAGS register's most commonly checked bits are ZF (zero flag, set when a result is zero), SF (sign flag, mirrors the result's most significant bit), CF (carry flag, set on unsigned overflow/borrow), and OF (overflow flag, set on signed overflow). Conditional jumps combine these differently depending on whether the preceding comparison should be treated as signed or unsigned: je/jne check ZF alone, jl/jle/jg/jge (signed) combine SF and OF, while jb/jbe/ja/jae (unsigned) use CF, making the mnemonic choice — not just the cmp — determine correctness.