Language basics

31 steps in 9 sets of Assembly.

Assembly is the floor every other language stands on: instructions the processor executes directly, registers instead of variables, flags instead of booleans, and syscalls where a runtime would be. This tour speaks NASM for x86-64 Linux — the dialect of most tutorials and of osdev culture — and every seed is a complete program that assembles, links, and runs, most of them handing their computed answer out as the exit code: the machine's own way of returning a value.

The route runs from a five-line exit through the whole working vocabulary: mov and the register views, add through div with rdx's double duty, data sections with db/dw/dq and equ, the cmp-and-jump loop shape, scaled addressing over arrays, push/pop and the call/ret pair, lea's free arithmetic, byte-at-a-time string work, cmov and setcc for branch-free logic, two's complement, jump tables, and the rbp stack frame every compiler emits. The Encore is three honest programs: a decimal printer built on div, FizzBuzz spoken in remainders, and a nested-loop triangle.

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First programs

The smallest programLoad the exit syscall's number and make the call: five lines.
sys_writefd, buffer, length in rdi, rsi, rdx — then syscall.
Registers and movCopy between registers; 32-bit writes zero the top half.

Arithmetic

add and subIn-place arithmetic: the first operand is the destination.
mul and divMultiplication and division live in rax; rdx catches the rest.
Two's complementneg flips sign; signed jumps read it correctly.

Data

Data sizesdb, dw, dq: bytes, words, quadwords, declared and measured.
Loads and storesbyte, word, qword at memory — and movzx to widen cleanly.
equ and timesAssemble-time constants and repetition.
memcpy by handLoad a byte, store a byte, walk both pointers.

Control flow

cmp and jumpscmp sets the flags; the j-family branches on them.
The loop shapeA label, a compare, a conditional jump back: every loop ever.
dec and jnzThe tightest countdown: decrement until the zero flag.
Jump tablesIndex into addresses instead of chaining compares.

Bits & flags

Bitwiseand, or, xor, shl: arithmetic below arithmetic.
Shift and maskPull packed fields out of a value.
test and setccFlag questions become byte answers.
cmovPick a value from flags without branching.

The stack

push and popThe stack grows down; last in, first out swaps for free.
call and retcall pushes the return address; ret pops and jumps back.
Stack framesrbp anchors locals at fixed offsets: the classic prologue.

Memory & strings

Scaled addressingbase + index * size reaches array slots in one operand.
leaCompute an address without loading it: free arithmetic.
strlen by handWalk bytes until the zero terminator.
The register shuffleFibonacci in three registers, no memory at all.

Talking to Linux

Digits to textA number becomes printable by adding '0'.
stdout and stderrFile descriptors are numbers: 1 and 2 are two streams.
argc and argvAt _start the stack holds argc, then the argv pointers.

Encore

print_decimal.asmUnsigned to decimal: divide by ten, build the digits backward.
fizzbuzz.asmThe classic, spoken in remainders and sys_write.
triangle.asmNested loops draw a hash triangle one row at a time.