# Targets

hdass separates two things that assemblers usually tangle together:

- the **architecture** — which instructions exist and how registers work;
- the **assembler syntax** — how those instructions are written to a file.

A target is a pairing of the two, chosen with `-t`:

| `-t` | architecture | assembler | notes |
| --- | --- | --- | --- |
| `nasm` (default) | x86-64 | NASM | Intel syntax, `nasm -f elf64` |
| `fasm` | x86-64 | fasm | Intel syntax, `fasm` (one step) |
| `arm64` | AArch64 | GNU as | `aarch64-linux-gnu-as` |

For x86-64, `nasm` and `fasm` emit the **same instruction bodies** and differ only
in framing (file header, sections, constant and data syntax). `arm64` is a
separate instruction selector: different registers, three-operand arithmetic,
`ldr`/`str` memory, `cmp`+`b.cond` branches and `svc #0` syscalls.

`masm` (x86-64) and a 32-bit `arm` target are planned.

## What each architecture supports

The language is the same; not every construct lowers on every architecture yet.

| Feature | x86-64 | AArch64 |
| --- | --- | --- |
| Moves, arithmetic (`+ - * /`), compound assignment | ✅ | ✅ |
| `if` / `else` / `while`, `goto`, labels | ✅ | ✅ |
| Conditional select (`a if c else b`) | ✅ `cmov` | ✅ `csel` |
| Calls, `syscall` | ✅ | ✅ |
| Memory load/store (`^`), sized and signed | ✅ | partial (`ldr`/`str`) |
| Raw instruction statement | ✅ | ✅ |
| Modulo (`%`), division remainder | ✅ | ❌ not yet |
| Floating point (`xmm`) | ✅ | ❌ not yet |
| `stack` buffers | ✅ | ❌ not yet |
| Bare-metal directives (`format`, `org`, `boot`, `bits 16`) | ✅ | — (x86/BIOS concept) |

Unsupported constructs emit a `; TODO` comment instead of incorrect instructions.

## The portable register model

Architecture-native register names (`rax` on x86-64, `x0` on AArch64) lock a
program to one architecture. To write for both, enable
[`logical_registers`](language.md#logical_registers): `r1`–`r14` are the
general-purpose registers, mapped per target.

| logical | `r1` | `r2` | `r3` | `r4` | `r5` | `r6` | `r7` | `r8` | `r9` | `r10` | … |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| x86-64 | rax | rbx | rcx | rdx | rsi | rdi | r8 | r9 | r10 | r11 | … |
| AArch64 | x0 | x1 | x2 | x3 | x4 | x5 | x6 | x7 | x8 | x9 | … |

AArch64 is simply `rN → x(N-1)`. The raw [instruction statement](language.md#raw-instructions)
is architecture-locked too: its mnemonics are whatever you write.

## Syscall ABIs differ

Even with logical registers, a *syscall* is not portable: Linux uses different
call numbers, argument registers and trap instructions per architecture. So a
program still carries architecture-specific ABI constants.

| | x86-64 | AArch64 |
| --- | --- | --- |
| syscall number in | `rax` (logical `r1`) | `x8` (logical `r9`) |
| arguments in | `rdi rsi rdx r10 r8 r9` | `x0 x1 x2 x3 x4 x5` |
| trap (`syscall`) | `syscall` | `svc #0` |
| `exit` number | `60` | `93` |
| `write` number | `1` | `64` |

C works the same way: portable source, per-platform syscalls.

## OS independence

The emitted instructions aren't tied to an OS; only the syscall numbers and the
`[entry]`/link convention are. The examples and toolchain here target Linux (ELF,
`ld`, and `qemu-aarch64` for ARM); see [getting started](getting-started.md).