# ASM instructions

## Introduction

While the Shelter kernel is made in C, somes basic CPU functions require using ASM instructions. For that, we define wrappers that can be used anywhere in the kernel. These wrappers are defined inside the header file `shelter/lib/include/cpu/asm.h` which act as the only place in the entire kernel where the `__asm__` should be used. The subsystem prefix is `sh_asm_`.

## Overview

Here is a list of all the wrappers available. They are all defined as `static inline` function:
- `inb`: take a `sh_uint16` in input for specifying the port and return the inputed byte
- `outb`: take a `sh_uint16` for specifying the port and a `sh_uint8` for specifying the byte to output
- `rdtsc`: take no arguments and return the current TSC as a `sh_uint64`
- `invlpg`: take a `void *` as an address and return nothing
- `lgdt`: take a `void *` as an address and return nothing
- `ltr`: take a `sh_uint16` as a TSS selector and return nothing
- `sidt`: take a `sh_idt_IDTR*` as a pointer and return nothing
- `sti`: take no arguments and return nothing
- `cli`: take no arguments and return nothing
- `cpuid`: take 2 `sh_uint32`: `leaf`, `subleaf`, and 2 `sh_uint32*`: `eax`, `ebx`, `ecx`, `edx`, return nothing
- `mfence`: take no arguments and return nothing
- `lfence`: take no arguments and return nothing
- `sfence`: take no arguments and return nothing
