Create a 16-bit 'Hello World'

Author: phil-opp

real_mode/.cargo/config

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+[build]
+target = "x86_64-bootloader-real-mode.json"

real_mode/.gitignore

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+/target/
+**/*.rs.bk

real_mode/Cargo.lock

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+[package]
+name = "real_mode"
+version = "0.1.0"
+authors = ["Philipp Oppermann <[email protected]>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+
+[profile.release]
+opt-level = "z"

real_mode/Cargo.toml

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+# 16-bit Rust (Experiment)
+
+This is an experiment to translate the 16-bit code of the bootloader from assembly to Rust.
+
+## Building
+
+To build the project, use cargo-xbuild:
+
+```
+cargo xbuild --release
+```
+
+The BIOS only loads the first 512 bytes of our executable into memory, so the amount of code that this binary can contain is very limited. This is also the reason why this can only be built in release mode.
+
+If the code does not fit into 512 bytes, the linker will throw the following error:
+
+> rust-lld: error: linker.ld:16: unable to move ___location counter backward for: .bootloader
+
+## Creating a Disk Image
+
+The output of `cargo xbuild` is an ELF binary, which can't be loaded directly by the BIOS. To boot our project, we must therefore convert it into a flat binary first. This works with the following `objcopy` command:
+
+```
+objcopy -I elf32-i386 -O binary target/x86_64-bootloader-real-mode/release/real_mode image.bin
+```
+
+This creates a file named `image.bin` in the root folder of the project, which is a bootable disk image.
+
+## Running it in QEMU
+
+To run the disk image in QEMU, execute the following command:
+
+```
+qemu-system-x86_64 -drive format=raw,file=image.bin
+```

real_mode/README.md

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+ENTRY(_start)
+
+SECTIONS {
+    . = 0x500;
+    _stack_start = .;
+    . = 0x7c00;
+    _stack_end = .;
+
+    .bootloader :
+    {
+        *(.boot-first-stage)
+        *(.text .text.*)
+        *(.rodata .rodata.*)
+        *(.data .data.*)
+        *(.got)
+        . = 0x7c00 + 510;
+        SHORT(0xaa55)       /* magic number for bootable disk */
+    }
+    _bootloader_end = .;
+}

real_mode/linker.ld

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+.section .boot-first-stage, "awx"
+.global _start
+.intel_syntax noprefix
+.code16
+
+# This stage initializes the stack, enables the A20 line, loads the rest of
+# the bootloader from disk, and jumps to stage_2.
+
+_start:
+    # zero segment registers
+    xor ax, ax
+    mov ds, ax
+    mov es, ax
+    mov ss, ax
+    mov fs, ax
+    mov gs, ax
+
+    # clear the direction flag (e.g. go forward in memory when using
+    # instructions like lodsb)
+    cld
+
+    # initialize stack
+    mov sp, 0x7c00
+
+    call rust_main
+
+spin:
+    hlt
+    jmp spin

real_mode/src/boot.s

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+#![feature(asm, global_asm)]
+#![no_std]
+#![no_main]
+
+use core::panic::PanicInfo;
+
+global_asm!(include_str!("boot.s"));
+
+#[no_mangle]
+pub extern "C" fn rust_main() {
+    println(b"Hello from Rust!");
+    panic!()
+}
+
+fn println(s: &[u8]) {
+    print(s);
+    print_char(b'\n');
+}
+
+fn print(s: &[u8]) {
+    for &c in s {
+        print_char(c);
+    }
+}
+
+fn print_char(c: u8) {
+    let ax = u16::from(c) | 0x0e00;
+    unsafe {
+        asm!("int 0x10" :: "{ax}"(ax) :: "intel" );
+    }
+}
+
+#[panic_handler]
+pub fn panic(_info: &PanicInfo) -> ! {
+    println(b"PANIC!");
+    loop {}
+}
+

real_mode/src/main.rs

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+{
+	"arch": "x86",
+	"cpu": "i386",
+	"data-layout": "e-m:e-p:32:32-f64:32:64-f80:32-n8:16:32-S128",
+	"dynamic-linking": false,
+	"executables": true,
+	"linker-flavor": "ld.lld",
+	"linker": "rust-lld",
+	"llvm-target": "i386-unknown-none-code16",
+	"pre-link-args": {
+		"ld.lld": [
+			"-Tlinker.ld"
+		]
+	},
+	"max-atomic-width": 64,
+	"position-independent-executables": false,
+	"disable-redzone": true,
+	"relro-level": "full",
+	"target-c-int-width": "32",
+	"target-pointer-width": "32",
+	"target-endian": "little",
+	"panic-strategy": "abort",
+	"os": "none",
+	"vendor": "unknown"
+}