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# Tutorial 04 - Zero Overhead Abstraction
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## tl;dr
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All hand-written assembly is replaced by Rust code from the [cortex-a] crate, which provides
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zero-overhead abstractions and wraps the `unsafe` parts.
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[cortex-a]: https://github.com/rust-embedded/cortex-a
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## Diff to previous
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```diff
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diff -uNr 03_hacky_hello_world/Cargo.toml 04_zero_overhead_abstraction/Cargo.toml
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--- 03_hacky_hello_world/Cargo.toml
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+++ 04_zero_overhead_abstraction/Cargo.toml
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@@ -7,7 +7,10 @@
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# The features section is used to select the target board.
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[features]
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default = []
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-bsp_rpi3 = []
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-bsp_rpi4 = []
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+bsp_rpi3 = ["cortex-a"]
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+bsp_rpi4 = ["cortex-a"]
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[dependencies]
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+
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+# Optional dependencies
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+cortex-a = { version = "3.0.x", optional = true }
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diff -uNr 03_hacky_hello_world/src/_arch/aarch64/cpu/smp.rs 04_zero_overhead_abstraction/src/_arch/aarch64/cpu/smp.rs
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--- 03_hacky_hello_world/src/_arch/aarch64/cpu/smp.rs
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+++ 04_zero_overhead_abstraction/src/_arch/aarch64/cpu/smp.rs
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@@ -0,0 +1,22 @@
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+// SPDX-License-Identifier: MIT OR Apache-2.0
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+//
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+// Copyright (c) 2018-2020 Andre Richter <andre.o.richter@gmail.com>
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+
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+//! Architectural symmetric multiprocessing.
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+
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+use cortex_a::regs::*;
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+
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+//--------------------------------------------------------------------------------------------------
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+// Public Code
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+//--------------------------------------------------------------------------------------------------
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+
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+/// Return the executing core's id.
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+#[inline(always)]
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+pub fn core_id<T>() -> T
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+where
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+ T: From<u8>,
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+{
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+ const CORE_MASK: u64 = 0b11;
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+
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+ T::from((MPIDR_EL1.get() & CORE_MASK) as u8)
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+}
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diff -uNr 03_hacky_hello_world/src/_arch/aarch64/cpu.rs 04_zero_overhead_abstraction/src/_arch/aarch64/cpu.rs
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--- 03_hacky_hello_world/src/_arch/aarch64/cpu.rs
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+++ 04_zero_overhead_abstraction/src/_arch/aarch64/cpu.rs
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@@ -4,8 +4,34 @@
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//! Architectural processor code.
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-// Assembly counterpart to this file.
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-global_asm!(include_str!("cpu.S"));
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+use crate::{bsp, cpu};
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+use cortex_a::{asm, regs::*};
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+
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+//--------------------------------------------------------------------------------------------------
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+// Boot Code
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+//--------------------------------------------------------------------------------------------------
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+
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+/// The entry of the `kernel` binary.
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+///
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+/// The function must be named `_start`, because the linker is looking for this exact name.
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+///
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+/// # Safety
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+///
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+/// - Linker script must ensure to place this function at `0x80_000`.
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+#[naked]
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+#[no_mangle]
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+pub unsafe extern "C" fn _start() -> ! {
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+ use crate::runtime_init;
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+
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+ // Expect the boot core to start in EL2.
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+ if bsp::cpu::BOOT_CORE_ID == cpu::smp::core_id() {
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+ SP.set(bsp::cpu::BOOT_CORE_STACK_START);
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+ runtime_init::runtime_init()
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+ } else {
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+ // If not core0, infinitely wait for events.
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+ wait_forever()
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+ }
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+}
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//--------------------------------------------------------------------------------------------------
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// Public Code
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@@ -14,13 +40,7 @@
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/// Pause execution on the core.
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#[inline(always)]
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pub fn wait_forever() -> ! {
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- unsafe {
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- loop {
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- llvm_asm!("wfe"
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- : // outputs
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- : // inputs
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- : // clobbers
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- : "volatile") // options
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- }
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+ loop {
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+ asm::wfe()
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}
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}
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diff -uNr 03_hacky_hello_world/src/_arch/aarch64/cpu.S 04_zero_overhead_abstraction/src/_arch/aarch64/cpu.S
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--- 03_hacky_hello_world/src/_arch/aarch64/cpu.S
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+++ 04_zero_overhead_abstraction/src/_arch/aarch64/cpu.S
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@@ -1,21 +0,0 @@
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-// SPDX-License-Identifier: MIT OR Apache-2.0
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-//
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-// Copyright (c) 2018-2020 Andre Richter <andre.o.richter@gmail.com>
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-
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-.section ".text._start"
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-
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-.global _start
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-
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-_start:
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- mrs x1, mpidr_el1 // Read Multiprocessor Affinity Register
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- and x1, x1, #3 // Clear all bits except [1:0], which hold core id
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- cbz x1, 2f // Jump to label 2 if we are core 0
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-1: wfe // Wait for event
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- b 1b // In case an event happened, jump back to 1
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-2: // If we are here, we are core0
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- ldr x1, =_start // Load address of function "_start()"
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- mov sp, x1 // Set start of stack to before our code, aka first
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- // address before "_start()"
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- bl runtime_init // Jump to the "runtime_init()" kernel function
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- b 1b // We should never reach here. But just in case,
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- // park this core aswell
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diff -uNr 03_hacky_hello_world/src/bsp/raspberrypi/cpu.rs 04_zero_overhead_abstraction/src/bsp/raspberrypi/cpu.rs
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--- 03_hacky_hello_world/src/bsp/raspberrypi/cpu.rs
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+++ 04_zero_overhead_abstraction/src/bsp/raspberrypi/cpu.rs
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@@ -0,0 +1,15 @@
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+// SPDX-License-Identifier: MIT OR Apache-2.0
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+//
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+// Copyright (c) 2018-2020 Andre Richter <andre.o.richter@gmail.com>
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+
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+//! BSP Processor code.
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+
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+//--------------------------------------------------------------------------------------------------
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+// Public Definitions
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+//--------------------------------------------------------------------------------------------------
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+
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+/// Used by `arch` code to find the early boot core.
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+pub const BOOT_CORE_ID: usize = 0;
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+
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+/// The early boot core's stack address.
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+pub const BOOT_CORE_STACK_START: u64 = 0x80_000;
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diff -uNr 03_hacky_hello_world/src/bsp/raspberrypi.rs 04_zero_overhead_abstraction/src/bsp/raspberrypi.rs
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--- 03_hacky_hello_world/src/bsp/raspberrypi.rs
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+++ 04_zero_overhead_abstraction/src/bsp/raspberrypi.rs
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@@ -5,3 +5,4 @@
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//! Top-level BSP file for the Raspberry Pi 3 and 4.
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pub mod console;
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+pub mod cpu;
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diff -uNr 03_hacky_hello_world/src/cpu/smp.rs 04_zero_overhead_abstraction/src/cpu/smp.rs
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--- 03_hacky_hello_world/src/cpu/smp.rs
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+++ 04_zero_overhead_abstraction/src/cpu/smp.rs
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@@ -0,0 +1,10 @@
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+// SPDX-License-Identifier: MIT OR Apache-2.0
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+//
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+// Copyright (c) 2018-2020 Andre Richter <andre.o.richter@gmail.com>
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+
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+//! Symmetric multiprocessing.
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+
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+#[cfg(target_arch = "aarch64")]
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+#[path = "../_arch/aarch64/cpu/smp.rs"]
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+mod arch_cpu_smp;
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+pub use arch_cpu_smp::*;
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diff -uNr 03_hacky_hello_world/src/cpu.rs 04_zero_overhead_abstraction/src/cpu.rs
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--- 03_hacky_hello_world/src/cpu.rs
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+++ 04_zero_overhead_abstraction/src/cpu.rs
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@@ -8,3 +8,5 @@
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#[path = "_arch/aarch64/cpu.rs"]
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mod arch_cpu;
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pub use arch_cpu::*;
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+
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+pub mod smp;
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diff -uNr 03_hacky_hello_world/src/main.rs 04_zero_overhead_abstraction/src/main.rs
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--- 03_hacky_hello_world/src/main.rs
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+++ 04_zero_overhead_abstraction/src/main.rs
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@@ -93,8 +93,7 @@
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//! - `crate::bsp::memory::*`
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#![feature(format_args_nl)]
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-#![feature(global_asm)]
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-#![feature(llvm_asm)]
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+#![feature(naked_functions)]
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#![feature(panic_info_message)]
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#![no_main]
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#![no_std]
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@@ -116,7 +115,8 @@
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///
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/// - Only a single core must be active and running this function.
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unsafe fn kernel_init() -> ! {
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- println!("[0] Hello from Rust!");
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+ println!("[0] Hello from pure Rust!");
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- panic!("Stopping here.")
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+ println!("[1] Stopping here.");
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+ cpu::wait_forever()
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}
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diff -uNr 03_hacky_hello_world/src/runtime_init.rs 04_zero_overhead_abstraction/src/runtime_init.rs
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--- 03_hacky_hello_world/src/runtime_init.rs
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+++ 04_zero_overhead_abstraction/src/runtime_init.rs
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@@ -50,8 +50,7 @@
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/// # Safety
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///
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/// - Only a single core must be active and running this function.
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-#[no_mangle]
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-pub unsafe extern "C" fn runtime_init() -> ! {
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+pub unsafe fn runtime_init() -> ! {
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zero_bss();
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crate::kernel_init()
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```
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