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08 - Hardware Device Drivers & Low-Level I/O

BangOS interfaces directly with hardware peripherals via x86_64 I/O port instructions (inb, outb, outw). All driver modules reside in the kernel/drivers/ directory:

  1. 16550 UART Serial Driver (COM1 Serial Console at 38400 baud).
  2. 8254 PIT Timer Driver (100 Hz Preemptive Scheduler Interrupts).
  3. 8259 PIC Driver (Programmable Interrupt Controller remapping).
  4. PS/2 Keyboard Driver (Scancode decoder).
  5. QEMU Hypervisor Driver (isa-debug-exit & ACPI poweroff ports).

🔌 Low-Level I/O Port Primitives

Communication with x86_64 legacy hardware uses dedicated bus instructions:

static inline void outb(uint16_t port, uint8_t val) {
    __asm__ volatile ("outb %0, %1" : : "a"(val), "Nd"(port));
}

static inline uint8_t inb(uint16_t port) {
    uint8_t ret;
    __asm__ volatile ("inb %1, %0" : "=a"(ret) : "Nd"(port));
    return ret;
}

static inline void io_wait(void) {
    // Write to unused port 0x80 to provide hardware bus settling delay
    __asm__ volatile ("outb %%al, $0x80" : : "a"(0));
}

📡 16550 UART Serial Driver (kernel/drivers/uart.c)

The 16550 Universal Asynchronous Receiver-Transmitter (UART) provides the primary console interface for BangOS over serial port COM1 (0x3F8).

I/O Port Map:

  • COM1 + 0 (0x3F8): Receiver Buffer / Transmitter Holding Register / Divisor Latch Low.
  • COM1 + 1 (0x3F9): Interrupt Enable Register / Divisor Latch High.
  • COM1 + 2 (0x3FA): FIFO Control Register (FCR).
  • COM1 + 3 (0x3FB): Line Control Register (LCR).
  • COM1 + 4 (0x3FC): Modem Control Register (MCR).
  • COM1 + 5 (0x3FD): Line Status Register (LSR).

Initialization Sequence:

void uart_init(void) {
    outb(COM1_PORT + 1, 0x00); // Disable UART interrupts
    outb(COM1_PORT + 3, 0x80); // Enable DLAB (Divisor Latch Access Bit)
    outb(COM1_PORT + 0, 0x03); // Divisor 3 -> 38400 baud (115200 / 3)
    outb(COM1_PORT + 1, 0x00);
    outb(COM1_PORT + 3, 0x03); // 8 Data Bits, No Parity, 1 Stop Bit (8N1)
    outb(COM1_PORT + 2, 0x07); // Enable FIFO with 1-byte trigger threshold
    outb(COM1_PORT + 4, 0x0B); // Set RTS/DSR lines active
}

Formatted Kernel Printf (kprintf)

kprintf() provides freestanding variadic output across Ring 0 supporting:

  • %s (Strings), %d (Signed integers), %u (Unsigned integers), %x (Hexadecimal integers), %p (Pointers), %c (Characters), %% (Literal percent).

⏱️ 8254 PIT Timer & 8259 PIC Driver (kernel/drivers/pit.c)

The 8254 Programmable Interval Timer (PIT) generates periodic hardware interrupts to drive the multitasking scheduler.

1. 8259 PIC Remapping

Legacy PC motherboards map hardware IRQs 0..7 to CPU vectors 8..15 (which overlap with CPU exception vectors). pic_remap() shifts the vector offsets:

  • Master PIC: Vectors 0x20..0x27 (32..39).
  • Slave PIC: Vectors 0x28..0x2F (40..47).

2. PIT Frequency Programming

The PIT oscillator runs at a base clock of 1,193,182 Hz. BangOS programs Channel 0 in Mode 3 (Square Wave Generator) for 100 Hz:

$$\text{Divisor} = \frac{1,193,182\text{ Hz}}{100\text{ Hz}} = 11,931\text{ (0x2E9B)}$$

void pit_init(uint32_t frequency_hz) {
    uint32_t divisor = PIT_BASE_FREQ / frequency_hz;

    // Channel 0, access lobyte/hibyte, Mode 3 (Square Wave), 16-bit binary
    outb(PIT_COMMAND, 0x36);
    io_wait();
    outb(PIT_CHANNEL0, (uint8_t)(divisor & 0xFF));
    io_wait();
    outb(PIT_CHANNEL0, (uint8_t)((divisor >> 8) & 0xFF));
    io_wait();

    pit_ticks = 0;
}

⌨️ PS/2 Keyboard Driver (kernel/drivers/keyboard.c)

The PS/2 keyboard controller communicates via:

  • Port 0x64 (Status Register): Bit 0 indicates data availability in output buffer.
  • Port 0x60 (Data Register): Delivers scancodes.

Scancode Decoding:

  • Make Codes (Key Press): Scancodes with bit 7 clear (!(scancode & 0x80)).
  • Break Codes (Key Release): Scancodes with bit 7 set (scancode & 0x80).
  • BangOS translates Scan Code Set 1 into ASCII characters via scancode_map[].
int keyboard_has_char(void) {
    if (inb(KEYBOARD_STATUS_PORT) & 1) {
        uint8_t scancode = inb(KEYBOARD_DATA_PORT);
        if (!(scancode & 0x80)) { // Key press
            if (scancode < sizeof(scancode_map)) {
                return scancode_map[scancode];
            }
        }
    }
    return 0;
}

⚡ QEMU Hypervisor Ports (kernel/drivers/qemu.c)

To enable deterministic, headless testing in CI pipelines and clean system shutdown:

  1. isa-debug-exit Port (0xF4): Writing an exit code to port 0xF4 causes QEMU to terminate immediately with exit code (val << 1) | 1. 

    void qemu_exit(uint8_t code) {
    outb(QEMU_DEBUG_EXIT_PORT, code);
}

  2. ACPI Poweroff Port (0x604): Writing 0x2000 to port 0x604 initiates standard ACPI system power down in QEMU. 

    void qemu_poweroff(void) {
    outw(0x604, 0x2000);
}

💾 ATA / IDE PIO Storage Driver (kernel/drivers/ata.c)

BangOS supports standard ATA/IDE disk drives operating in PIO mode across Primary (0x1F0) and Secondary (0x170) channels.

Features:

  • Port Probing & Identification: Issues 0xEC (IDENTIFY), decodes model string, serial numbers, sector capacity, and checks for LBA48 support.
  • LBA28 / LBA48 Sector I/O: Performs 16-bit word transfers via 0x1F0 with hardware status busy-wait and DRQ polling.
  • Block Device Layer (kernel/drivers/block.c): Exposes registered drives (/dev/ata0, /dev/ata1) as abstract block_dev_t devices to the VFS and filesystems.

For detailed hardware register maps and I/O diagrams, see Chapter 12 - ATA Storage Driver.

🌐 VirtIO Network Controller & PCI Enumerator (kernel/drivers/virtio_net.c)

BangOS includes a native driver for the OASIS VirtIO Legacy PCI Network Controller (0x1AF4:0x1000).

Features:

  • PCI Bus Scanning (kernel/drivers/pci.c): Enumerates 256 buses via ports 0xCF8/0xCFC, decodes Vendor/Device IDs, and maps Base Address Registers (BAR0).
  • Split Virtqueue Architecture: Configures 16-buffer RX ring (Virtqueue 0) and TX ring (Virtqueue 1) in contiguous physical memory.
  • Packet I/O: Zero-copy packet reception with MAC address parsing and direct handoff to the in-kernel TCP/IP stack.

For detailed memory layouts and register specifications, see Chapter 14 - VirtIO Network Driver.