55  Serial Backend

This tour explores DOOM’s serial networking backend, covering interrupt handling, UART initialization and detection, packet framing, network dispatch, connection handshaking, and modem control.

55.1 Initialization

DOOM initializes serial communication by hooking interrupts and configuring the serial port hardware.

  • LaunchDOOM sets up an interrupt handler and spawns DOOM:
    → sersrc/DOOMNET.C (lines 76–102)

  • GetUart() sets comport from CLI parameters:
    → sersrc/PORT.C (52–59)

  • BIOS and ISA detection of UART settings:
    → PORT.C (61–67)

  • Checks for MCA or ISA, selects UART/IRQ:
    → PORT.C (69–74)

  • CLI overrides port/IRQ settings:
    → PORT.C (81–86)

  • InitPort sets up UART I/O and IRQ:
    → PORT.C (109–113)

  • Initializes UART speed and clears interrupts:
    → PORT.C (117–125)

  • Detects 16550 FIFO vs 8250 UARTs:
    → PORT.C (147–153)

  • Installs interrupt handler (ISR):
    → PORT.C (193–200)

55.2 UART Interrupt Handling

  • isr_8250() handles basic UART interrupts:
    → PORT.C (287–295)

  • isr_16550() handles FIFO UARTs:
    → PORT.C (348–356)

  • read_byte() and write_byte() manage TX/RX queues:
    → PORT.C (256–262), (268–272)

55.3 Packet Framing

ReadPacket and WritePacket handle framed data over serial:

  • ReadPacket extracts bytes via read_byte() and signals completeness:
    → SERSETUP.C (135–145)

  • write_buffer enqueues bytes via write_byte():
    → SERSETUP.C (50–51)

  • WritePacket builds and sends framed packets:
    → SERSETUP.C (181–188)

55.4 Network ISR

  • NetISR handles serial send/receive for DOOM:
    → SERSETUP.C (212–218)

55.5 Connection Handshake

  • Connect() assigns player 0 or 1 deterministically:
    → SERSETUP.C (240–248)

55.6 Modem Control

  • Commands and responses are sent with pacing:
    • Setup: → SERSETUP.C (341–347)
    • Waits for response: → SERSETUP.C (371–378)

55.7 Queues

  • que_t is a 2048-byte circular buffer for serial communication:
    → SERSETUP.C (403–410)

We’ve now explored DOOM’s serial backend: interrupt hooks, UART detection, packet framing, and modem handshaking—all classic DOS-era techniques.