0x378

0x378 is the conventional I/O port base address used by the first parallel port (LPT1) on IBM PC compatibles. This address identifies the hardware interface that connects a computer to devices such as printers through a parallel interface, a common arrangement in the 1980s and 1990s.

In the classic parallel port layout, three 8-bit registers are accessed via consecutive I/O ports starting at 0x378. The data register resides at the base address (0x378) and provides the D0–D7 data lines to the connected device. The status register is at base+1 (0x379) and exposes input lines from the peripheral, such as signals indicating readiness, paper status, or errors. The control register is at base+2 (0x37A) and controls host-to-device handshake signals, such as strobe and other control lines. Many of the status and control lines have inverted logic in practice, depending on the hardware and device.

Historically, 0x378 and its neighboring ports formed the standard interface for the PC’s parallel printer ports. LPT1 typically shared its I/O range with other legacy peripherals, and additional parallel ports (LPT2, LPT3) were mapped to other base addresses such as 0x278 and 0x3BC. The exact mapping could be configured by the system BIOS, but 0x378 remained the canonical base address in many systems.

In modern computing, dedicated parallel ports are less common, with USB printers and networked devices becoming prevalent. When present, access to 0x378-based ports is usually provided through operating system drivers or specialized libraries that manage the I/O port operations and ensure proper privileges. Programming typically involves low-level I/O access, using port I/O instructions in environments that permit direct hardware access or through higher-level driver interfaces in contemporary OSes.