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Devices Drivers

Device drivers take care of device-specific functionality isolating the rest of Software from these details by presenting a common interface to them. This interface is the Device Manager service (DEVMAN).

A device driver consists of three separate routines:

  * Initialization Routine (INIT)
  * Device Service Routine (DSR)
  * Interrupt Service Routine (ISR)

The INIT routine initializes device hardware (for example, programmable chips) and data structures associated with the device driver.

The DSR routine is called to initiate operations on the controlled device. In most cases, the DSR sends commands to the device, returning immediately after that. The device's response could take some time and it is captured through interrupts.

The ISR routine services the interrupts coming from the device in question. Possibly the first thing to do is to query some device's control register to learn what caused the interrupt.

The ISR also performs part of the useful job. For instance, when reading from a Tape Drive, it receives the incoming bytes into a given buffer.

In source, device drivers are written in separate files with extension ".DEV".

VTCOM.DEV is the device driver for the VTCOM Tape Drive Simulation Software running at MS-DOS PC connected via Serial Port (RS-232) to LC-81. This program simulates a Tape Drive according to the "LC-81 Tape Drives Specification". (See the specification in section "Specs").

The LC-81 serial port is based on UART chip 16C550. The INIT routine initializes the chip to use FIFO and to interrupt for each byte received.

Since there is no EXT-BUS in this case, Drive's registers C and F can not be accessed directly. Access to those registers is orchestrated by the mean of 3-bytes "Escape Sequences". This method, however, presents a limitation for interrupts associated to the WRITE operation: it is not practical to write one byte at the time (as specified for Tape Drives) so the ISR routine writes a whole block continuously in a loop before returning.

We will cover VTCOM Operations and Escape Sequences in details later in section "VTCOM Tape Drive Simulation Software".