The ./src/base/serial directory contains Mark Rejhon's 16550A UART emulation code.
The files in this subdirectory are:
The Makefile for this subdir.
Serial initialization code.
Serial Port I/O emulation code.
Serial Interrupts emulation code.
BIOS int14 emulation code.
FOSSIL driver emulation code.
DOS part of FOSSIL emulation.
Include file for above files only.
General include file for all DOSEMU code.
Redistribution of these files is permitted under the terms of the GNU Public License (GPL). See end of this file for more information.
Information on a 16550 is based on information from HELPPC.EXE 2.1 and results from National Semiconductor's COMTEST.EXE diagnostics program. This code aims to emulate a 16550A as accurately as possible, using just reasonably POSIX.2 compliant code. In some cases, this code does a better job than OS/2 serial emulation (in non-direct mode) done by its COM.SYS driver! There may be about 100 kilobytes of source code, but nearly 50% of this size are comments!
This 16550A emulator never even touches the real serial ports, it merely traps port writes, and does the I/O via file functions on a device in /dev. Interrupts are also simulated as necessary.
One of the most important things to know before programming this serial code, is to understand how the com array works.
Most of the serial variables are stored in the com array. The com array is a structure in itself. Take a look at the 'serial_t' struct declaration in the ../include/serial.h module for more info about this. Only the most commonly referenced global variables are listed here:
Number of serial ports active.
The base port address of emulated serial port.
The COM port number.
The PIC interrupt level (based on IRQ number)
Flag mouse (to enable extended features)
File descriptor for port device
Filename of port port device
Flag whether device has been locked
The arbritary example variable 'x' in com[x] can have a minimum value of 0 and a maximum value of (config.numser - 1). There can be no gaps for the value 'x', even though gaps between actual COM ports are permitted. It is strongly noted that the 'x' does not equal the COM port number. This example code illustrates the fact, and how the com[x] array works:
for (i = 0; i < config.numser; i++) s_printf("COM port number %d has a base address of %x", com[i].real_comport, com[i].base_port);
If you require heavy debugging output for serial operations, please take a look in ./ser_defs.h for the following defines:
extra debug output on the most critical information.
super-heavy extra debug output, including all ports reads and writes, and every character received and transmitted!
additional debug output related to serial interrupt code, including flagging serial interrupts, or PIC-driven code.
heavy FOSSIL debug output, including all reads and writes.
super-heavy FOSSIL debug output, including all status checks.
The FOSSIL emulation requires a memory-resident DOS module, FOSSIL.COM. If you don't load it, the code in fossil.c does nothing, permitting you to use another FOSSIL driver such as X00 or BNU.
The emulation isn't as complete as it could be. Some calls aren't implemented at all. However, the programs I've tried don't seem to use these calls. Check fossil.c if you're interested in details.
I've done only minimal testing on this code, but at least the performance seems to be quite good. Depending on system load, I got Zmodem speeds ranging from 1500 to nearly 3800 cps over a 38400 bps null-modem cable when sending data to another PC. Zmodem receive, however, was only about 2200 cps, since Linux tried to slow down the sender with flow control (I'm not sure why).
Most of the code in the DOSEMU 'serial' subdirectory is: Copyright (C) 1995 by Mark D. Rejhon <email@example.com> with the following exceptions:
FOSSIL driver emulation is Copyright (C) 1995 by Pasi Eronen <firstname.lastname@example.org>.
UART definitions were derived from /linux/include/linux/serial.h Copyright (C) 1992 by Theodore Ts'o.
All of this serial code is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.