ser.txt

来自「汇编编程艺术」· 文本 代码 · 共 707 行 · 第 1/2 页

Interrupt-Driven Serial Port I/O Package
========================================

One major problem the the PC's BIOS is the lack of good interrupt driven
I/O support for the serial port.  The BIOS provides a mediocre set of polled
I/O facilities, but completely drops the ball on interrupt driven I/O.

This set of routines in the standard library provides polled I/O support
to read and set the registers on the 8250 (or other comparable chip, e.g.,
16450) as well as read and write data (polled).  In addition, there are
a pair of routines to initialize and disable the interrupt system as well
as perform I/O using interrupts.

Typical polled I/O session:

1. Initialize chip using polled I/O routines.
2. Read and write data using ComRead and ComWrite routines.

Typical interrupt driven I/O session:

1. Initialize chip using polled I/O routines.
2. Read and write data using ComIn and ComOut routines.

Of course, all the details of serial communications cannot be discussed
here- it's far too broad a subject.  These routines, like most in the
library, assume you know what you're doing.  They just make it a little
easier on you.  If you don't understand anything about serial communications,
you *might* be able to use these routines, but they were not written with
that audience in mind.  There are several good references on serial communi-
cations; "C Programmer's Guide to Serial Communications" comes to mind.  If
you've never looked at the 8250 or comparable chips before, you might want
to take a look at a reference such as this one if the routines in this
section don't make much sense.

Note: This routines are set up to use the COM1: hardware port.  See the
source listings if you want to access a different serial port.  Perhaps in
a future release we will modify this code to work with any serial port.


Routine:  ComBaud
-----------------

Author:		      	Randall Hyde

Category:		Serial Communications

Registers on entry:   	AX-	BPS (baud rate): 110, 150, 300, 600, 1200,
						 2400, 4800, 9600, 19200

Registers on return:  	None

Flags affected:       	None

Example of Usage:
			mov	ax, 9600	;Set system to 9600 bps
			ComBaud
Description:

ComBaud programs the serial chip to change its "baud rate" (technically,
it's "bits per second" not baud rate).  You load AX with the appropriate
bps value and call ComBaud, as above.  Note: if AX is not one of the legal
values, ComBaud defaults to 19.2kbps.

Include:	ser.a or stdlib.a

Routine:  ComStop
-----------------

Author:		      	Randall Hyde

Category:             	Serial Communications

Registers on entry:   	AX-	# of stop bits (1 or 2)

Registers on return:  	None

Flags affected:       	None

Example of Usage:
			mov	ax, 2		;Set system to send 2 stop bits
			ComStop
Description:

ComStop programs the serial chip to transmit the specifed number of stop
bits when sending data.  You load AX with the appropriate value and call
ComStop, as above.  Note that this only affects the output data stream. The
serial chip on the PC will always work with one incoming stop bit, regardless
of the setting.  Since additional stop bits slow down your data transmission
(by about 10%) and most devices work fine with one stop bit, you should
normally program the chip with one stop bit unless you encounter some
difficulties.  The setting of this value depends mostly on the system you
are connecting to.

Include:	ser.a or stdlib.a

Routine:  ComSize
-----------------

Author:		      	Randall Hyde

Category:             	Serial Communications

Registers on entry:   	AX-	# of data bits to transmit (5, 6, 7, or 8)

Registers on return:  	None

Flags affected:       	None

Example of Usage:
			mov	ax, 8	;Set system to send 8 data bits
			ComSize
Description:

ComSize programs the serial chip to transmit the specifed number of data
bits when sending data.  You load AX with the appropriate value and call
ComSize, as above.  The setting of this value depends mostly on the system
you are connecting to.

Include:	ser.a or stdlib.a

Routine:  ComParity
-------------------

Author:		      	Randall Hyde

Category:             	Serial Communications

Registers on entry:   	AX-    Bits 0, 1, and 2 are defined as follows:
			       bit 0- 1 to enable parity, 0 to disable.
			       bit 1- 0 for odd parity, 1 for even.
			       bit 2- Stuck parity bit.  If 1 and bit 0 is 1, then the parity bit
				      is always set to the inverse of bit 1.

Registers on return:  	None

Flags affected:       	None

Example of Usage:
			mov	ax, 0	;Set NO parity
			ComParity
			 .
			 .
			 .
			mov	ax, 11b	;Set even parity
			ComParity
Description:

ComParity programs the serial chip to use various forms of parity error
checking.  If bit zero of AX is zero, then this routine disables parity
checking and transmission.  In this case, ComParity ignores the other
two bits (actually, the 8250 ignores them, ComParity just passes them
through).  If bit zero is a one, and bit two is a zero, then bit #1
defines even/odd parity during transmission and receiving.  If bit #0
is a one and bit two is a one, then the 8250 will always transmit bit #1
as the parity bit (forced on or off).

Include:	ser.a or stdlib.a

Routine:  ComRead
-----------------

Author:		      	Randall Hyde

Category:             	Serial Communications

Registers on entry:   	None

Registers on return:  	AL-	Character read from port

Flags affected:       	None

Example of Usage:
			ComRead
			mov	Buffer, al

Description:

ComRead polls the port to see if a character is available in the on-chip
data register.  If not, it waits until a character is available.  Once
a character is available, ComRead reads it and returns this character in
the AL register.

Warning: do *not* use this routine while operating in the interrupt mode.
This routine is for polled I/O only.

Include:	ser.a or stdlib.a

Routine:  ComWrite
------------------

Author:		      	Randall Hyde

Category:             	Serial Communications

Registers on entry:   	AL-	Character to write to port

Registers on return:  	None

Flags affected:       	None

Example of Usage:
			mov	al, 'a'
			ComWrite

Description:

ComWrite polls the port to see if the transmitter is busy.  If so, it waits
until the current transmission is through.  Once the 8250 is done with the
current character, ComWrite will put the character in AL into the 8250
transmit register.

Warning: do *not* use this routine while operating in the interrupt mode.
This routine is for polled I/O only.

Include:	ser.a or stdlib.a

Routine:  ComTstIn
------------------

Author:		      	Randall Hyde

Category:             	Serial Communications

Registers on entry:   	None

Registers on return:  	AL=0 if no char available, 1 if char available

Flags affected:       	None

Example of Usage:

		Wait4Data:	ComTstIn
				cmp	al, 0
				je	Wait4Data

Description:

ComTstIn polls the port to see if any input data is available.  If so,
it returns a one in AL, else it returns a zero.

Warning: do *not* use this routine while operating in the interrupt mode.
This routine is for polled I/O only.

Include:	ser.a or stdlib.a

Routine:  ComTstOut
-------------------

Author:		      	Randall Hyde

Category:             	Serial Communications

Registers on entry:   	None

Registers on return:  	AL = 1 if xmitr available, 0 if not

Flags affected:       	None

Example of Usage:

		WriteData:	<Do Something>
				ComTstOut
				cmp	al, 0
				je	WriteData
				mov	al, 'a'
				ComWrite

Description:

ComTstIn polls the port to see if the transmitter is currently busy.  If so,
it returns a zero in AL, else it returns a one.

Warning: do *not* use this routine while operating in the interrupt mode.
This routine is for polled I/O only.

Include:	ser.a or stdlib.a

Routine:  ComGetLSR
-------------------

Author:		      	Randall Hyde

Category:             	Serial Communications

Registers on entry:   	None

Registers on return:  	AL = LSR value

Flags affected:       	None

Example of Usage:

			ComGetLSR
			<do something with value in LSR>
Description:

Reads the LSR (line status register) and returns this value in AL.  The
LSR using the following layout.

 Line Status Register (LSR):

 bit 0-	Data Ready
 bit 1-	Overrun error
 bit 2-	Parity error
 bit 3-	Framing error
 bit 4-	Break Interrupt
 bit 5-	Transmitter holding register is empty.
 bit 6-	Transmit shift register is empty.
 bit 7-	Always zero.

Warning: In general, it is not a good idea to call this routine while
the interrupt system is active.  It won't hurt anything, but the value
you get back may not reflect properly upon the last/next character you
read.

Include:	ser.a or stdlib.a

Routine:  ComGetMSR
-------------------

Author:		      	Randall Hyde

Category:             	Serial Communications

Registers on entry:   	None

Registers on return:  	AL = MSR value

Flags affected:       	None

Example of Usage:

			ComGetMSR
			<do something with value in MSR>
Description:

The MSR (modem status register) bits are defined as follows:

 Modem Status Register (MSR):

 bit 0-	Delta CTS
 bit 1-	Delta DSR
 bit 2-	Trailing edge ring indicator
 bit 3-	Delta carrier detect
 bit 4-	Clear to send
 bit 5-	Data Set Ready
 bit 6-	Ring indicator
 bit 7-	Data carrier detect


Warning: In general, it is not a good idea to call this routine while
the interrupt system is active.  It won't hurt anything, but the value
you get back may not reflect properly upon the last/next character you
read.