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\ Lesson 11 - Terminal Program Using Interrupts

\ The Forth Course

\ by Richard E. Haskell

\ Dept. of Computer Science and Engineering

\ Oakland University, Rochester, MI 48309

comment:

Lesson 11

TERMINAL PROGRAM USING INTERRUPTS

11.1 8086/8088 INTERRUPTS 11-2

11.2 THE 8250 ACE CHIP 11-3

11.3 A QUEUE DATA STRUCTURE 11-5

11.4 SENDING CHARACTERS TO THE SCREEN

AND/OR TO DISK 11-8

11.5 DOWNLOADING FILES 11-10

11.6 MAIN TERMINAL PROGRAM 11-12

11.1 8086/8088 INTERRUPTS

In this lesson we will write a terminal program using interrupts that we can use to communicate with other computers or to download Forth code to Forth chips such as the 68HC11 single-chip microcomputer that contains Max-Forth.

We will want to communicate at baud rates up to 9600 baud. This means that we will need to use interrupts to store incoming characters without losing them while scrolling the screen. We will write an interrupt service routine that is called every time a character is received in the serial port. This interrupt service routine will read the character and store it in a queue. The main terminal program will then alternate between checking the keyboard for key pressings and checking the queue for received characters. When a key is pressed the character will be transmitted out the serial port. When a character is in the queue (i.e. has been received in the serial port) it will be displayed on the screen and, optionally, stored on disk.

The segment and offset addresses of the interrupt service routine must be stored in the interrupt vector table at the beginning of segment zero in memory. The DOS functions 25H (set interrupt vector) and 35H (get interrupt vector) can be used for this purpose. The following Forth words make this easy.

comment;

PREFIX HEX

\ Get interrupt vector

CODE get.int.vector ( int# -- seg offset )

POP AX

PUSH ES

PUSH BX \ AL = interrupt number

MOV AH, # 35 \ DOS service 35H

INT 21 \ ES:BX = segment:offset

MOV DX, ES \ of interrupt handler

MOV AX, BX

POP BX

POP ES

2PUSH

END-CODE

\ Set interrupt vector

CODE set.int.vector ( segment offset int# -- )

POP AX \ AL = interrupt number

POP DX \ DX = offset addr

POP BX \ BX = segment addr

MOV AH, # 25 \ DOS service 25H

PUSH DS \ save DS

MOV DS, BX \ DS:DX -> int handler

INT 21 \ DOS INT 21H

POP DS \ restore DS

END-CODE

\ Store interrupt vector of routine at addr

: store.int.vector ( addr int# -- )

?CS: -ROT set.int.vector ;

\ We will need words from Lessons 7, 8 and 10. Therefore,

\ we will FLOAD these files.

DECIMAL

fload lesson7

fload lesson8

fload lesson10

comment:

11.2 THE 8250 ACE CHIP

Serial communication is handled by the 8250 Asynchronous Communication Element (ACE) chip. The interrupt line from this chip goes to IRQ4 of the Priority Interrupt Controller (PIC) chip for COM1 and to IRQ3 of the PIC for COM2. OUT2 of the modem control register of the 8250 must be set to enable the output buffer of the 8250 IRQ line.

comment;

HEX

300 CONSTANT COM1 \ base address for COM1

200 CONSTANT COM2 \ base address for COM2

0C CONSTANT INT#1 \ interrupt number for COM1

0B CONSTANT INT#2 \ interrupt number for COM2

EF CONSTANT ENABLE4 \ interrupt 4 enable mask

10 CONSTANT DISABLE4 \ interrupt 4 disable mask

F7 CONSTANT ENABLE3 \ interrupt 3 enable mask

08 CONSTANT DISABLE3 \ interrupt 3 disable mask

\ Default COM1

COM1 VALUE COM \ current COM base address

INT#1 VALUE INT# \ interrupt # for current COM

ENABLE4 VALUE ENABLE \ enable mask for current COM

DISABLE4 VALUE DISABLE \ disable mask for current COM

\ The following values are added to the base COM address to obtain

\ the corresponding register addresses:

F8 CONSTANT txdata \ transmit data reg (write only)

F8 CONSTANT recdat \ receive data reg (read only)

FC CONSTANT mcr \ modem control reg

F9 CONSTANT ier \ interrupt enable reg

FD CONSTANT lsr \ line status reg

21 CONSTANT imask \ mask reg in PIC

20 CONSTANT eoi \ end of int value

20 CONSTANT ocw2 \ PIC ocw2

VARIABLE int.vec.addr \ save int vector offset address

VARIABLE int.vec.seg \ save int vector segment address

DECIMAL

\ We will use the BIOS INT 14H (20 decimal) initialize communications

\ port routine (AH = 0) to set the baud rate. This MUST be done

\ before the modem control register bits are set to enable interrupts

\ because the INT 14H call will undo them!

\ The following table contains the control register masks

\ for baud rates of 300, 1200, 2400, 4800 and 9600

\ with no parity, 8 data bits and 1 stop bit.

CREATE baud.table 67 , 131 , 163 , 195 , 227 ,

\ Index Baud rate

\ 0 300

\ 1 1200

\ 2 2400

\ 3 4800

\ 4 9600

CODE INIT-COM ( mask -- )

POP AX

MOV AH, # 0

MOV DX, # 0

INT 20

END-CODE

\ Default 9600 baud

\ Modify this word if you want to change the baud rate from the screen.

: get.baud# ( -- n )

4 ;

: set.baud.rate ( -- )

get.baud# 2*

baud.table + @

INIT-COM ;

comment:

11.3 A QUEUE DATA STRUCTURE

A circular queue will be used to store the received characters in an interrupt service routine

The following pointers are used to define the queue

comment;

VARIABLE front \ pointer to front of queue (oldest data at front+1)

VARIABLE rear \ pointer to rear of queue (most recent data at rear)

VARIABLE qmin \ pointer to first byte in queue

VARIABLE qmax \ pointer to last byte in queue

VARIABLE qbuff.seg \ segment of queue

10000 CONSTANT qsize \ size of queue in bytes

\ Initialize queue

: initq ( -- )

qsize alloc.mem qbuff.seg ! \ allocate memory for queue

0 front ! \ front = 0

0 rear ! \ rear = 0

0 qmin ! \ qmin = 0

qsize 1- qmax ! ; \ qmax = qsize - 1

\ Check queue

: checkq ( -- n tf | ff )

front @ rear @ <> \ if front = rear

IF \ then empty

INLINE

CLI \ disable interrupts

END-INLINE

1 front +! \ inc front

front @ qmax @ > \ if front > qmax

qmin @ front ! \ then front = qmin

THEN

qbuff.seg @ front @ C@L \ get byte

TRUE \ set true flag

INLINE

STI \ enable interrupts

END-INLINE

ELSE

FALSE \ set false flag

THEN ;

\ Store byte in AL in queue

LABEL qstore

PUSH SI

PUSH ES

MOV SI, qbuff.seg

MOV ES, SI \ ES = qbuff.seg

INC rear WORD \ inc rear

MOV SI, rear \ if rear > qmax

CMP SI, qmax

JBE 2 $

MOV SI, qmin \ then rear = qmin

MOV rear SI

2 $: CMP SI, front \ if front = rear

JNE 4 $ \ then full

DEC SI \ dec rear

CMP SI, qmin \ if rear < qmin

JAE 3 $ \ then rear = qmax

MOV SI, qmax

MOV rear SI

3 $: POP ES

POP SI

RET

4 $: MOV ES: 0 [SI], AL \ else store at rear

POP ES

POP SI

RET

END-CODE

\ Interrupt service routine

\ This routine gets data from the receive serial port

\ and stores it in the queue.

LABEL INT.SRV ( -- )

PUSH AX

PUSH DX

PUSH DS

MOV AX, CS

MOV DS, AX \ DS = CS

MOV DX, # COM \ if data is ready

ADD DX, # lsr

IN AL, DX

TEST AL, # 1

JE 1 $

MOV DX, # COM

ADD DX, # recdat

IN AL, DX \ read it

CALL qstore

1 $: MOV AL, # eoi

MOV DX, # ocw2

OUT DX, AL \ clear eoi

POP DS

POP DX

POP AX

IRET

END-CODE

\ Set up interrupts

: int.setup ( -- )

12 COM mcr + PC! \ modem cr out2 lo

1 COM ier + PC! \ enable recv int

INT# get.int.vector \ save old int vector

int.vec.addr ! int.vec.seg !

INT.SRV INT# store.int.vector ; \ set new int vector

\ Terminal initialization routine

: init.term ( -- )

initq \ initialize queue

int.setup \ set up interrupts

imask PC@

ENABLE AND \ enable irq4 (COM1 default)

imask PC! ;

: disable.term ( -- )

imask PC@

DISABLE OR \ disable irq4 (COM1 default)

imask PC!

0 COM mcr + PC! \ 0 -> modem control reg

int.vec.seg @ \ restore original

int.vec.addr @ \ interrupt vector

INT# set.int.vector ;

comment:

11.4 SENDING CHARACTERS TO THE SCREEN AND/OR TO DISK

Characters in the queue will be displayed on the screen and, optionally, sent to a disk file.

comment;

FALSE VALUE ?>disk \ flag to "send to disk"

0 VALUE col.at \ saved cursor position

0 VALUE row.at

VARIABLE t_handle \ terminal file handle

CREATE edit_buff 70 ALLOT \ temporary edit buffer

: $HCREATE ( addr -- f ) \ create file for counted string at addr

SEQHANDLE HCLOSE DROP

SEQHANDLE $>HANDLE

SEQHANDLE HCREATE ;

: file.open.error ( -- )

33 12 65 14 box&fill

." Could not open file!!"

KEY DROP ;

\ The following word will display a window on the screen in which

\ to enter a filename, which will then be opened. Data coming in

\ the serial port will be sent to this file. This word will be

\ called by pressing function key F1.

: select.nil.file ( -- )

20 4 60 7 box&fill

." Enter a filename"

" " ">$

edit_buff OVER C@ 1+ CMOVE

21 6 edit_buff 30 lineeditor

edit_buff $HCREATE

file.open.error

ELSE

SEQHANDLE >HNDLE @

DUP handl ! t_handle !

TRUE !> ?>disk

THEN

THEN ;

: >term ( -- )

t_handle @ handl ! ;

\ Pressing function key F1 will turn 'data capture' on

: disk.on.nil ( -- )

IBM-AT? !> row.at !> col.at

SAVESCR

select.nil.file

RESTSCR

col.at row.at AT ;

\ Pressing function key F2 will turn 'data capture' off

: disk.off ( -- )

t_handle @ ?DUP

close.file

0 t_handle !

THEN

FALSE !> ?>disk ;

\ Transmit ascii code out serial port

: XMT ( ascii -- )

COM \ use base address in COM

BEGIN

DUP lsr + \ wait for bit 5 in line status

PC@ 32 AND \ register (TDRE) to be set

UNTIL

txdata + PC! ; \ send data

\ Pressing CTRL P will toggle the printer on and off

: ?PRINT ( -- )

PRINTING C@ NOT PRINTING C! ;

\ Send character to the screen

: do.emit ( n -- )

DUP 13 = \ if CR

DROP CR \ do a carriage return

ELSE

DUP 8 = \ keep backspace

OVER 32 >= OR \ ignore other control characters

EMIT

ELSE

DROP

THEN

THEN ;

: ?EMIT ( n -- )

127 AND \ mask parity bit

DUP 13 = \ ignore control char

OVER 10 = OR \ other than CR and LF

OVER 8 = OR \ or backspace

OVER 32 >= OR

?>disk \ if data capture on

DUP >term send.byte \ send to disk

THEN

do.emit \ send to screen

ELSE

DROP

THEN ;

comment:

11.5 DOWNLOADING FILES

The following words can be used to download a file containing MaxForth code to the 68HC11. MaxForth will load a line at a time, compiling the words in the dictionary. After loading a line it will send a line-feed (ASCII 10) back to the PC.

comment;

VARIABLE wait.count

\ Transmit a string given its address and length

: xmt.str ( addr cnt -- ) \ XMT string + CR

0 DO

DUP I + C@

DUP do.emit XMT

LOOP

DROP

13 XMT ;

\ Wait for one of two particular characters to be received

: wait.for ( ascii -- )

0 wait.count !

BEGIN

checkq \ asc n tf | asc ff

IF \ asc n | asc

DUP ?EMIT \ asc n

OVER = \ asc f

0 wait.count !

ELSE

1 wait.count +! FALSE \ char ff

THEN

FALSE \ wait.count @ 32000 = \ char f f

CONTROL G EMIT 2DROP \ beep -- no response

CR ." No response..."

KEY DROP

2R> 2DROP \ exit wait.for

2R> 2DROP \ exit file.download

EXIT \ exit DO-KEY

THEN

UNTIL

DROP ;

\ Empty the queue buffer

: mt.qbuff ( -- )

BEGIN

checkq

WHILE

?EMIT

REPEAT ;

\ Download a file to the 68HC11

: file.download ( -- )

GETFILE

DARK

$HOPEN

file.open.error

ELSE

." File: " .SEQHANDLE CR

SEQHANDLE >HNDLE @ handl !

BEGIN

get.next.byte

eof @ NOT

WHILE

DUP XMT

BEGIN

CHECKQ

UNTIL

?EMIT

13 = \ IF CR, THEN WAIT FOR

IF \ LF TO BE SENT BACK

\ GET.NEXT.BYTE DROP \ DROP LF

BEGIN

CHECKQ

UNTIL

DUP DUP ?EMIT

10 = SWAP 63 = OR

UNTIL

THEN

mt.qbuff \ empty qbuff

REPEAT

THEN

ELSE

2R> 2DROP

EXIT \ exit DO-KEY

THEN ;

: s-rec.dwload ( -- )

GETFILE

DARK

$HOPEN

file.open.error

ELSE

." File: " .SEQHANDLE CR

SEQHANDLE >HNDLE @ handl !

" LOAD T" xmt.str

BEGIN

handl @

1 PAD read.file

WHILE

PAD C@ DUP XMT

do.emit

mt.qbuff \ empty qbuff

REPEAT

THEN

ELSE

2R> 2DROP

EXIT \ exit DO-KEY

THEN ;

comment:

11.6 MAIN TERMINAL PROGRAM

Pressing the ESC key will exit the terminal word HOST

comment;

: ESC.HOST ( -- )

disable.term \ disable all interrupts

disk.off \ close file if necessary

qbuff.seg @ release.mem \ release queue buffer

DARK