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Help with COM1 serial port interrupt in x86 assembly

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Robert Pengelly

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Nov 11, 2023, 3:43:07 AM11/11/23
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Do anyone know how I get the COM1 serial port interrupt to work in a 16-bit operating system? I have the following:

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Disable all interrupts.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
mov dx, HEX (03F9)
xor al, al
out dx, al

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Enable DLAB (set baud rate divisor).
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
mov dx, HEX (03FB)
mov al, HEX (80)
out dx, al

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Set divisor.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
mov dx, HEX (03F8)
mov al, HEX (03) ; 3 (lo byte) 38400 baud.
out dx, al

mov dx, HEX (03F9)
xor al, al ; (hi byte)
out dx, al

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 8 bits, no parity, one stop bit.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
mov dx, HEX (03FB)
mov al, HEX (03)
out dx, al

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Enable FIF0, clear them, with 14-byte threshold.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
mov dx, HEX (03FA)
mov al, HEX (C7)
out dx, al

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; IRQs enabled, RTS/DSR set.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
mov dx, HEX (03FC)
mov al, HEX (0B)
out dx, al

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Set in loopback mode, test the serial chip.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
mov dx, HEX (03FC)
mov al, HEX (1E)
out dx, al

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Test serial chip (send byte 0xAE and check if serial
;; returns same byte).
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
mov dx, HEX (03F8)
mov al, HEX (AE)
out dx, al

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Check if serial is faulty (i.e. not same byte as sent).
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
mov dx, HEX (03F8)
xor ax, ax
in al, dx

cmp al, HEX (AE)
jne .bad

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; If serial is not faulty set it in normal operation mode
;; (not-loopback with IRQs enabled and OUT#1 and OUT#2 bits enabled).
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
mov dx, HEX (03FC)
mov al, HEX (0F)
out dx, al

which as far as I can tell is the assembly equivalent of the example shown towards the bottom of https://wiki.osdev.org/Serial_Ports but I can't figure out how to get an interrupt working.

Branimir Maksimovic

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Nov 11, 2023, 7:51:06 PM11/11/23
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You have to tell PIC and UART to trigger interrupt. Just out instructions, I guess that now since there
is internet, you can find info easilly. Last time I programmed that was 1994...


--

7-77-777, Evil Sinner!
https://www.linkedin.com/in/branimir-maksimovic-6762bbaa/

Borax Man

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Nov 11, 2023, 8:21:28 PM11/11/23
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On Sun, 12 Nov 2023 00:30:40 GMT
Branimir Maksimovic <branimir....@icloud.com> wrote:

> You have to tell PIC and UART to trigger interrupt. Just out instructions, I guess that now since there
> is internet, you can find info easilly. Last time I programmed that was 1994...

Yes, you will need to write an interrupt handler which gets called
when the interrupt is triggered. I haven't done this for a serial
port, but an example of an interrupt handler I wrote using the PC
timer is below. Its a bit more complex than it needs to be, because
it also makes sound through the PC speaker.

OurInt08 is the interrupt handler.

This section saves the old interrupts then sets the interrupt vector to point to our handler. This is just for demonstration purposes.

cli ;Turn off interrupts!
mov word ax, [es:08h*4]
mov word [cs:OldInt8], ax
mov word ax, [es:08h*4 + 2]
mov word [cs:OldInt8+2], ax

mov ax, OurInt08
mov word [es:08h*4], ax
mov ax,cs
mov word [es:(08h*4)+2], ax

Then don't forget to restore the old hander...


mov ax, word [cs:OldInt8]
mov word [es:08h*4], ax
mov word ax, word [cs:OldInt8+2]
mov word [es:08h*4 + 2], ax


The full program is below and it will compile with FASM.


; A working example of how to use an interrupt handler
; in DOS. This source file is for FASM.
;

FORMAT MZ
; DOS 16-bit EXE format
STACK 400h
HEAP 0 ; No extra heap required.
ENTRY CODE:Init

; Quick example of using interrupts and chaning the PIT
; tick rate
use16

freq = 42940
inputlen = 160

SEGMENT CODE USE16

Init:
mov ax,DATA2
mov ds,ax
mov fs,ax ; Set FS to point to data segment.
; DS will change during the program
; but FS won't as DOS doesn't touch this.
mov ax, oldscreen ; Create far pointer.
mov word [dptr],ax
mov word [dptr+2],ds

mov ah,09h
mov dx,intro
int 21h ; Print introduction message.

mov cx,inputlen ; Input length
xor ax,ax
mov ah,3fh
mov bx,0
mov dx,input ; Where to store
int 21h
jc endp
cmp ax,8
jg inputerror ; Anything more than 6 characters we reject

mov bx, ax ; BX will store the offset as we go through the input data.
cmp [bx+input-1],0ah ; If this is not the last character
; It means we didn't fill all the input
; and some may be lost. Best to end with
; an error. Besides, if it was a valid
; number, it would have been well over the
; maximum number of stars we could handle.
jne inputerror

xor bx, bx ; Start index at 0.
sub ax, 2 ; We don't want the CR/LF duo at the end.
mov cx, ax ; CX will be used to compare the index
; to determine whether we have reached the end of the user data
; we want to convert.
xor ax, ax ; Start tally at 0.
mov si, 10 ; Multiply each number we get by 10, as we push it up
; as a significant digit.
mov di, input
.getnum:
cmp byte [di+bx], '0' ; If less than '0', its not a numeral.
jl inputerror
cmp byte [di+bx], '9' ; If more than '9', its not a numeral.'
jg inputerror
sub byte [di+bx], 30h ; Sub 30h to convert the ascii numeral
; to the number
mul si ; Multiply what we have added so far by 10
push dx
mov dl, byte [di+bx]
add ax,dx ; And add the next digit
pop dx
inc bx
cmp bx, cx
jl .getnum ; If more digits, go again.
mov cx, ax
begindemo:
mov al, 0x36
out 0x43, al ;tell the PIT which channel we're setting
mov ax, cx
out 0x40, al ;send low byte
mov al, ah
out 0x40, al ;send high byte
xor dx,dx
mov es,dx

cli ;Turn off interrupts!
mov word ax, [es:08h*4]
mov word [cs:OldInt8], ax
mov word ax, [es:08h*4 + 2]
mov word [cs:OldInt8+2], ax

mov ax, OurInt08
mov word [es:08h*4], ax
mov ax,cs
mov word [es:(08h*4)+2], ax
mov ax, 0B800h
mov es,ax
call savescreen
; The interrupt may update the screen prior to us saving it.
call soundon
sti

inputloop:

mov cx,inputlen ; Input name
mov ax,3f00h ; Input call
xor bx,bx ; From keyboard (stdin)
mov dx,keybuff ; Save at keybuff
int 21h
cmp [fs:keybuff],'q'
jne inputloop
xor dx,dx
mov es,dx
call soundonoff

mov es,dx
cli
mov ax, word [cs:OldInt8]
mov word [es:08h*4], ax
mov word ax, word [cs:OldInt8+2]
mov word [es:08h*4 + 2], ax

xor ax,ax
out 0x43, al ;tell the PIT which channel we're setting
out 0x40, al ;send low byte
out 0x40, al ;send high byte

call restorescreen

mov ax,DATA3
mov ds,ax
mov ax,0b800h
mov es,ax

mov ah,09h
mov dx,data3msg
int 21h ; Print introduction


mov cx,800
mov bx,1
loop55:
mov byte [es:bx],2
add bx,2
dec cx
jnz loop55
endp:
mov ax,4c00h ; Send exit code to dos
int 21h ; Send command to DOS

soundon:
push ax
in al,61h ;-Bits D1-D0 of "port 61h" are unmasked
or al,03h ; or masked !
out 61h,al
pop ax
ret
soundonoff:
push ax
in al,61h ;-Bits D1-D0 of "port 61h" are unmasked
xor al,03h ; or masked !
out 61h,al
pop ax
ret


OurInt08:
; This interrupt handler could be called in any context
; so we cannot assume anything about the state of the registers
; including the segment registers. As we are calling for user input
; this is likely to be called during DOS function 3F, and DOS
; may very well have set DS and ES to different values.
pusha
pushf
add word [fs:elapsed], tickrate
jnc skipint

cli
mov al,0b6h
mov dx, 043h
out dx,al

mov ax,[fs:elapsed]
mov dx, 042h
out dx,al
mov al, ah
out dx,al
sti
popf
popa
jmp far [cs:OldInt8] ; using INT 08H emulation
skipint:
push 0b800h
pop gs
mov bx,(160*20)-2
loop1:
inc word [gs:bx]
sub bx,2
jnc loop1
ll2:
mov al,20h ; EOI code for PIC
out 20h,al ; Send
popf
popa
iret

savescreen:
push ax
mov cx,2000-2
xor si,si
les di, [dptr]
cld
push 0b800h
pop ds

rep movsw
push ds
pop es
mov ax,DATA2
mov ds,ax

pop ax
ret

restorescreen:
cld
push ds
mov ax,DATA2
mov ds,ax
push 0b800h
pop es
push ax
mov cx,2000-2
lds si, [dptr] ; This is unnecessary, we don't need to store the pointer
; But I wanted to use the lds and les commands as an example
xor di,di
rep movsw
pop ax
pop ds
ret
inputerror:
mov dx,invalid ;Tell user about invalid input
mov ah,09h
int 21h
mov ah,10h
int 16h ; Wait for keypress
jmp begindemo


OldInt8 dd 0


SEGMENT DATA3 USE16
data3msg db "Now we are at the next part of the demonstration."
db 13,10,"I will change some text colours.",13,10,24h


SEGMENT DATA2 USE16
invalid db "Invalid input or number too high. ",\
"Using default number of stars.",0ah,0dh,\
"Press any key.",0ah,0dh,24h

numstars dw 10000
elapsed dw 0
intro db "Interrupt handler demo, by Borax Man",13,10
db "September, 2020",13,10
db 13,10,"Type 'q' and hit enter to exit while demo is running.",13,10,10
db "Enter a ticrate between 0 - 65536",13,10
db "Numbers lower than 3000 may cause problems on older machines",13,10,24h
input rb 10

keybuff rb inputlen ; This must be immediately after message
dptr: rb 4
oldscreen: rb 4000


SEGMENT ENDSEG


--

Branimir Maksimovic

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Nov 12, 2023, 10:21:21 PM11/12/23
to
On 2023-11-12, Borax Man <rotf...@nospicedham.hotmail.com> wrote:
> On Sun, 12 Nov 2023 00:30:40 GMT
> Branimir Maksimovic <branimir....@icloud.com> wrote:
>
>> You have to tell PIC and UART to trigger interrupt. Just out instructions, I guess that now since there
>> is internet, you can find info easilly. Last time I programmed that was 1994...
>
> Yes, you will need to write an interrupt handler which gets called
> when the interrupt is triggered. I haven't done this for a serial
> port, but an example of an interrupt handler I wrote using the PC
> timer is below. Its a bit more complex than it needs to be, because
> it also makes sound through the PC speaker.
>

Here it is nicely explained;
https://www.activexperts.com/serial-port-component/tutorials/uart/

Terje Mathisen

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Nov 13, 2023, 10:37:14 AM11/13/23
to
Branimir Maksimovic wrote:
> On 2023-11-12, Borax Man <rotf...@nospicedham.hotmail.com> wrote:
>> On Sun, 12 Nov 2023 00:30:40 GMT
>> Branimir Maksimovic <branimir....@icloud.com> wrote:
>>
>>> You have to tell PIC and UART to trigger interrupt. Just out instructions, I guess that now since there
>>> is internet, you can find info easilly. Last time I programmed that was 1994...
>>
>> Yes, you will need to write an interrupt handler which gets called
>> when the interrupt is triggered. I haven't done this for a serial
>> port, but an example of an interrupt handler I wrote using the PC
>> timer is below. Its a bit more complex than it needs to be, because
>> it also makes sound through the PC speaker.
>>
>
> Here it is nicely explained;
> https://www.activexperts.com/serial-port-component/tutorials/uart/
>

Borax Man is lucky indeed!

I first had to implement this stuff in 1982, writing the interrupt
handler code inside DEBUG.COM and then copy the generated bytes into my
mainline (Modula2) program.

I repeated that exact process a year or two later with Turbo Pascal 1.0.

Having a proper 16-byte buffered uart makes it all much simpler!

Terje

--
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"

Borax Man

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Nov 14, 2023, 7:35:05 AM11/14/23
to
On 2023-11-13, Terje Mathisen <terje.m...@nospicedham.tmsw.no> wrote:
> Branimir Maksimovic wrote:
> Borax Man is lucky indeed!
>
> I first had to implement this stuff in 1982, writing the interrupt
> handler code inside DEBUG.COM and then copy the generated bytes into my
> mainline (Modula2) program.
>
> I repeated that exact process a year or two later with Turbo Pascal 1.0.
>
> Having a proper 16-byte buffered uart makes it all much simpler!
>
> Terje
>

My first experience with assembly was typing in assembly source code
listings from Australian Personal Computer magazine. They were
essentially debug scripts. You'd feed it into debug and it would
create a .COM file. Had little idea what the code actually meant, but
I sought information on how to write my own 'debug scripts' so I could
create my own machine language programs.

Then I found MASM and it made things quite a bit easier.

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