Magnum One/Magnum One (Mid-American Digital) (Disc Manufacturing).iso/d8/pdriver5.arc/SLIP8250.ASM

Assembly Source File | 1989-12-17 | 24KB | 948 lines

version    equ    3

    include    defs.asm

;Ported from Phil Karn's asy.c and slip.c, a C-language driver for the IBM-PC
;8250 by Russell Nelson.  Any bugs are due to Russell Nelson.

;  Copyright, 1988, 1989, Russell Nelson

;   This program 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, version 1.
;
;   This program is distributed in the hope that it will be useful,
;   but WITHOUT ANY WARRANTY; without even the implied warranty of
;   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;   GNU General Public License for more details.
;
;   You should have received a copy of the GNU General Public License
;   along with this program; if not, write to the Free Software
;   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.

code    segment    byte public
    assume    cs:code, ds:code

;8250 definitions
;Control/status register offsets from base address
THR    equ    0        ;Transmitter holding register
RBR    equ    0        ;Receiver buffer register
DLL    equ    0        ;Divisor latch LSB
DLM    equ    1        ;Divisor latch MSB
IER    equ    1        ;Interrupt enable register
IIR    equ    2        ;Interrupt ident register
FCR    equ    2        ;16550 FIFO control register
LCR    equ    3        ;Line control register
MCR    equ    4        ;Modem control register
LSR    equ    5        ;Line status register
MSR    equ    6        ;Modem status register

;8250 Line Control Register
LCR_5BITS    equ    0    ;5 bit words
LCR_6BITS    equ    1    ;6 bit words
LCR_7BITS    equ    2    ;7 bit words
LCR_8BITS    equ    3    ;8 bit words
LCR_NSB        equ    4    ;Number of stop bits
LCR_PEN        equ    8    ;Parity enable
LCR_EPS        equ    10h    ;Even parity select
LCR_SP        equ    20h    ;Stick parity
LCR_SB        equ    40h    ;Set break
LCR_DLAB    equ    80h    ;Divisor Latch Access Bit

;16550 Line Control Register
FCR_RCVR    equ    002h    ;Reset Receive FIFO
FCR_XMIT    equ    004h    ;Reset Transmit FIFO
FCR_1        equ    001h    ;One byte FIFO
FCR_4        equ    041h    ;Four byte FIFO
FCR_8        equ    081h    ;Eight byte FIFO
FCR_14        equ    0c1h    ;Fourteen byte FIFO

;8250 Line Status Register
LSR_DR    equ    1    ;Data ready
LSR_OE    equ    2    ;Overrun error
LSR_PE    equ    4    ;Parity error
LSR_FE    equ    8    ;Framing error
LSR_BI    equ    10h    ;Break interrupt
LSR_THRE equ    20h    ;Transmitter line holding register empty
LSR_TSRE equ    40h    ;Transmitter shift register empty

;8250 Interrupt Identification Register
IIR_IP        equ    1    ;0 if interrupt pending
IIR_ID        equ    6    ;Mask for interrupt ID
IIR_RLS        equ    6    ;Receiver Line Status interrupt
IIR_RDA        equ    4    ;Receiver data available interrupt
IIR_THRE    equ    2    ;Transmitter holding register empty int
IIR_MSTAT    equ    0    ;Modem status interrupt

;8250 interrupt enable register bits
IER_DAV    equ    1    ;Data available interrupt
IER_TxE    equ    2    ;Tx buffer empty interrupt
IER_RLS    equ    4    ;Receive line status interrupt
IER_MS    equ    8    ;Modem status interrupt

;8250 Modem control register
MCR_DTR    equ    1    ;Data Terminal Ready
MCR_RTS    equ    2    ;Request to Send
MCR_OUT1 equ    4    ;Out 1 (not used)
MCR_OUT2 equ    8    ;Master interrupt enable (actually OUT 2)
MCR_LOOP equ    10h    ;Loopback test mode

;8250 Modem Status Register
MSR_DCTS equ    1    ;Delta Clear-to-Send
MSR_DDSR equ    2    ;Delta Data Set Ready
MSR_TERI equ    4    ;Trailing edge ring indicator
MSR_DRLSD equ    8    ;Delta Rx Line Signal Detect
MSR_CTS equ    10h    ;Clear to send
MSR_DSR equ    20h    ;Data set ready
MSR_RI    equ    40h    ;Ring indicator
MSR_RLSD equ    80h    ;Received line signal detect

;Slip Definitions
FR_END        equ    0c0h        ;Frame End
FR_ESC        equ    0dbh        ;Frame Escape
T_FR_END    equ    0dch        ;Transposed frame end
T_FR_ESC    equ    0ddh        ;Transposed frame escape

    public    int_no
int_no        db    4,0,0,0        ; interrupt number.
io_addr        dw    03f8h,0        ; I/O address for COM1.
baud_rate    dw    12c0h,0        ; We support baud rates higher than 65535.
baudclk        label    word
        dd    115200        ;1.8432 Mhz / 16

    public    driver_class, driver_type, driver_name
driver_class    db    6,0,0,0        ;from the packet spec
driver_type    db    0,0,0,0        ;from the packet spec
driver_name    db    'SLIP8250',0    ;name of the driver.

  ifdef debug
    public recv_buf_size, recv_buf,    recv_buf_end, recv_buf_head
    public recv_buf_tail, recv_pkt_ready
  endif
recv_buf_size    dw    3000        ;receive buffer size
recv_buf    dw    ?        ;->receive buffer
recv_buf_end    dw    ?        ;->after end of buffer
recv_buf_head    dw    ?        ;->next character to get
recv_buf_tail    dw    ?        ;->next character to store
recv_pkt_ready    dw    0        ; flag indicating a packet is ready

  ifdef debug
    public send_buf
  endif
send_buf_size    dw    3000        ;send buffer size
send_buf    dw    ?        ;->send buffer
send_buf_end    dw    ?        ;->after end of buffer
send_buf_head    dw    ?        ;->next character to get
send_buf_tail    dw    ?        ;->next character to store

  ifdef debug
    public packet_sem, pkt_send_sem, xmit_time
  endif
packet_sem    dw    0        ; semaphore for    packets received
pkt_send_sem    dw    0        ; semaphore for    packets xmitted
asyrxint_cnt    dw    0        ; loop counter in asyrxint
xmit_time    dw    0        ; loop timer for asyrxint

    public    rcv_modes
rcv_modes    dw    4        ;number    of receive modes in our table.
        dw    0,0,0,rcv_mode_3


    public    send_pkt
;
; mod 7/25/89 John Grover
; - operates with interrupts on. Xmits one byte per interrupt
; - only turns transmitter buffer empty interrupt off when
; - all bytes of all packets are transmitted.

send_pkt:
;enter with ds:si -> packet, cx = packet length.
;exit with nc if ok, or else cy if error, dh set to error number.
;called from telnet layer via software interrupt
    assume    ds:nothing

    push    cs
    pop    es
    mov    di,send_buf_tail
    sti                ; enable interrupts

    mov    al,FR_END        ;Flush out any line garbage
    call    send_char

;Copy input to output, escaping special characters
send_pkt_1:
    lodsb
    cmp    al,FR_ESC        ;escape FR_ESC with FR_ESC and T_FR_ESC
    jne    send_pkt_2
    mov    al,FR_ESC
    call    send_char
    mov    al,T_FR_ESC
    jmp    short send_pkt_3
send_pkt_2:
    cmp    al,FR_END        ;escape FR_END with FR_ESC and T_FR_END
    jne    send_pkt_3
    mov    al,FR_ESC
    call    send_char
    mov    al,T_FR_END
send_pkt_3:
    call    send_char
    loop    send_pkt_1
    mov    al,FR_END        ;terminate it with a FR_END
    call    send_char
    mov    send_buf_tail,di

    inc    pkt_send_sem        ; increment the semaphore
    cmp    pkt_send_sem, 1        ; see if we need to enable
                    ; xmit buffer empty interrupt
    ja    send_pkt_end

;Enable modem status and (maybe) transmitter buffer empty interrupt.

    loadport
    setport    MSR
    mov    ah, IER_MS        ; always enable modem status interrupt
    in    al, dx            ; check if clear to send
    test    al, MSR_CTS
    jz    send_pkt_4        ; no - won't enable xmit buffer empty interrupt

    or    ah,IER_TxE        ; yes - enable xmit buffer empty interrupt

send_pkt_4:
    loadport
    setport    IER
    call    setbit            ; enable
    cli
send_pkt_end:
    clc
    ret

;
; mod 7/25/89 John Grover
; - utilizes buffer pointers to ascertain if the
; - buffer has room for    another character
;

send_char:
;stuff the character in al into the transmit buffer, but only if there
;is enough room, otherwise ignore the char.
    assume    ds:nothing

    cmp    di, send_buf_head    ;are we out of buffer?
    jne    send_char_ok        ;no - continue
    cmp    pkt_send_sem, 0        ;maybe - if no packets then no
    jne    send_char_1        ;if there are packets then yes

send_char_ok:
    stosb                ;store the char.
    cmp    di,send_buf_end        ;do we need to wrap around?
    jne    send_char_1        ;no.
    mov    di,send_buf        ;yes - reload with beginning.
send_char_1:
    ret


    public    get_address
get_address:
;get the address of the interface.
;enter with es:di -> place to get the address, cx = size of address buffer.
;exit with nc, cx = actual size of address, or cy if buffer not big enough.
    assume    ds:code
    mov    cx,0
    clc
    ret


    public    set_address
set_address:
;set the address of the interface.
;enter with es:di -> place to get the address, cx = size of address buffer.
;exit with nc, cx = actual size of address, or cy if buffer not big enough.
    assume    ds:nothing
    clc
    ret


rcv_mode_3:
;receive mode 3 is the only one we support, so we don't have to do anything.
    ret


    public    set_multicast_list
set_multicast_list:
;enter with es:di ->list of multicast addresses, cx = number of bytes.
;return nc if we set all of them, or cy,dh=error if we didn't.
    mov    dh,NO_MULTICAST
    stc
    ret


    public    get_multicast_list
get_multicast_list:
;return with nc, es:di ->list of multicast addresses, cx = number of bytes.
;return    cy, NO_ERROR if we don't remember all of the addresses ourselves.
;return cy, NO_MULTICAST if we don't implement multicast.
    mov    dh,NO_MULTICAST
    stc
    ret


    public    reset_interface
reset_interface:
;reset the interface.
    assume    ds:code
    ret


;called    when we    want to determine what to do with a received packet.
;enter with cx = packet length, es:di -> packet type.
    extrn    recv_find: near

;called after we have copied the packet into the buffer.
;enter with ds:si ->the packet, cx = length of the packet.
    extrn    recv_copy: near

    extrn    count_in_err: near
    extrn    count_out_err: near

    public    recv

;
; mod 7/25/89 John Grover
;
; - added code to check modem status change interrupt. If CTS is
; - low  turn off transmitter buffer empty interrupt. If CTS is
; - high turn it on.

recv:
;called from the recv isr.  All registers have been saved, and ds=cs.
;Upon exit, the interrupt will be acknowledged.
    assume    ds:code
recv_2:
    loadport
    setport    IIR
    in    al,dx            ;any interrupts at all?
    test    al,IIR_IP
    jne    recv_1            ;no.
    and    al,IIR_ID
    cmp    al,IIR_RDA        ;Receiver interrupt
    jne    recv_3
    call    asyrxint
    jmp    recv_2
recv_3:
    cmp    al,IIR_THRE        ;Transmit interrupt
    jne    recv_5
    call    asytxint
    jmp    recv_2
recv_5:
;process IIR_MSTAT here.
;  If CTS and packet ready then
;    enable the    transmit buffer empty interrupt
;  else
;    disable the transmit buffer empty interrupt
;
    cmp    al, IIR_MSTAT
    jne    recv_4
    setport    MSR            ; make sure of CTS status
    mov    ah, IER_TxE        ; get ready to alter xmit buffer empty interrupt
    in    al, dx
    test    al, MSR_CTS        ; is CTS bit set
    jz    recv_5_1        ; no - disable xmit buffer empty int
    cmp    pkt_send_sem, 0        ; yes - is there a packet to xmit
    jbe    recv_2            ; no - all done here
    setport    IER            ; yes - enable xmit buffer empty int
    call    setbit
    jmp    recv_2

recv_5_1:
    setport    IER
    call    clrbit
    jmp    recv_2

recv_4:
;process IIR_RLS here
recv_1:
    ret


;Process 8250 receiver interrupts
;
; mod 7/25/89 John Grover
; - this branches off when bps < 9600. See asyrxint_a.
; - Above 9600 bps we go into a loop to process a packet at
; - a time. If not data ready for a certain amount of time,
; - the process exits and waits for the next byte. This certain
; - amount of time to wait depends on the bps and CPU processor speed
; - and is determined in the initialization of the driver.
; - Upon receiving the FR_END character for the first frame in the
; - buffer a semaphore is set which tells recv_frame to run.

asyrxint:

    push    ds            ; get set up for the routine
    pop    es
        xor     bx, bx
        cmp     baud_rate, 9600         ; below 9600 we're strictly
        jbe     asyrxint_a              ; interrupt driven
        mov     bx, xmit_time
asyrxint_a:
    mov    di,recv_buf_tail
    xor    bp, bp            ; set flag to indicate 1st char
                    ; processed
        mov     si, packet_sem          ; optimization
        loadport
        mov     ah, LSR_DR

asyrxint_again:
    xor    cx, cx            ; initialize counter
    setport    LSR
asyrxint_in:
    in    al,dx            ; check for data ready
    test    al,LSR_DR
    jnz    asyrxint_gotit        ; yes - break out of loop
    inc    cx            ; no - increase loop counter
    cmp    cx, bx            ; timeout?
    jae    asyrxint_exit        ; yes - leave
    jmp    asyrxint_in        ; no - keep looping

asyrxint_gotit:
    setport    RBR
    in    al,dx

;Process incoming data;
; If buffer is full, we have no choice but
; to drop the character

    cmp    di,recv_buf_head    ; check for buffer collision
    jne    asyrxint_ok        ; none - continue
        or      si, si                  ; maybe - if there are packets
    jnz    asyrxint_exit        ; yes exit

asyrxint_ok:
    stosb

    cmp    di,recv_buf_end        ; did we hit the end of the buffer?
    jne    asyrxint_3        ; no.
    mov    di,recv_buf        ; yes - wrap around.

asyrxint_3:
    cmp    al,FR_END        ; might    this be    the end of a frame?
    jne    asyrxint_reset        ; no - reset flag and loop
        inc     si                      ; yes - indicate packet ready
        cmp     si, 1                   ; determine if semaphore is <> 1
        jne     asyrxint_chk_flg        ; yes - recv_frame must be active
        inc     recv_pkt_ready          ; no - set flag to start recv_frame

asyrxint_chk_flg:
        cmp     bp, 0                   ; was this the first char?
        jne     asyrxint_1              ; no - exit handler
asyrxint_reset:
    inc    bp            ; set 1st character flag
    jmp    asyrxint_again        ; get another character

asyrxint_exit:
asyrxint_1:
    mov    recv_buf_tail,di
        mov     packet_sem, si

    ret


; --------------------------------------------------------------
;
;  recv_exiting
;
    public    recv_exiting
recv_exiting:
        cmp     recv_pkt_ready, 1       ; is a packet ready?
        jne     recv_isr_exit           ; no - skip to end
    push    ax
    push    bx
    push    cx
    push    dx
    push    ds
    push    es
    push    bp
    push    di
    push    si
    push    cs            ; point ds properly
    pop    ds
    mov    recv_pkt_ready,    0    ; reset flag
        sti                             ; enable interrupts

    call    recv_frame

        cli
    pop    si
    pop    di
    pop    bp
    pop    es
    pop    ds
    pop    dx
    pop    cx
    pop    bx
    pop    ax
recv_isr_exit:
    ret


; --------------------------------------------------------------
;
;  recv_frame
;
; mod 7/25/89 John Grover
;
; - recv_frame now operates with interrupts on. It is triggered
; - by the recv_pkt_ready flag and continues until all bytes
; - in all packets in the buffer have been transmitted to the upper
; - layer.
;
  ifdef debug
    public recv_frame
  endif
recv_frame:
    cmp    packet_sem, 0        ; should we do this?
    jz    recv_frame_end        ; no - exit
    mov    si,recv_buf_head    ;process characters.
    xor    cx,cx            ;count up the size here.
recv_frame_1:

    call    recv_char        ;get a char.
    je    recv_frame_2        ;go if no more chars.
    cmp    al,FR_ESC        ;an escape?
    je    recv_frame_1        ;yes - don't count this char.
    inc    cx            ;no - count this one.
    jmp    recv_frame_1
recv_frame_2:

    jcxz    recv_frame_3        ;count zero? yes - just free the frame.
;we don't need to set the type because none are defined for SLIP.
    push    si            ;save si in case we reject it.
    push    bx
    mov    di,0            ;but we avoid any segment end bullshit.
    call    recv_find        ;look up our type.
    pop    bx
    pop    si

    mov    ax,es            ;is this pointer null?
    or    ax,di
    je    recv_frame_3        ;yes - just free the frame.

    push    cx
    push    es            ;remember where the buffer pointer is.
    push    di

    mov    si,recv_buf_head    ;process characters.
recv_frame_4:
    call    recv_char
    je    recv_frame_6        ;yes - we're all done.
    cmp    al,FR_ESC        ;an escape?
    jne    recv_frame_5        ;no - just store it.

    call    recv_char        ;get the next character.
    je    recv_frame_6
    cmp    al,T_FR_ESC
    mov    al,FR_ESC        ;assume T_FR_ESC
    je    recv_frame_5        ;yup, that's it    - store FR_ESC
    mov    al,FR_END        ;nope, store FR_END
recv_frame_5:
    stosb                ;store the byte.
    jmp    recv_frame_4
recv_frame_6:
    mov    recv_buf_head,si    ;we're skipped to the end.

    pop    si            ;now give the frame to the client.
    pop    ds
    pop    cx
    assume    ds:nothing

    call    recv_copy
    push    cs
    pop    ds
    assume    ds:code
    jmp    recv_frame_end

recv_frame_3:
    mov    recv_buf_head,si    ;remember the new starting point.
recv_frame_end:
    dec    packet_sem
    cmp    packet_sem, 0        ; are there more packets ready?
        ja      recv_frame              ; yes - execute again
    ret


; --------------------------------------------------------------
;
;  recv_char
;
; mod 7/25/89 John Grover
; - Now    uses buffer pointers to determine if there are
; - characters left.
;

recv_char:
;enter with si -> receive buffer, bx = receive count.  Wrap around if needed.
;return with nz, al = next char.  Return zr if there are no more chars in
;  this frame.
;
    lodsb
    cmp    si,recv_buf_end
    jb    recv_char_1
    mov    si,recv_buf
recv_char_1:
    mov    bx, recv_buf_tail
    cmp    si, bx
    je    recv_char_2
    cmp    al,FR_END
recv_char_2:
    ret


;Handle 8250 transmitter interrupts

; --------------------------------------------------------------
;
;  asytxint
;
; mod 7/25/89
; - Transmits one character and then exits. Upon last character
; - to transmit modem status and transmitter buffer empty interrupt
; - are disabled.
;

asytxint:

asytxint_2:

;    loadport            ;can we load another character?
;    setport    LSR
;    in    al,dx
;    test    al,LSR_THRE
;    je    asytxint_1        ;no.

    mov    si,send_buf_head    ; yes - load one
    lodsb                ;
    cmp    si,send_buf_end        ;have we hit the end yet?
    jne    asytxint_3        ;no.
    mov    si,send_buf        ;yes - wrap around
asytxint_3:
    loadport
    setport    THR
    out    dx,al
    mov    send_buf_head, si    ; store our location


    cmp    si, send_buf_tail    ; more to xmit?
    jne    asytxint_1        ; yes just exit

;No more characters to transmit -- disable transmit interrupts.

    setport    IER            ;Disable transmit and modem
    mov    ah,IER_TxE or IER_MS    ; status interrupts
    call    clrbit
    mov    pkt_send_sem, 0        ; indicate we're finished
asytxint_1:


       ret




;Set bit(s) in I/O port
setbit:
;enter with dx = port, ah = bit to set.
    in    al,dx
    or    al,ah
    out    dx,al
    ret


;Clear bit(s) in I/O port
clrbit:
;enter with dx = port, ah = bit to set.
    in    al,dx
    not    al            ;perform an and-not using DeMorgan's.
    or    al,ah
    not    al
    out    dx,al
    ret


;any code after this will not be kept after initialization.
end_resident    label    byte

    public    usage_msg
usage_msg    db    "usage: SLIP8250 packet_int_no [driver_class] [int_no] [io_addr] [baud_rate]",CR,LF
        db    "   [send_buf_size] [recv_buf_size]",CR,LF
        db    "   The driver_class should be SLIP, KISS, AX.25, or a number.",CR,LF,'$'

    public    copyright_msg
copyright_msg    db    "Packet driver for SLIP8250, version ",'0'+majver,".",'0'+version,CR,LF
        db    "Portions Copyright 1988 Phil Karn",CR,LF,'$'

approximate_msg    db    "Warning: This baud rate can only be approximated using the 8250",CR,LF
        db    "because it is not an even divisor of 115200",CR,LF,'$'

class_name    db    "Interface class ",'$'
kiss_name    db    "KISS",CR,LF,'$'
ax25_name    db    "AX.25",CR,LF,'$'
slip_name    db    "SLIP",CR,LF,'$'
int_no_name    db    "Interrupt number ",'$'
io_addr_name    db    "I/O port ",'$'
baud_rate_name    db    "Baud rate ",'$'
send_buf_name    db    "Send buffer size ",'$'
recv_buf_name    db    "Receive buffer size ",'$'
unusual_com1    db    "That's unusual!  Com1 (0x3f8) usually uses interrupt 4!",CR,LF,'$'
unusual_com2    db    "That's unusual!  Com2 (0x2f8) usually uses interrupt 3!",CR,LF,'$'

    extrn    set_recv_isr: near

;enter with si -> argument string, di -> word to store.
;if there is no number, don't change the number.
    extrn    get_number: near

;enter with si -> argument string.
;skip spaces and tabs.  Exit with si -> first non-blank char.
    extrn    skip_blanks: near


    public    parse_args
parse_args:
    call    skip_blanks
    mov    al,[si]            ;now classify the argument.
    or    al,20h            ;convert to lower case (assuming letter).
    cmp    al,'k'
    jne    parse_args_2
    mov    driver_class,10        ;KISS, from packet spec.
    mov    dx,offset kiss_name
    jmp    short parse_args_1
parse_args_2:
    cmp    al,'s'
    jne    parse_args_3
    mov    driver_class,6        ;SLIP, from packet spec.
    mov    dx,offset slip_name
    jmp    short parse_args_1
parse_args_3:
    cmp    al,'a'
    jne    parse_args_4
    mov    driver_class,9        ;AX.25, from packet spec.
    mov    dx,offset ax25_name
    jmp    short parse_args_1
parse_args_4:
    mov    di,offset driver_class
    mov    bx,offset class_name
    call    get_number
    jmp    short parse_args_6
parse_args_1:
    push    dx
    mov    dx,offset class_name
    mov    ah,9
    int    21h
    pop    dx
    mov    ah,9
    int    21h
parse_args_5:
    mov    al,[si]            ;skip to the next blank or CR.
    cmp    al,' '
    je    parse_args_6
    cmp    al,CR
    je    parse_args_6
    inc    si            ;skip the character.
    jmp    parse_args_5
parse_args_6:
    mov    di,offset int_no
    mov    bx,offset int_no_name
    call    get_number
    mov    di,offset io_addr
    mov    bx,offset io_addr_name
    call    get_number
    cmp    io_addr,03f8h        ;is this com1?
    jne    ia_com2
    mov    dx,offset unusual_com1
    cmp    int_no,4        ;com1 usually uses int 1.
    jne    ia_unusual
    jmp    short ia_usual
ia_com2:
    cmp    io_addr,02f8h        ;is this com2?
    jne    ia_usual        ;no.
    mov    dx,offset unusual_com2
    cmp    int_no,3        ;com2 usually uses int 3.
    je    ia_usual
ia_unusual:
    mov    ah,9
    int    21h
ia_usual:
    mov    di,offset baud_rate
    mov    bx,offset baud_rate_name
    call    get_number
    mov    di,offset send_buf_size
    mov    bx,offset send_buf_name
    call    get_number
    mov    di,offset recv_buf_size
    mov    bx,offset recv_buf_name
    call    get_number
    ret


; --------------------------------------------------------------
;
;  etopen
;
; mod 7/25/89 John Grover
; - Contains a loop to determine a pseudo timeout for asyrxint.
; - The value is determined by transmitting characters in a
; - loop whose clock cycles are nearly the same as the "sister"
; - loop in asyrxint. The per character, maximum time used
; - basis which is then multiplied by a factor to achieve a timeout
; - value for the particular bps and CPU speed of the host.

    public    etopen
etopen:
    pushf
    cli

    loadport            ;Purge the receive data buffer
    setport    RBR
    in    al,dx

    ;Enable FIFO, set FIFO trigger level to 8
    mov    al,FCR_8
    setport    FCR
    out    dx,al

    ;Set line control register: 8 bits, no parity
    mov    al,LCR_8BITS
    setport    LCR
    out    dx,al

    ;Turn on receive interrupt enable in 8250, leave transmit
    ; and modem status interrupts turned off for now
    mov    al,IER_DAV
    setport    IER
    out    dx,al

    ;Set modem control register: assert DTR, RTS, turn on 8250
    ; master interrupt enable (connected to OUT2)

    mov    al,MCR_DTR or MCR_RTS or MCR_OUT2
    setport    MCR
    out    dx,al

;compute the divisor given the baud rate.
    mov    dx,baudclk+2
    mov    ax,baudclk
    mov    bx,0
asy_speed_1:
    inc    bx
    sub    ax,baud_rate
    sbb    dx,baud_rate+2
    jnc    asy_speed_1
    dec    bx
    add    ax,baud_rate
    adc    dx,baud_rate+2
    or    ax,dx
    je    asy_speed_2

    mov    dx,offset approximate_msg
    mov    ah,9
    int    21h

asy_speed_2:

    loadport            ;Purge the receive data buffer
    setport    RBR
    in    al,dx

    mov    ah,LCR_DLAB        ;Turn on divisor latch access bit
    setport    LCR
    call    setbit

    mov    al,bl            ;Load the two bytes of the divisor.
    setport    DLL
    out    dx,al
    mov    al,bh
    setport    DLM
    out    dx,al

    mov    ah,LCR_DLAB        ;Turn off divisor latch access bit
    setport    LCR
    call    clrbit

    call    set_recv_isr        ;Set interrupt vector to SIO handler

;set up the various pointers.
    mov    dx,offset end_resident
    mov    send_buf,dx
    mov    send_buf_head,dx
    mov    send_buf_tail,dx
    add    dx,send_buf_size
    mov    send_buf_end,dx

    mov    recv_buf,dx
    mov    recv_buf_head,dx
    mov    recv_buf_tail,dx
    add    dx,recv_buf_size
    mov    recv_buf_end,dx


    ; the following code attempts to determine a pseudo timeout
    ; value    to use in the loop that waits for an incoming character
    ; in asyrxint. The value returned in xmit_time is the number of
    ; loops processed between characters - therefore the loop used below
    ; is and should    remain similar to the loop used in asyrxint.

    push    dx
    xor    ax, ax            ; we'll send a 0
    mov    ah, LSR_THRE
    mov    cx, 10h            ; take the highest of 16 runs
    xor    si, si            ; will hold highest value

xmit_time_start:

    xor    di, di            ; initialize counter
    loadport
    setport    THR            ; xmit a character
    out    dx, al
    setport    LSR               ; set up    to check for an empty buffer

    ; next is the loop actually being timed

xmit_time_top:
    in    al, dx
    test    al, ah
    jnz    xmit_time_done
    inc    di
    cmp    cx, cx            ; these next few instructions do nothing
    jmp    xmit_time_1        ;  except maintain similarity with the
                    ;  "sister" loop in asyrxint
xmit_time_1:
    jmp    xmit_time_top

xmit_time_done:                ; end of timed loop



    cmp    si, di            ; compare highest value with new value
    ja    xmit_time_end        ; no bigger - just loop
    mov    si, di            ; bigger - save it

xmit_time_end:
    loop    xmit_time_start        ; bottom of outer loop

    shl    si, 1            ; we'll wait 8 characters worth
    shl    si, 1
    shl    si, 1
    mov    xmit_time, si        ; retain largest value
    pop    dx

    ; end of pseudo timer determination


    popf
    clc                ;indicate no errors.
    ret

code    ends

    end