The 640 MEG Shareware Studio 2/The 640 Meg Shareware Studio CD-ROM Volume II (Data Express)(1993).ISO/pascal/async12.zip/ASYNC12.ASM

Assembly Source File | 1989-05-04 | 22KB | 749 lines

;********************************************************
;*                            *
;*    Low-level serial communications handlers    *
;*        For ASYNC11 - Turbo Pascal V5.0        *
;*                            *
;*    Copyright (C) 1988, Rising Edge Data Services     *
;*                            *
;*   Assembled with the Borland TURBO ASSEMBER (V1.0)    *
;*                            *
;*  Author: M. Schultz        Ver 1.1 - Thu 05/04/89    *
;*                            *
;********************************************************

;    Status byte definition (C_Status):

;    7   6   5   4   3   2   1   0
;    |   |   |   |   |   |   |   |____ Input buffer empty
;    |   |   |   |   |   |   |________ Input buffer full
;    |   |   |   |   |   |____________ Output buffer empty
;    |   |   |   |   |________________ Output buffer full
;    |   |   |   |____________________ Input buffer overflow
;    |   |   |________________________ Output buffer overflow
;    |   |____________________________ Hard handshake active (xmit stopped)
;    |________________________________ Soft handshake active (xmit stopped)

;    Control byte definition (C_Ctrl):

;    7   6   5   4   3   2   1   0
;    |   |   |   |   |   |   |   |____ Enable RTS handshake
;    |   |   |   |   |   |   |________ Enable CTS handshake
;    |   |   |   |   |   |____________ Enable software handshake
;    |   |   |   |   |________________
;    |   |   |   |____________________
;    |   |   |________________________
;    |   |____________________________
;    |________________________________ XL3 communications mode

    IDEAL

    SEGMENT    DATA     WORD PUBLIC

;    Externally accessed (TP program) variables
;    Note:     Most of these variables are declared as arrays in the main
;        program; i.e. C_InSize : Array[1..4] Of Word

    EXTRN    C_InBufPtr :DWORD    ;Pointer to input buffers
    EXTRN    C_OutBufPtr:DWORD    ;Pointer to output buffers
    EXTRN    C_InSize   :WORD    ;Size (bytes) of input buffers
    EXTRN    C_OutSize  :WORD    ;Size (bytes) of output buffers
    EXTRN    C_InHead   :WORD    ;Input (receive) head pointers
    EXTRN    C_OutHead  :WORD    ;Output (transmit) head pointers
    EXTRN    C_InTail   :WORD    ;Input (receive) tail pointers
    EXTRN    C_OutTail  :WORD    ;Output (transmit) tail pointers
    EXTRN    C_RTSOn    :WORD    ;Point at which RTS line is asserted
    EXTRN    C_RTSOff   :WORD    ;Point at which RTS line is dropped
    EXTRN    C_StartChar:BYTE    ;Start character for soft handshake
    EXTRN    C_StopChar :BYTE    ;Stop character for soft handshake
    EXTRN    C_Status   :BYTE    ;Status byte (see above)
    EXTRN    C_Ctrl     :BYTE    ;Control byte (see above)
    EXTRN    C_PortOpen :BYTE    ;Port-open flags
    EXTRN    C_PortAddr :WORD    ;Base address of ports
    EXTRN    C_MaxCom   :BYTE    ;Highest port # defined (single byte)
    EXTRN    C_XL3Ptr   :BYTE    ;String index, detection of XL3 ID
    EXTRN    C_Temp     :WORD    ;Used for debugging

;    8250 register offsets

IER    EQU    1            ;Interrupt enable register
IIR    EQU    2            ;Interrupt identification 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
SCR    EQU    7            ;8250 scratch register

    ENDS    DATA

;    Code segment declaration

    SEGMENT    CODE    BYTE PUBLIC

    ASSUME    CS:CODE,DS:DATA

;    Externally accessable procedures defined here

        PUBLIC  INT_Handler
    PUBLIC    ComReadCh
    PUBLIC    ComReadChW
    PUBLIC    ComWriteCh
    PUBLIC    ComWriteChW

;********************************************************
;*                            *
;*    Subroutines that are used internally        *
;*                            *
;********************************************************

; Subroutine:    ChkPort
; Function:    Check port parameter(s), ensure that port is OPEN
; Entry:    AL <- Port # (1 - C_MaxCom)
; Exit:        AL -> Adjusted port # (0 - 3)
;        AH -> Status register
;        BX -> Byte array index
;        DX -> Base address of port
;        Carry flag SET if parameters & port are OK

    PROC    ChkPort        FAR

;    Determine if port # is valid

    CMP    AL,[C_MaxCom]        ;Port # > Maximum port # ?
    JA    ChkErr            ; Yes, exit w/error
    CMP    AL,0            ;Port # = 0 (invalid port #)
    JZ    ChkErr            ; Yes, exit w/error
    DEC    AL            ;AL <- Adjusted port #

;    Check if port open

    XOR    BH,BH            ;
    MOV    BL,AL            ;BX <- Byte array index
    MOV    AH,[C_PortOpen+BX]    ;AH <- Port-open flag
    CMP    AH,0            ;Port open ?
    JZ    ChkErr            ; No, exit w/error

;    Get status register and base port address in DX

    MOV    AH,[C_Status+BX]    ;AH <- Status register
    SHL    BL,1            ;BX <- Word array index
    MOV    DX,[C_PortAddr+BX]    ;DX <- Port address
    SHR    BL,1            ;BX <- Byte array index
    STC                ;Set carry (valid return)
    RET                ;Exit

;    Here if error

ChkErr:    CLC                ;Clear carry (invalid return)
    RET                ;Exit

    ENDP    ChkPort

;********************************************************

; Subroutine:    PutChar
; Function:    Place character in transmit buffer (transmit a character)
; Entry:    AL <- Port # (0-3)
;        AH <- Status register
;        BX <- Byte array pointer
;        CH <- Character to put in buffer
;        DX <- Base address of port
; Destroyed:    ES,SI,DI

; Note:    Port address passed is NOT checked for validity.
;    Subroutine ChkPort will create the proper entry environment

    PROC    PutChar        FAR

;    Check for buffer overflow

    TEST    AH,00001000b        ;Buffer full ?
    JZ    PutCh1            ; No, continue
    OR    AH,00100000b        ;Set buffer-overflow flag
    JMP    PutChX            ;Exit

;    Increment head pointer

PutCh1:    SHL    BL,1            ;BX <- Word array index
    MOV    DI,[C_OutHead+BX]    ;DI <- Output head pointer
    MOV    SI,[C_OutTail+BX]    ;SI <- Output tail pointer
    INC    DI            ;Bump head pointer
    CMP    DI,[C_OutSize+BX]    ;Head >= Buffer size ?
    JB    PutCh2            ; No, continue
    XOR    DI,DI            ; Yes, reset pointer
PutCh2:    MOV    [C_OutHead+BX],DI    ;Save head pointer

;    Place character in buffer

    SHL    BL,1            ;BX <- Pointer array index
    LES    BX,[C_OutBufPtr+BX]    ;ES:BX <- Pointer to output buffer
    MOV    [ES:BX+DI],CH        ;Place character in buffer

;    Check for full buffer

    CMP    DI,SI            ;Head = Tail (buffer full) ?
    JNZ    PutCh3            ; No, buffer not full
    OR    AH,00001000b        ;Set buffer-full flag

;    Determine if transmitter should be activated

PutCh3:    XOR    BH,BH            ;
    MOV    BL,AL            ;BX <- Byte array index

    TEST    AH,11000000b        ;Any inhibits (soft/hard hshake) ?
    JNZ    PutChX            ; Yes, do not activate xmit interrupt
;    TEST    AH,00000100b        ;Buffer empty ?
;    JZ    PutChX            ; No, xmit interrupt is on already

    AND    AH,11111011b        ;Reset buffer-empty flag
    ADD    DL,IER            ;Point to interrupt enable register
    IN    AL,DX            ;AL <- Interrupt enable register
    OR    AL,00000010b        ;Enable transmit interrupt
    OUT    DX,AL            ;Update port
    SUB    DL,IER            ;Point back to base

;    Here to exit

PutChX:    MOV    [C_Status+BX],AH    ;Save status byte
    MOV    AL,BL            ;
    RET                ;Exit

    ENDP    PutChar

;********************************************************
;*                            *
;*    Interrupt service routine for INT3,INT4        *
;*                            *
;*        INT3 typically used by COM2, COM4        *
;*         INT4 typically used by COM1, COM3        *
;*                            *
;********************************************************

    PROC    INT_Handler     FAR

    PUSH    AX            ;Save environment
    PUSH    BX            ;
    PUSH    CX            ;
    PUSH    DX            ;
    PUSH    SI            ;
    PUSH    DI            ;
    PUSH    DS            ;
    PUSH    ES            ;
    PUSH    BP            ;

    MASM
    MOV    AX,SEG DATA        ;AX <- Current data segment
    IDEAL
    MOV    DS,AX            ;Set new data segment

INT_Id:    XOR    BX,BX            ;BL <- Port # (start at 0)

;    Identify active port

IntID1:    MOV    AL,[C_PortOpen+BX]    ;AL <- Port-open flag
    CMP    AL,0            ;Port open ?
    JZ    IntID2            ; No, don't check
    SHL    BL,1            ;BX <- Word array index
    MOV    DX,[C_PortAddr+BX]    ;DX <- Base address of port
    SHR    BL,1            ;BX <- Byte array index
    ADD    DL,IIR            ;Add in offset for IIR
    IN    AL,DX            ;AL <- COMn IIR
    TEST    AL,00000001b        ;Interrupt active on this port ?
    JZ    INT_Active        ; (Bit 0 = 0 if active)
IntID2:    INC    BL            ;Bump port #
    CMP    BL,[C_MaxCom]        ;All ports checked ?
    JB    IntID1            ; No, continue

;    Here to leave interrupt handler

    MOV    AL,20h            ;AL <- EOI Acknowledge code
    OUT    20h,AL            ;To 8259 PIC

    POP    BP            ;Restore envirionment
    POP    ES            ;
    POP    DS            ;
    POP    DI            ;
    POP    SI            ;
    POP    DX            ;
    POP    CX            ;
    POP    BX            ;
    POP    AX            ;

    IRET                ;Interrupt exit

;    Active port (8250) found.
;    Determine cause of interrupt and execute appropriate handler.
;    Upon entry into routine, registers are set as follows:
;    BX <- Byte array index
;    DX <- Port address

INT_Active:
    DEC    DX            ;
    DEC    DX            ;DX <- Base address of COM port

    CMP    AL,0            ;IIR = 0 (Modem status change) ?
    JZ    ComMsc            ; Yes
    CMP    AL,2            ;IIR = 2 (Character transmitted) ?
    JZ    ComXmt            ; Yes
    CMP    AL,4            ;IIR = 4 (Character received) ?
    JNZ    Next            ;
    JMP    ComRcv            ; Yes
Next:    CMP    AL,6            ;IIR = 6 (Line status change) ?
    JZ    ComLsc            ; Yes
    JMP    INT_Id            ;Check other ports & exit

;*******************************************************
;        Line status change
;        NOTE: Currently unused
;*******************************************************

ComLsc:    ADD    DL,LSR            ;Point to line status register
    IN    AL,DX            ;Get LSR

;    The following code (which disables then enables the transmit
;    interrupt) is required to compensate for some "buggy" 8250's and
;    8250-emulating gate arrays.

    SUB    DL,LSR            ;Back to base
    ADD    DL,IER            ;Point to interrupt enable register
    IN    AL,DX            ;Get IER
    MOV    AH,AL            ;Save IER
    AND    AL,11111101b        ;Mask transmit interrupt
    OUT    DX,AL            ;Send modified to port
    MOV    AL,AH            ;
    OUT    DX,AL            ;Send original to port

    JMP    INT_Id            ;Exit

;*******************************************************
;        Modem status change
;*******************************************************

ComMsc:    NOP

;    Get modem status & control/status registers

Msc0:    ADD    DL,MSR            ;DX <- 8250 MSR port address
    IN    AL,DX            ;AL <- 8250 Modem status
    SUB    DL,MSR            ;Back to base

    MOV    CL,[C_Status+BX]    ;CL <- Status register
    MOV    CH,[C_Ctrl+BX]        ;CH <- Control register

;    Control CTS status flag based on CTS status and handshake enable

    TEST    CH,00000010b        ;CTS handshake enabled ?
    JZ    Msc2            ; No, ignore CTS status
    TEST    AL,00010000b        ;CTS asserted ?
    JNZ    Msc1            ; Yes, enable transmit
    OR    CL,01000000b        ;CTS inactive - hard handshake on
    JMP    Msc2            ;
Msc1:    AND    CL,10111111b        ;CTS active - hard handshake off
Msc2:    MOV    [C_Status+BX],CL    ;Save status flags

;    Determine if transmitter should be enabled

Msc3:    ADD    DL,IER            ;DX <- 8250 IER port address
    IN    AL,DX            ;AL <- Interrupt enable register
    OR    AL,00000010b        ;Enable transmitter by default
    TEST    CL,11000100b        ;Enable transmitter ?
    JZ    Msc4            ; Yes
    AND    AL,11111101b        ; No - disable transmitter
Msc4:    OUT    DX,AL            ;Update IER
    JMP    INT_Id            ;Exit

;*******************************************************
;        Character transmitted
;*******************************************************

; Register usage:
; AH : Status register
; BX : Array pointer
; DX : Port address
; SI : Output tail pointer
; DI : Output head pointer

;    Get status and control registers

ComXmt:    MOV    AH,[C_Status+BX]    ;AH <- Status register
    TEST    AH,00000100b        ;Buffer empty ?
    JNZ    Xmt2            ; Yes, stop transmitter

;    Bump tail pointer

    PUSH    BX            ;Save byte array pointer
    SHL    BL,1            ;BX <- Word array pointer
    MOV    SI,[C_OutTail+BX]    ;SI <- Output tail pointer
    MOV    DI,[C_OutHead+BX]    ;DI <- Output head pointer

    INC    SI            ;Bump tail pointer
    CMP    SI,[C_OutSize+BX]    ;Tail < Buffer size ?
    JB    Xmt1            ; Yes, proceed normally
    XOR    SI,SI            ; No, reset to 0
Xmt1:    MOV    [C_OutTail+BX],SI    ;Save tail pointer

;    Send character

    SHL    BL,1            ;BX <- Pointer array pointer
    LES    BX,[C_OutBufPtr+BX]    ;ES:BX <- Pointer to output buffer
    MOV    AL,[ES:BX+SI]        ;AL <- Character from buffer
    OUT    DX,AL            ;Send
    POP    BX            ;Recover byte array pointer

;    Determine if buffer is empty
;    Reset output-buffer-full flag & exit

    CMP    DI,SI            ;Head = Tail (buffer empty) ?
    JNZ    Xmt2            ; No, continue normally
    OR    AH,00000100b        ;Set buffer-empty flag
Xmt2:    AND    AH,11010111b        ;Reset FULL and OVERFLOW flags
    MOV    [C_Status+BX],AH    ;Save status flags
    JMP    Msc0            ;Exit (check xmit mask)

;*******************************************************
;        Character received
;*******************************************************

; Register usage:
; AL : Status register
; AH : Control register
; BX : Array index
; CL : Character received
; CH : Temporary storage
; DX : Port address
; SI : Input tail pointer
; DI : Input head pointer

ComRcv:    IN    AL,DX            ;
    MOV    CL,AL            ;CL <- Received character

;    Check for software handshake

    MOV    AL,[C_Status+BX]    ;AL <- Status byte
    MOV    AH,[C_Ctrl+BX]        ;AH <- Control byte
    TEST    AH,00000100b        ;Software handshake enabled ?
    JZ    Rcv3            ; No, don't check software handshake
    CMP    CL,[C_StopChar+BX]    ;STOP TRANSMIT character ?
    JZ    Rcv1            ; Yes
    CMP    CL,[C_StartChar+BX]    ;START TRANSMIT character ?
    JNZ    Rcv3            ; No

;    Soft-handshake character received.
;    Activate or deactive transmitter depending on status

    AND    AL,01111111b        ;Reset soft-handshake flag (start)
    JMP    Rcv2            ;Save status & exit
Rcv1:    OR    AL,10000000b        ;Set soft-handshake flag (stop)
Rcv2:    MOV    CL,AL            ;Routines in MSC2 require status in CL
    JMP    Msc2            ;Exit (xmit interrupt controlled here)

;    Check for XL3 verification sequence

;    Note : This code is here to provide the capability to communicate
;    with a Pyrotronics XL3 fire alarm panel (since this is why these
;    routines were developed in the first place).  In almost all cases
;    they will never be needed by the "normal" user.

Rcv3:    TEST    AH,10000000b        ;XL3 mode active ?
    JZ    Rcv3e            ; No, ignore
    MOV    DI,BX            ;Save array index
    MOV    BL,[C_XL3Ptr+BX]    ;BX <- Current check position
    CMP    CL,[BYTE CS:XL3Str+BX]    ;Check current char vs. escape sequence
    JNZ    Rcv3d            ;Continue normally if no match

;    Possible XL3 string match

    MOV    CH,BL            ;CH <- String index
    MOV    BX,DI            ;Recover array index
    INC    CH            ;Bump index
    CMP    CH,OFFSET XL3Len    ;End of string ?
    JAE    Rcv3a            ; Yes, send answerback
    MOV    [C_XL3Ptr+BX],CH    ;Save string index
    JMP    INT_Id            ;Exit

;    XL3 identify sequence detected.  Send answerback.

Rcv3a:    MOV    SI,OFFSET AnsBak    ;SI <- Pointer to answerback string
    MOV    AH,AL            ;Put status byte in AH (for PutChar)
    MOV    AL,BL            ;Put port # in AL (for PutChar)
Rcv3b:    MOV    CH,[CS:SI]        ;CH <- Char from answerback string
    CMP    CH,0            ;End of string ?
    JZ    Rcv3c            ; Yes, exit now
    PUSH    SI            ;Save SI (modified by PutChar)
    CALL    PutChar            ;Transmit character
    POP    SI            ;Recover SI
    INC    SI            ;Point to next char in string
    JMP    Rcv3b            ;Continue until string sent

Rcv3c:    MOV    [C_XL3Ptr+BX],0        ;Reset string index
    JMP    INT_Id            ;Exit

;    XL3 identify string mismatch - reset pointer & continue

Rcv3d:    MOV    BX,DI            ;Recover array index
    MOV    [C_XL3Ptr+BX],0        ;Reset string index

;    Clear buffer empty flag / check for buffer overflow

Rcv3e:    AND    AL,11111110b        ;Clear buffer-empty flag
    TEST    AL,00000010b        ;Buffer full ?
    JZ    Rcv4            ; No, continue
    OR    AL,00010000b        ;Set overflow flag
    JMP    Rcv10            ;Exit

;    Bump receive buffer pointer

Rcv4:    SHL    BL,1            ;BX <- Word array index
    MOV    DI,[C_InHead+BX]    ;DI <- Input head pointer
    MOV    SI,[C_InTail+BX]    ;SI <- Input tail pointer

    INC    DI            ;Bump buffer pointer
    CMP    DI,[C_InSize+BX]    ;Head > buffer size ?
    JB    Rcv5            ; No, continue
    XOR    DI,DI            ; Yes, reset pointer

;    Store character in buffer

Rcv5:    PUSH    BX            ;Save word array pointer
    MOV    [C_InHead+BX],DI    ;Save updated input head pointer
    SHL    BL,1            ;BX <- Doubleword array index
    LES    BX,[C_InBufPtr+BX]    ;ES:BX <- Pointer to buffer
    MOV    [ES:BX+DI],CL        ;Save character in buffer
    POP    BX            ;Recover word array pointer

;    Check for full buffer

    CMP    SI,DI            ;Tail = Head ?
    JNZ    Rcv6            ;If Tail <> Head, buffer is not full
    OR    AL,00000010b        ;Set buffer-full flag
    JMP    Rcv8            ;Reset RTS and exit

;    Check for near-full buffer (buffer used >= RTSOff)

Rcv6:    CMP    SI,DI            ;Tail <= Head ?
    JBE    Rcv7            ; Yes, use standard formula
    SUB    SI,DI            ;SI <- Tail - Head
    MOV    DI,[C_InSize+BX]    ;DI <- Input buffer size
Rcv7:    SUB    DI,SI            ;DI <- Head - Tail (amt used)
    CMP    DI,[C_RTSOff+BX]    ;Used < Limit ?
    JB    Rcv9            ; Yes, leave RTS on

;    Buffer is (near) full, force RTS off & exit

Rcv8:    TEST    AH,00000001b        ;RTS handshake enabled ?
    JZ    Rcv9            ; No, exit now

    MOV    CH,AL            ;Keep status byte
    ADD    DL,MCR            ;DX <- Address of modem control reg.
    IN    AL,DX            ;AL <- MCR
    AND    AL,11111101b        ;Disable RTS
    OUT    DX,AL            ;Update MCR
    SUB    DL,MCR            ;DX <- Base of port
    MOV    AL,CH            ;Recover status byte

;    Exit - receive

Rcv9:    SHR    BL,1            ;BX <- Byte array index
Rcv10:    MOV    [C_Status+BX],AL    ;Save status byte

;    The following code corrects a "bug" present in some 8250's

    ADD    DL,IER            ;DX <- Interrupt Enable Register
    IN    AL,DX            ;AL <- IER
    MOV    AH,AL            ;Save IER
    AND    AL,11111101b        ;Mask off transmit interrupt
    OUT    DX,AL            ;Send to IER
    MOV    AL,AH            ;
    OUT    DX,AL            ;Restore original IER state

    JMP    INT_Id            ;Check for pending INTs and exit

    ENDP    INT_Handler

;********************************************************
;*                            *
;*    Start of Pascal low-level procedures        *
;*                            *
;********************************************************
;*                            *
;*    Function ComReadCh(ComPort:Byte) : Char        *
;*                            *
;********************************************************

    PROC    ComReadCh     FAR

;    Check port # for validity

    CLI                ;Interrupts disabled

    MOV    BX,SP            ;BX <- Stack pointer
    MOV    AL,[SS:BX+4]        ;AL <- Port #
    CALL    ChkPort            ;Check port # for validity
    JNC    ComRd1            ; Invalid port, exit

;    Check buffer, return null if empty

    MOV    AH,[C_Status+BX]    ;AH <- Status byte
    TEST    AH,00000001b        ;Buffer empty ?
    JZ    ComRd2            ; No, continue normally

;    Buffer empty, port not open or port # invalid - exit

ComRd1:    MOV    CH,0            ;Return null (port error or empty)
ComRdX:    MOV    AL,CH            ;AL <- Char from buffer
    STI                ;Enable interrupts
    RET    2            ;Exit

;    Increment tail pointer
;    NOTE: Entry point for ComReadChW

ComRd2:    SHL    BL,1            ;BX <- Word array index
    MOV    SI,[C_InTail+BX]    ;SI <- Tail pointer
    MOV    DI,[C_InHead+BX]    ;DI <- Input head pointer
    INC    SI            ;Bump tail pointer
    CMP    SI,[C_InSize+BX]    ;Tail past end of buffer ?
    JB    ComRd3            ; No, continue
    XOR    SI,SI            ; Yes, reset pointer
ComRd3:    MOV    [C_InTail+BX],SI    ;Save updated tail pointer

;    Get character from buffer

    SHL    BL,1            ;BX <- Pointer array index
    LES    BX,[C_InBufPtr+BX]    ;ES:BX <- Pointer to input buffer
    MOV    CH,[ES:BX+SI]        ;CH <- Character from buffer

;    Clear FULL and OVERFLOW flags
;    Check for empty buffer

    XOR    BH,BH            ;
    MOV    BL,AL            ;Byte array index
    AND    AH,11101101b        ;Reset FULL and OVERFLOW status flags
    CMP    DI,SI            ;Head = Tail (buffer empty ?)
    JNZ    ComRd4            ; No, continue normally
    OR    AH,00000001b        ; Yes, set empty flag
ComRd4:    MOV    [C_Status+BX],AH    ;Save status byte

;    Check for RTS assert (Used <= RTSOn)
;    Variable USED (# of used bytes in buffer) calculated as :
;      IF (Head >= Tail) THEN
;        Used = Head-Tail
;      ELSE
;        Used = BufferSize - (Tail-Head)

    SHL    BL,1            ;BX <- Word array index
    CMP    DI,SI            ;Head >= Tail ?
    JAE    ComRd5            ; Yes, use alternate formula
    SUB    SI,DI            ;SI <- Tail - Head
    MOV    DI,[C_InSize+BX]    ;DI <- Input buffer size
ComRd5:    SUB    DI,SI            ;DI <- Amount of buffer used
    CMP    DI,[C_RTSOn+BX]        ;Used > Limit ?
    JA    ComRdX            ; Yes, not ready for receive

;    Here to assert RTS

    SHR    BL,1            ;BX <- Byte array index
    MOV    AL,[C_Ctrl+BX]        ;AL <- Control byte
    TEST    AL,00000001b        ;RTS handshaking enabled ?
    JZ    ComRdX            ; No, exit now

    ADD    DL,MCR            ;DX <- 8250 MCR port address
    IN    AL,DX            ;AL <- 8250 MCR
    OR    AL,00000010b        ;Assert RTS
    OUT    DX,AL            ;Send to port
    JMP    ComRdX            ;Exit

    ENDP    ComReadCh

;********************************************************
;*                            *
;*    Function ComReadChW(ComPort:Byte) : Char    *
;*                            *
;********************************************************

    PROC    ComReadChW    FAR

;    Check for valid port

    MOV    BX,SP            ;BX <- Stack pointer
    MOV    AL,[SS:BX+4]        ;AL <- Port #
    CALL    ChkPort            ;Port # valid ?
    JNC    ComRWX            ; No, exit now

;    Wait for character

ComRW1:    MOV    AH,[C_Status+BX]    ;AL <- Status byte
    TEST    AH,00000001b        ;Input buffer empty ?
    JNZ    ComRW1            ; Yes, continue waiting

    CLI                ;Disable interrupts
    JMP    ComRd2            ;Proceed with normal read

;    Here if error

ComRWX:    MOV    AL,0            ;Return null result
    RET    2            ;Exit

    ENDP    ComReadChW

;********************************************************
;*                            *
;*    Procedure ComWriteCh(ComPort:Byte; Var Ch:Char)    *
;*                            *
;********************************************************

    PROC    ComWriteCh    FAR

;    Check port # for validity

    MOV    BX,SP            ;Point BX at parameters
    MOV    AL,[SS:BX+6]        ;AL <- Port #
    MOV    CH,[SS:BX+4]        ;CH <- Character to send
    CALL    ChkPort            ;Check port for validity
    JNC    ComWr1            ;Exit if not valid

    CLI                ;Disable interrupts
    CALL    PutChar            ;Place character in appropriate buffer
    STI                ;Enable interrupts

ComWr1:    RET    4            ;Exit

    ENDP    ComWriteCh

;********************************************************
;*                            *
;*   Procedure ComWriteChW(ComPort:Byte; Var Ch:Char)    *
;*                            *
;********************************************************

    PROC    ComWriteChW    FAR

;    Check port # for validity

    MOV    BX,SP            ;Point BX at parameters
    MOV    AL,[SS:BX+6]        ;AL <- Port #
    MOV    CH,[SS:BX+4]        ;CH <- Character to send
    CALL    ChkPort            ;Check port for validity
    JNC    ComWW2            ;Exit if not valid

;    Wait for buffer to open up

ComWW1:    MOV    AH,[C_Status+BX]    ;AH <- Status byte
    TEST    AH,00101000b        ;Buffer filled ?
    JNZ    ComWW1            ; Yes, loop until open

    CLI                ;Turn off interrupts
    CALL    PutChar            ;Place character in buffer
    STI                ;Enable interrupts

ComWW2:    RET    4            ;Exit

    ENDP    ComWriteChW

;********************************************************

XL3Len    EQU    3            ;Length of XL3 string
XL3Str:    DB    1Bh,5Bh,63h        ;ESC [ c    (From XL3)
AnsBak:    DB    1Bh,5Bh,38h,32h,30h,00h    ;ESC [ 8 2 0    (To XL3)

    ENDS    CODE
    END