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──────────────────────────────────────────────────────────────────────────── Chapter 7 Printer and Serial Port MS-DOS supports printers, plotters, modems, and other hard-copy output or communication devices with device drivers for parallel ports and serial ports. Parallel ports are so named because they transfer a byte──8 bits── in parallel to the destination device over eight separate physical paths (plus additional status and handshaking signals). The serial port, on the other hand, communicates with the CPU with bytes but sends data to or receives data from its destination device serially──a bit at a time──over a single physical connection. Parallel ports are typically used for high-speed output devices, such as line printers, over relatively short distances (less than 50 feet). They are rarely used for devices that require two-way communication with the computer. Serial ports are used for lower-speed devices, such as modems and terminals, that require two-way communication (although some printers also have serial interfaces). A serial port can drive its device reliably over much greater distances (up to 1000 feet) over as few as three wires── transmit, receive, and ground. The most commonly used type of serial interface follows a standard called RS-232. This standard specifies a 25-wire interface with certain electrical characteristics, the use of various handshaking signals, and a standard DB-25 connector. Other serial-interface standards exist──for example, the RS-422, which is capable of considerably higher speeds than the RS-232── but these are rarely used in personal computers (except for the Apple Macintosh) at this time. MS-DOS has built-in device drivers for three parallel adapters, and for two serial adapters on the PC or PC/AT and three serial adapters on the PS/2. The logical names for these devices are LPT1, LPT2, LPT3, COM1, COM2, and COM3. The standard printer (PRN) and standard auxiliary (AUX) devices are normally aliased to LPT1 and COM1, but you can redirect PRN to one of the serial ports with the MS-DOS MODE command. As with keyboard and video display I/O, you can manage printer and serial-port I/O at several levels that offer different degrees of flexibility and hardware independence: ■ MS-DOS handle-oriented functions ■ MS-DOS traditional character functions ■ IBM ROM BIOS driver functions In the case of the serial port, direct control of the hardware by application programs is also common. I will discuss each of these I/O methods briefly, with examples, in the following pages. Printer Output The preferred method of printer output is to use the handle write function (Int 21H Function 40H) with the predefined standard printer handle (4). For example, you could write the string hello to the printer as follows: ────────────────────────────────────────────────────────────────────────── msg db 'hello' ; message for printer msg_len equ $-msg ; length of message . . . mov ah,40h ; function 40h = write file or device mov bx,4 ; BX = standard printer handle mov cx,msg_len ; CX = length of string mov dx,seg msg ; DS:DX = string address mov ds,dx mov dx,offset msg int 21h ; transfer to MS-DOS jc error ; jump if error . . . ────────────────────────────────────────────────────────────────────────── If there is no error, the function returns the carry flag cleared and the number of characters actually transferred to the list device in register AX. Under normal circumstances, this number should always be the same as the length requested and the carry flag indicating an error should never be set. However, the output will terminate early if your data contains an end-of-file mark (Ctrl-Z). You can write independently to several list devices (for example, LPT1, LPT2) by issuing a specific open request (Int 21H Function 3DH) for each device and using the handles returned to access the printers individually with Int 21H Function 40H. You have already seen this general approach in Chapters 5 and 6. An alternative method of printer output is to use the traditional Int 21H Function 05H, which transfers the character in the DL register to the printer. (This function is sensitive to Ctrl-C interrupts.) For example, the assembly-language code sequence at the top of the following page would write the the string hello to the line printer. ────────────────────────────────────────────────────────────────────────── msg db 'hello' ; message for printer msg_len equ $-msg ; length of message . . . mov bx,seg msg ; DS:BX = string address mov ds,bx mov bx,offset msg mov cx,msg_len ; CX = string length next: mov dl,[bx] ; get next character mov ah,5 ; function 05h = printer output int 21h ; transfer to MS-DOS inc bx ; bump string pointer loop next ; loop until string done . . . ────────────────────────────────────────────────────────────────────────── Programs that run on IBM PC─compatible machines can obtain improved printer throughput by bypassing MS-DOS and calling the ROM BIOS printer driver directly by means of Int 17H. Section III of this book, "IBM ROM BIOS and Mouse Functions Reference," documents the Int 17H functions in detail. Use of the ROM BIOS functions also allows your program to test whether the printer is off line or out of paper, a capability that MS-DOS does not offer. For example, the following sequence of instructions calls the ROM BIOS printer driver to send the string hello to the line printer: ────────────────────────────────────────────────────────────────────────── msg db 'hello' ; message for printer msg_len equ $-msg ; length of message . . . mov bx,seg msg ; DS:BX = string address mov ds,bx mov bx,offset msg mov cx,msg_len ; CX = string length mov dx,0 ; DX = printer number next: mov al,[bx] ; AL = character to print mov ah,0 ; function 00h = printer output int 17h ; transfer to ROM BIOS inc bx ; bump string pointer loop next ; loop until string done . . . ────────────────────────────────────────────────────────────────────────── Note that the printer numbers used by the ROM BIOS are zero-based, whereas the printer numbers in MS-DOS logical-device names are one-based. For example, ROM BIOS printer 0 corresponds to LPT1. Finally, the most hardware-dependent technique of printer output is to access the printer controller directly. Considering the functionality already provided in MS-DOS and the IBM ROM BIOS, as well as the speeds of the devices involved, I cannot see any justification for using direct hardware control in this case. The disadvantage of introducing such extreme hardware dependence for such a low-speed device would far outweigh any small performance gains that might be obtained. The Serial Port MS-DOS support for serial ports (often referred to as the auxiliary device in MS-DOS manuals) is weak compared with its keyboard, video-display, and printer support. This is one area where the application programmer is justified in making programs hardware dependent to extract adequate performance. Programs that restrict themselves to MS-DOS functions to ensure portability can use the handle read and write functions (Int 21H Functions 3FH and 40H), with the predefined standard auxiliary handle (3) to access the serial port. For example, the following code writes the string hello to the serial port that is currently defined as the AUX device: ────────────────────────────────────────────────────────────────────────── msg db 'hello' ; message for serial port msg_len equ $-msg ; length of message . . . mov ah,40h ; function 40h = write file or device mov bx,3 ; BX = standard aux handle mov cx,msg_len ; CX = string length mov dx,seg msg ; DS:DX = string address mov ds,dx mov dx,offset msg int 21h ; transfer to MS-DOS jc error ; jump if error . . . ────────────────────────────────────────────────────────────────────────── The standard auxiliary handle gives access to only the first serial port (COM1). If you want to read or write COM2 and COM3 using the handle calls, you must issue an open request (Int 21H Function 3DH) for the desired serial port and use the handle returned by that function with Int 21H Functions 3FH and 40H. Some versions of MS-DOS have a bug in character-device handling that manifests itself as follows: If you issue a read request with Int 21H Function 3FH for the exact number of characters that are waiting in the driver's buffer, the length returned in the AX register is the number of characters transferred minus one. You can circumvent this problem by always requesting more characters than you expect to receive or by placing the device handle into binary mode using Int 21H Function 44H. MS-DOS also supports two traditional functions for serial-port I/O. Int 21H Function 03H inputs a character from COM1 and returns it in the AL register; Int 21H Function 04H transmits the character in the DL register to COM1. Like the other traditional calls, these two are direct descendants of the CP/M auxiliary-device functions. For example, the following code sends the string hello to COM1 using the traditional Int 21H Function 04H: ────────────────────────────────────────────────────────────────────────── msg db 'hello' ; message for serial port msg_len equ $-msg ; length of message . . . mov bx,seg msg ; DS:BX = string address mov ds,bx mov bx,offset msg mov cx,msg_len ; CX = length of string mov dl,[bx] ; get next character mov ah,4 ; function 04h = aux output int 21h ; transfer to MS-DOS inc bx ; bump pointer to string loop next ; loop until string done . . . ────────────────────────────────────────────────────────────────────────── MS-DOS translates the traditional auxiliary-device functions into calls on the same device driver used by the handle calls. Therefore, it is generally preferable to use the handle functions in the first place, because they allow very long strings to be read or written in one operation, they give access to serial ports other than COM1, and they are symmetrical with the handle video-display, keyboard, printer, and file I/O methods described elsewhere in this book. Although the handle or traditional serial-port functions allow you to write programs that are portable to any machine running MS-DOS, they have a number of disadvantages: ■ The built-in MS-DOS serial-port driver is slow and is not interrupt driven. ■ MS-DOS serial-port I/O is not buffered. ■ Determining the status of the auxiliary device requires a separate call to the IOCTL function (Int 21H Function 44H)──if you request input and no characters are ready, your program will simply hang. ■ MS-DOS offers no standardized function to configure the serial port from within a program. For programs that are going to run on the IBM PC or compatibles, a more flexible technique for serial-port I/O is to call the IBM ROM BIOS serial-port driver by means of Int 14H. You can use this driver to initialize the serial port to a desired configuration and baud rate, examine the status of the controller, and read or write characters. Section III of this book, "IBM ROM BIOS and Mouse Functions Reference," documents the functions available from the ROM BIOS serial-port driver. For example, the following sequence sends the character X to the first serial port (COM1): ────────────────────────────────────────────────────────────────────────── . . . mov ah,1 ; function 01h = send character mov al,'X' ; AL = character to transmit mov dx,0 ; DX = serial-port number int 14h ; transfer to ROM BIOS and ah,80h ; did transmit fail? jnz error ; jump if transmit error . . . ────────────────────────────────────────────────────────────────────────── As with the ROM BIOS printer driver, the serial-port numbers used by the ROM BIOS are zero-based, whereas the serial-port numbers in MS-DOS logical-device names are one-based. In this example, serial port 0 corresponds to COM1. Unfortunately, like the MS-DOS auxiliary-device driver, the ROM BIOS serial-port driver is not interrupt driven. Although it will support higher transfer speeds than the MS-DOS functions, at rates greater than 2400 baud it may still lose characters. Consequently, most programmers writing high-performance applications that use a serial port (such as telecommunications programs) take complete control of the serial-port controller and provide their own interrupt driver. The built-in functions provided by MS-DOS, and by the ROM BIOS in the case of the IBM PC, are simply not adequate. Writing such programs requires a good understanding of the hardware. In the case of the IBM PC, the chips to study are the INS8250 Asynchronous Communications Controller and the Intel 8259A Programmable Interrupt Controller. The IBM technical reference documentation for these chips is a bit disorganized, but most of the necessary information is there if you look for it. The TALK Program The simple terminal-emulator program TALK.ASM (Figure 7-1) is an example of a useful program that performs screen, keyboard, and serial-port I/O. This program recapitulates all of the topics discussed in Chapters 5 through 7. TALK uses the IBM PC's ROM BIOS video driver to put characters on the screen, to clear the display, and to position the cursor; it uses the MS-DOS character-input calls to read the keyboard; and it contains its own interrupt driver for the serial-port controller. ────────────────────────────────────────────────────────────────────────── name talk page 55,132 .lfcond ; List false conditionals too title TALK--Simple terminal emulator ; ; TALK.ASM--Simple IBM PC terminal emulator ; ; Copyright (c) 1988 Ray Duncan ; ; To assemble and link this program into TALK.EXE: ; ; C>MASM TALK; ; C>LINK TALK; ; stdin equ 0 ; standard input handle stdout equ 1 ; standard output handle stderr equ 2 ; standard error handle cr equ 0dh ; ASCII carriage return lf equ 0ah ; ASCII linefeed bsp equ 08h ; ASCII backspace escape equ 1bh ; ASCII escape code dattr equ 07h ; display attribute to use ; while in emulation mode bufsiz equ 4096 ; size of serial-port buffer echo equ 0 ; 0 = full-duplex, -1 = half-duplex equ -1 false equ 0 com1 equ true ; use COM1 if nonzero com2 equ not com1 ; use COM2 if nonzero pic_mask equ 21h ; 8259 interrupt mask port pic_eoi equ 20h ; 8259 EOI port if com1 com_data equ 03f8h ; port assignments for COM1 com_ier equ 03f9h com_mcr equ 03fch com_sts equ 03fdh com_int equ 0ch ; COM1 interrupt number int_mask equ 10h ; IRQ4 mask for 8259 endif if com2 com_data equ 02f8h ; port assignments for COM2 com_ier equ 02f9h com_mcr equ 02fch com_sts equ 02fdh com_int equ 0bh ; COM2 interrupt number int_mask equ 08h ; IRQ3 mask for 8259 endif _TEXT segment word public 'CODE' assume cs:_TEXT,ds:_DATA,es:_DATA,ss:STACK talk proc far ; entry point from MS-DOS mov ax,_DATA ; make data segment addressable mov ds,ax mov es,ax ; initialize display for ; terminal emulator mode... mov ah,15 ; get display width and int 10h ; current display mode dec ah ; save display width for use mov columns,ah ; by the screen-clear routine cmp al,7 ; enforce text display mode je talk2 ; mode 7 ok, proceed cmp al,3 jbe talk2 ; modes 0-3 ok, proceed mov dx,offset msg1 mov cx,msg1_len jmp talk6 ; print error message and exit talk2: mov bh,dattr ; clear screen and home cursor call cls call asc_enb ; capture serial-port interrupt ; vector and enable interrupts mov dx,offset msg2 ; display message mov cx,msg2_len ; 'terminal emulator running' mov bx,stdout ; BX = standard output handle mov ah,40h ; function 40h = write file or device int 21h ; transfer to MS-DOS talk3: call pc_stat ; keyboard character waiting? jz talk4 ; nothing waiting, jump call pc_in ; read keyboard character cmp al,0 ; is it a function key? jne talk32 ; not function key, jump call pc_in ; function key, discard 2nd ; character of sequence jmp talk5 ; then terminate program talk32: ; keyboard character received if echo push ax ; if half-duplex, echo call pc_out ; character to PC display pop ax endif call com_out ; write char to serial port talk4: call com_stat ; serial-port character waiting? jz talk3 ; nothing waiting, jump call com_in ; read serial-port character cmp al,20h ; is it control code? jae talk45 ; jump if not call ctrl_code ; control code, process it jmp talk3 ; check keyboard again talk45: ; noncontrol char received, call pc_out ; write it to PC display jmp talk4 ; see if any more waiting talk5: ; function key detected, ; prepare to terminate... mov bh,07h ; clear screen and home cursor call cls mov dx,offset msg3 ; display farewell message mov cx,msg3_len talk6: push dx ; save message address push cx ; and message length call asc_dsb ; disable serial-port interrupts ; and release interrupt vector pop cx ; restore message length pop dx ; and address mov bx,stdout ; handle for standard output mov ah,40h ; function 40h = write device int 21h ; transfer to MS-DOS mov ax,4c00h ; terminate program with int 21h ; return code = 0 talk endp com_stat proc near ; check asynch status; returns ; Z = false if character ready ; Z = true if nothing waiting push ax mov ax,asc_in ; compare ring buffer pointers cmp ax,asc_out pop ax ret ; return to caller stat endp com_in proc near ; get character from serial- ; port buffer; returns ; new character in AL push bx ; save register BX com_in1: ; if no char waiting, wait mov bx,asc_out ; until one is received cmp bx,asc_in je com_in1 ; jump, nothing waiting mov al,[bx+asc_buf] ; character is ready, ; extract it from buffer inc bx ; update buffer pointer cmp bx,bufsiz jne com_in2 xor bx,bx ; reset pointer if wrapped com_in2: mov asc_out,bx ; store updated pointer pop bx ; restore register BX ret ; and return to caller com_in endp com_out proc near ; write character in AL ; to serial port push dx ; save register DX push ax ; save character to send mov dx,com_sts ; DX = status port address com_out1: ; check if transmit buffer in al,dx ; is empty (TBE bit = set) and al,20h jz com_out1 ; no, must wait pop ax ; get character to send mov dx,com_data ; DX = data port address out dx,al ; transmit the character pop dx ; restore register DX ret ; and return to caller com_out endp pc_stat proc near ; read keyboard status; returns ; Z = false if character ready ; Z = true if nothing waiting ; register DX destroyed mov al,in_flag ; if character already or al,al ; waiting, return status jnz pc_stat1 mov ah,6 ; otherwise call MS-DOS to mov dl,0ffh ; determine keyboard status int 21h jz pc_stat1 ; jump if no key ready mov in_char,al ; got key, save it for mov in_flag,0ffh ; "pc_in" routine pc_stat1: ; return to caller with ret ; Z flag set appropriately pc_stat endp pc_in proc near ; read keyboard character, ; return it in AL ; DX may be destroyed mov al,in_flag ; key already waiting? or al,al jnz pc_in1 ; yes, return it to caller call pc_stat ; try to read a character jmp pc_in pc_in1: mov in_flag,0 ; clear char-waiting flag mov al,in_char ; and return AL = character ret pc_in endp pc_out proc near ; write character in AL ; to the PC's display mov ah,0eh ; ROM BIOS function 0eh = ; "teletype output" push bx ; save register BX xor bx,bx ; assume page 0 int 10h ; transfer to ROM BIOS pop bx ; restore register BX ret ; and return to caller pc_out endp cls proc near ; clear display using ; char attribute in BH ; registers AX, CX, ; and DX destroyed mov dl,columns ; set DL,DH = X,Y of mov dh,24 ; lower right corner mov cx,0 ; set CL,CH = X,Y of ; upper left corner mov ax,600h ; ROM BIOS function 06h = ; "scroll or initialize ; window" int 10h ; transfer to ROM BIOS call home ; set cursor at (0,0) ret ; and return to caller cls endp clreol proc near ; clear from cursor to end ; of line using attribute ; in BH, registers AX, CX, ; and DX destroyed call getxy ; get current cursor position mov cx,dx ; current position = "upper ; left corner" of window; mov dl,columns ; "lower right corner" X is ; max columns, Y is same ; as upper left corner mov ax,600h ; ROM BIOS function 06h = ; "scroll or initialize ; window" int 10h ; transfer to ROM BIOS ret ; return to caller clreol endp home proc near ; put cursor at home position mov dx,0 ; set (X,Y) = (0,0) call gotoxy ; position the cursor ret ; return to caller home endp gotoxy proc near ; position the cursor ; call with DL = X, DH = Y push bx ; save registers push ax mov bh,0 ; assume page 0 mov ah,2 ; ROM BIOS function 02h = ; set cursor position int 10h ; transfer to ROM BIOS pop ax ; restore registers pop bx ret ; and return to caller gotoxy endp getxy proc near ; get cursor position, ; returns DL = X, DH = Y push ax ; save registers push bx push cx mov ah,3 ; ROM BIOS function 03h = ; get cursor position mov bh,0 ; assume page 0 int 10h ; transfer to ROM BIOS pop cx ; restore registers pop bx pop ax ret ; and return to caller getxy endp ctrl_code proc near ; process control code ; call with AL = char cmp al,cr ; if carriage return je ctrl8 ; just send it cmp al,lf ; if linefeed je ctrl8 ; just send it cmp al,bsp ; if backspace je ctrl8 ; just send it cmp al,26 ; is it cls control code? jne ctrl7 ; no, jump mov bh,dattr ; cls control code, clear call cls ; screen and home cursor jmp ctrl9 ctrl7: cmp al,escape ; is it Escape character? jne ctrl9 ; no, throw it away call esc_seq ; yes, emulate CRT terminal jmp ctrl9 ctrl8: call pc_out ; send CR, LF, or backspace ; to the display ctrl9: ret ; return to caller ctrl_code endp esc_seq proc near ; decode Televideo 950 escape ; sequence for screen control call com_in ; get next character cmp al,84 ; is it clear to end of line? jne esc_seq1 ; no, jump mov bh,dattr ; yes, clear to end of line call clreol jmp esc_seq2 ; then exit esc_seq1: cmp al,61 ; is it cursor positioning? jne esc_seq2 ; no jump call com_in ; yes, get Y parameter sub al,33 ; and remove offset mov dh,al call com_in ; get X parameter sub al,33 ; and remove offset mov dl,al call gotoxy ; position the cursor esc_seq2: ; return to caller ret esc_seq endp asc_enb proc near ; capture serial-port interrupt ; vector and enable interrupt ; save address of previous ; interrupt handler... mov ax,3500h+com_int ; function 35h = get vector int 21h ; transfer to MS-DOS mov word ptr oldvec+2,es mov word ptr oldvec,bx ; now install our handler... push ds ; save our data segment mov ax,cs ; set DS:DX = address mov ds,ax ; of our interrupt handler mov dx,offset asc_int mov ax,2500h+com_int ; function 25h = set vector int 21h ; transfer to MS-DOS pop ds ; restore data segment mov dx,com_mcr ; set modem-control register mov al,0bh ; DTR and OUT2 bits out dx,al mov dx,com_ier ; set interrupt-enable mov al,1 ; register on serial- out dx,al ; port controller in al,pic_mask ; read current 8259 mask and al,not int_mask ; set mask for COM port out pic_mask,al ; write new 8259 mask ret ; back to caller asc_enb endp asc_dsb proc near ; disable interrupt and ; release interrupt vector in al,pic_mask ; read current 8259 mask or al,int_mask ; reset mask for COM port out pic_mask,al ; write new 8259 mask push ds ; save our data segment lds dx,oldvec ; load address of ; previous interrupt handler mov ax,2500h+com_int ; function 25h = set vector int 21h ; transfer to MS-DOS pop ds ; restore data segment ret ; back to caller asc_dsb endp asc_int proc far ; interrupt service routine ; for serial port sti ; turn interrupts back on push ax ; save registers push bx push dx push ds mov ax,_DATA ; make our data segment mov ds,ax ; addressable cli ; clear interrupts for ; pointer manipulation mov dx,com_data ; DX = data port address in al,dx ; read this character mov bx,asc_in ; get buffer pointer mov [asc_buf+bx],al ; store this character inc bx ; bump pointer cmp bx,bufsiz ; time for wrap? jne asc_int1 ; no, jump xor bx,bx ; yes, reset pointer asc_int1: ; store updated pointer mov asc_in,bx sti ; turn interrupts back on mov al,20h ; send EOI to 8259 out pic_eoi,al pop ds ; restore all registers pop dx pop bx pop ax iret ; return from interrupt asc_int endp _TEXT ends _DATA segment word public 'DATA' in_char db 0 ; PC keyboard input char in_flag db 0 ; <>0 if char waiting columns db 0 ; highest numbered column in ; current display mode (39 or 79) msg1 db cr,lf db 'Display must be text mode.' db cr,lf msg1_len equ $-msg1 msg2 db 'Terminal emulator running...' db cr,lf msg2_len equ $-msg2 msg3 db 'Exit from terminal emulator.' db cr,lf msg3_len equ $-msg3 oldvec dd 0 ; original contents of serial- ; port interrupt vector asc_in dw 0 ; input pointer to ring buffer asc_out dw 0 ; output pointer to ring buffer asc_buf db bufsiz dup (?) ; communications buffer _DATA ends STACK segment para stack 'STACK' db 128 dup (?) STACK ends end talk ; defines entry point ────────────────────────────────────────────────────────────────────────── Figure 7-1. TALK.ASM: A simple terminal-emulator program for IBM PC─compatible computers. This program demonstrates use of the MS-DOS and ROM BIOS video and keyboard functions and direct control of the serial-communications adapter. The TALK program illustrates the methods that an application should use to take over and service interrupts from the serial port without running afoul of MS-DOS conventions. The program begins with some equates and conditional assembly statements that configure the program for half- or full-duplex and for the desired serial port (COM1 or COM2). At entry from MS-DOS, the main routine of the program──the procedure named talk──checks the status of the serial port, initializes the display, and calls the asc_enb routine to take over the serial-port interrupt vector and enable interrupts. The talk procedure then enters a loop that reads the keyboard and sends the characters out the serial port and then reads the serial port and puts the characters on the display──in other words, it causes the PC to emulate a simple CRT terminal. The TALK program intercepts and handles control codes (carriage return, linefeed, and so forth) appropriately. It detects escape sequences and handles them as a subset of the Televideo 950 terminal capabilities. (You can easily modify the program to emulate any other cursor-addressable terminal.) When one of the PC's special function keys is pressed, the program disables serial-port interrupts, releases the serial-port interrupt vector, and exits back to MS-DOS. There are several TALK program procedures that are worth your attention because they can easily be incorporated into other programs. These are listed in the table on the following page. Procedure Action ────────────────────────────────────────────────────────────────────────── asc_enb Takes over the serial-port interrupt vector and enables interrupts by writing to the modem-control register of the INS8250 and the interrupt-mask register of the 8259A. asc_dsb Restores the original state of the serial-port interrupt vector and disables interrupts by writing to the interrupt-mask register of the 8259A. asc_int Services serial-port interrupts, placing received characters into a ring buffer. com_stat Tests whether characters from the serial port are waiting in the ring buffer. com_in Removes characters from the interrupt handler's ring buffer and increments the buffer pointers appropriately. com_out Sends one character to the serial port. cls Calls the ROM BIOS video driver to clear the screen. clreol Calls the ROM BIOS video driver to clear from the current cursor position to the end of the line. home Places the cursor in the upper left corner of the screen. gotoxy Positions the cursor at the desired position on the display. getxy Obtains the current cursor position. pc_out Sends one character to the PC's display. pc_stat Gets status for the PC's keyboard. pc_in Returns a character from the PC's keyboard. ──────────────────────────────────────────────────────────────────────────