Art of Assembly: Chapter Twenty-One
dl register directly to the printer. Function 40h, with a file handle of 04h, also sends data to the printer. Since the chapter on DOS and BIOS fully describes these functions, we will not discuss them any further here.
There are eight variables in the BIOS variable space (segment 40h) the printer driver uses. The following table describes each of these variables:
| Address | Description |
|---|---|
| 40:08 | Base address of LPT1: device. |
| 40:0A | Base address of LPT2: device. |
| 40:0C | Base address of LPT3: device. |
| 40:0E | Base address of LPT4: device. |
| 40:78 | LPT1: time-out value. The printer port driver software should return an error if the printer device does not respond in a reasonable amount of time. This variable (if non-zero) determines how many loops of 65,536 iterations each a driver will wait for a printer acknowledge. If zero, the driver will wait forever. |
| 40:79 | LPT2: time-out value. See description above. |
| 40:7A | LPT3: time-out value. See description above. |
| 40:7B | LPT4: time-out value. See description above. |
You will notice a slight deviation in the handshake protocol in the following code. This printer driver does not wait for an acknowledge from the printer after sending a character. Instead, it checks to see if the printer has sent an acknowledge to the previous character before sending a character. This saves a small amount of time because the program printer then characters can continue to operating in parallel with the receipt of the acknowledge from the printer. You will also notice that this particular driver does not monitor the busy lines. Almost every printer in existence leaves this line inactive (not busy), so there is no need to check it. If you encounter a printer than does manipulate the busy line, the modification to this code is trivial. The following code implements the int 17h service:
; INT17.ASM
;
; A short passive TSR that replaces the BIOS' int 17h handler.
; This routine demonstrates the function of each of the int 17h
; functions that a standard BIOS would provide.
;
; Note that this code does not patch into int 2Fh (multiplex interrupt)
; nor can you remove this code from memory except by rebooting.
; If you want to be able to do these two things (as well as check for
; a previous installation), see the chapter on resident programs. Such
; code was omitted from this program because of length constraints.
;
;
; cseg and EndResident must occur before the standard library segments!
cseg segment para public 'code'
cseg ends
; Marker segment, to find the end of the resident section.
EndResident segment para public 'Resident'
EndResident ends
.xlist
include stdlib.a
includelib stdlib.lib
.list
byp equ <byte ptr>
cseg segment para public 'code'
assume cs:cseg, ds:cseg
OldInt17 dword ?
; BIOS variables:
PrtrBase equ 8
PrtrTimeOut equ 78h
; This code handles the INT 17H operation. INT 17H is the BIOS routine
; to send data to the printer and report on the printer's status. There
; are three different calls to this routine, depending on the contents
; of the AH register. The DX register contains the printer port number.
;
; DX=0 -- Use LPT1:
; DX=1 -- Use LPT2:
; DX=2 -- Use LPT3:
; DX=3 -- Use LPT4:
;
; AH=0 -- Print the character in AL to the printer. Printer status is
; returned in AH. If bit #0 = 1 then a timeout error occurred.
;
; AH=1 -- Initialize printer. Status is returned in AH.
;
; AH=2 -- Return printer status in AH.
;
;
; The status bits returned in AH are as follows:
;
; Bit Function Non-error values
; --- -------------------------- ----------------
; 0 1=time out error 0
; 1 unused x
; 2 unused x
; 3 1=I/O error 0
; 4 1=selected, 0=deselected. 1
; 5 1=out of paper 0
; 6 1=acknowledge x
; 7 1=not busy x
;
; Note that the hardware returns bit 3 with zero if an error has occurred,
; with one if there is no error. The software normally inverts this bit
; before returning it to the caller.
;
;
; Printer port hardware locations:
;
; There are three ports used by the printer hardware:
;
; PrtrPortAdrs --- Output port where data is sent to printer (8 bits).
; PrtrPortAdrs+1 --- Input port where printer status can be read (8 bits).
; PrtrPortAdrs+2 --- Output port where control information is sent to the
; printer.
;
; Data output port- 8-bit data is transmitted to the printer via this port.
;
; Input status port:
; bit 0: unused.
; bit 1: unused.
; bit 2: unused.
;
; bit 3: -Error, normally this bit means that the
; printer has encountered an error. However,
; with the P101 installed this is a data
; return line for the keyboard scan.
;
; bit 4: +SLCT, normally this bit is used to determine
; if the printer is selected or not. With the
; P101 installed this is a data return
; line for the keyboard scan.
;
; bit 5: +PE, a 1 in this bit location means that the
; printer has detected the end of paper. On
; many printer ports, this bit has been found
; to be inoperative.
;
; bit 6: -ACK, A zero in this bit position means that
; the printer has accepted the last character
; and is ready to accept another. This bit
; is not normally used by the BIOS as bit 7
; also provides this function (and more).
;
; bit 7: -Busy, When this signal is active (0) the
; printer is busy and cannot accept data.
; When this bit is set to one, the printer
; can accept another character.
;
;
;
; Output control port:
;
; Bit 0: +Strobe, A 0.5 us (minimum) active high pulse
; on this bit clocks the data latched into the
; printer data output port to the printer.
;
; Bit 1: +Auto FD XT - A 1 stored at this bit causes
; the printer to line feed after a line is
; printed. On some printer interfaces (e.g.,
; the Hercules Graphics Card) this bit is
; inoperative.
;
; Bit 2: -INIT, a zero on this bit (for a minimum of
; 50 us) will cause the printer to (re)init-
; ialize itself.
;
; Bit 3: +SLCT IN, a one in this bit selects the
; printer. A zero will cause the printer to
; go off-line.
;
; Bit 4: +IRQ ENABLE, a one in this bit position
; allows an interrupt to occur when -ACK
; changes from one to zero.
;
; Bit 5: Direction control on BI-DIR port. 0=output,
; 1=input.
; Bit 6: reserved, must be zero.
; Bit 7: reserved, must be zero.
MyInt17 proc far
assume ds:nothing
push ds
push bx
push cx
push dx
mov bx, 40h ;Point DS at BIOS vars.
mov ds, bx
cmp dx, 3 ;Must be LPT1..LPT4.
ja InvalidPrtr
cmp ah, 0 ;Branch to the appropriate code for
jz PrtChar ; the printer function
cmp ah, 2
jb PrtrInit
je PrtrStatus
; If they passed us an opcode we don't know about, just return.
InvalidPrtr: jmp ISR17Done
; Initialize the printer by pulsing the init line for at least 50 us.
; The delay loop below will delay well beyond 50 usec even on the fastest
; machines.
PrtrInit: mov bx, dx ;Get printer port value.
shl bx, 1 ;Convert to byte index.
mov dx, PrtrBase[bx] ;Get printer base address.
test dx, dx ;Does this printer exist?
je InvalidPrtr ;Quit if no such printer.
add dx, 2 ;Point dx at control reg.
in al, dx ;Read current status.
and al, 11011011b ;Clear INIT/BIDIR bits.
out dx, al ;Reset printer.
mov cx, 0 ;This will produce at least
PIDelay: loop PIDelay ; a 50 usec delay.
or al, 100b ;Stop resetting printer.
out dx, al
jmp ISR17Done
; Return the current printer status. This code reads the printer status
; port and formats the bits for return to the calling code.
PrtrStatus: mov bx, dx ;Get printer port value.
shl bx, 1 ;Convert to byte index.
mov dx, PrtrBase[bx] ;Base address of printer port.
mov al, 00101001b ;Dflt: every possible error.
test dx, dx ;Does this printer exist?
je InvalidPrtr ;Quit if no such printer.
inc dx ;Point at status port.
in al, dx ;Read status port.
and al, 11111000b ;Clear unused/timeout bits.
jmp ISR17Done
; Print the character in the accumulator!
PrtChar: mov bx, dx
mov cl, PrtrTimeOut[bx] ;Get time out value.
shl bx, 1 ;Convert to byte index.
mov dx, PrtrBase[bx] ;Get Printer port address
or dx, dx ;Non-nil pointer?
jz NoPrtr2 ; Branch if a nil ptr
; The following code checks to see if an acknowlege was received from
; the printer. If this code waits too long, a time-out error is returned.
; Acknowlege is supplied in bit #7 of the printer status port (which is
; the next address after the printer data port).
push ax
inc dx ;Point at status port
mov bl, cl ;Put timeout value in bl
mov bh, cl ; and bh.
WaitLp1: xor cx, cx ;Init count to 65536.
WaitLp2: in al, dx ;Read status port
mov ah, al ;Save status for now.
test al, 80h ;Printer acknowledge?
jnz GotAck ;Branch if acknowledge.
loop WaitLp2 ;Repeat 65536 times.
dec bl ;Decrement time out value.
jnz WaitLp1 ;Repeat 65536*TimeOut times.
; See if the user has selected no timeout:
cmp bh, 0
je WaitLp1
; TIMEOUT ERROR HAS OCCURRED!
;
; A timeout - I/O error is returned to the system at this point.
; Either we fall through to this point from above (time out error) or
; the referenced printer port doesn't exist. In any case, return an error.
NoPrtr2: or ah, 9 ;Set timeout-I/O error flags
and ah, 0F9h ;Turn off unused flags.
xor ah, 40h ;Flip busy bit.
; Okay, restore registers and return to caller.
pop cx ;Remove old ax.
mov al, cl ;Restore old al.
jmp ISR17Done
; If the printer port exists and we've received an acknowlege, then it's
; okay to transmit data to the printer. That job is handled down here.
GotAck: mov cx, 16 ;Short delay if crazy prtr
GALp: loop GALp ; needs hold time after ack.
pop ax ;Get char to output and
push ax ; save again.
dec dx ;Point DX at printer port.
pushf ;Turn off interrupts for now.
cli
out dx, al ;Output data to the printer.
; The following short delay gives the data time to travel through the
; parallel lines. This makes sure the data arrives at the printer before
; the strobe (the times can vary depending upon the capacitance of the
; parallel cable's lines).
mov cx, 16 ;Give data time to settle
DataSettleLp: loop DataSettleLp ; before sending strobe.
; Now that the data has been latched on the printer data output port, a
; strobe must be sent to the printer. The strobe line is connected to
; bit zero of the printer port. Also note that this clears bit 5 of the
; control port. This ensures that the port continues to operate as an
; output port if it is a bidirectional device. This code also clears bits
; six and seven which IBM claims should be left zero.
inc dx ;Point DX at the printer
inc dx ; control output port.
in al, dx ;Get current control bits.
and al, 01eh ;Force strobe line to zero and
out dx, al ; make sure it's an output port.
mov cx, 16 ;Short delay to allow data
Delay0: loop Delay0 ; to become good.
or al, 1 ;Send out the (+) strobe.
out dx, al ;Output (+) strobe to bit 0
mov cx, 16 ;Short delay to lengthen strobe
StrobeDelay: loop StrobeDelay
and al, 0FEh ;Clear the strobe bit.
out dx, al ;Output to control port.
popf ;Restore interrupts.
pop dx ;Get old AX value
mov al, dl ;Restore old AL value
ISR17Done: pop dx
pop cx
pop bx
pop ds
iret
MyInt17 endp
Main proc
mov ax, cseg
mov ds, ax
print
byte "INT 17h Replacement",cr,lf
byte "Installing....",cr,lf,0
; Patch into the INT 17 interrupt vector. Note that the
; statements above have made cseg the current data segment,
; so we can store the old INT 17 value directly into
; the OldInt17 variable.
cli ;Turn off interrupts!
mov ax, 0
mov es, ax
mov ax, es:[17h*4]
mov word ptr OldInt17, ax
mov ax, es:[17h*4 + 2]
mov word ptr OldInt17+2, ax
mov es:[17h*4], offset MyInt17
mov es:[17h*4+2], cs
sti ;Okay, ints back on.
; We're hooked up, the only thing that remains is to terminate and
; stay resident.
print
byte "Installed.",cr,lf,0
mov ah, 62h ;Get this program's PSP
int 21h ; value.
mov dx, EndResident ;Compute size of program.
sub dx, bx
mov ax, 3100h ;DOS TSR command.
int 21h
Main endp
cseg ends
sseg segment para stack 'stack'
stk byte 1024 dup ("stack ")
sseg ends
zzzzzzseg segment para public 'zzzzzz'
LastBytes byte 16 dup (?)
zzzzzzseg ends
end Main
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