Byte Port I/O Processing

Playing With The Speaker

Long before SoundBlaster (reg.trade mark) cards, the IBM PC came equiped with its own speaker. From a systems perspective, this speaker provides the ability to signal the user during the boot process, when (for example) the monitor is not working. Subsequently, programmers have made use of this (tiny, cheap, and tinny) speaker to play simple tunes and even simulate speech (poorly).

• How Speakers Work

  Sound is the result of air vibrating within a certain frequency range. A speaker causes air to vibrate by moving a portion of itself, its "cone", back and forth. The cone (generally) contains embedded wires which can be pulled back by an electic magnet. These wires are in the form of a spring, so if the magnet is turned off, the speaker cone returns to its original position.
  By turning the magnet on and off at an appropriate frequency, it is possible to make an audible noise.

• The 8255 Programmable Peripheral Interface Chip

  • The "8255 - Programmable Peripheral Interface" chip is connected to the 80x88/86 at Port Address 0061h. It controls several different components of the PC but for this specific example we are only concerned with its connection to the PC's speaker; bit 1 of this Port is connected to the power supply of the electro-magnet of the speaker. Turning bit 1 "on" and "off" causes the electro-magnet to pull the speaker cone back and then release it; if this is done at the right frequencies, the resulting air vibration will form an audible sound.

• Example of Code to Toggle the Speaker

  •         ; Turn on power to speaker magnet
            OR      AL,00000010b ; binary is useful for individual bit references
            OUT     61h,AL
            ; Wait for a short time
    

  •         ; Turn off power to speaker magnet
            AND     AL,1111101b
            OUT     61h,AL
            ; Wait again and then repeat
    

• FTP simple program to make a noise on your speaker

Character Printing

Warning: The following example is intended for demonstration purposes only. It works with dumb printers which have been connected in a fairly standard way; it may not work if you are using a serial instead of a parallel printer; it probably won't work if you are using an intelligent printer (e.g. something that can process PostScript commands).

• Finding the Printer Ports

  The actual port addresses used by a printer on an IBM PC vary from computer to computer. During the "boot" process, the initial load routines locate the actual printer ports and save this information (along with a lot of similar information) in an area of memory called the "BIOS communications area". Specifically, the first port address used by the printer is stored in memory in the word at address 0400:0008h.

• Functions of the Printer Ports

  The printer always uses 3 port addresses in numerically serial order:
  1. The "data" port used to specify the ASCII pattern of the character to be printed. This port address is the one specified in memory at 0400:0008h (or equivalently 0000:0408h as used below) and is sometimes referred to as the printer's "base" port address.
  2. The "status" port is used to determine the current status of the printer, if it's out of paper, etc. In the example below, the status is only used to check if the printer is still busy printing the previous pattern(s) sent to it. The port address of the status port is one greater than the port address of the data port.
  3. The "control" port is used to tell the printer when the value at the data port is valid to print (among other things). The control port address is two greater than the port address of the data port.

• Example of Code to Print a Character

•          SUB     AX,AX       ;set DS: to Interrupt Vector/BIOS Data area
           MOV     DS,AX
  
           MOV     DX,[0408h]  ;printer adaptor base port # (data port)
  
           MOV     AX,CS       ;restore DS: to this segment
           MOV     DS,AX
  
           INC     DX          ;printer status port (always data port # +1)
           IN      AL,DX       ;get status
           AND     AL,80h      ;test for not busy
           JZ      P_BUSY      ;  -it was busy
  
           DEC     DX          ;printer data port #
  
           MOV     AL,[BX]     ;get char to print
           OUT     DX,AL       ;put char on data port lines
           INC     DX          ;printer control port (always data port # +2)
           INC     DX
           MOV     AL,00001101b  ;strobe printer control port (on)
           OUT     DX,AL
           MOV     AL,00001100b  ;                            (off)
           OUT     DX,AL

• FTP a program that contains this code (only for the brave or foolish).