12.7 Handshaking

Many I/O devices cannot accept data at just any rate. For example, a Pentium-based PC is capable of sending several hundred million characters per second to a printer, but printers are incapable of printing that many characters each second. Likewise, an input device such as a keyboard will never transmit several million keystrokes per second to the system (because it operates at human speeds, not computer speeds). Because of the difference in capabilities between the CPU and many of the system peripherals, the CPU needs some mechanism to coordinate data transfer between the computer system and its peripheral devices.

One common way to coordinate data transfers is to send and receive status bits on a port separate from the data port. For example, a single bit sent by a printer could tell the system whether it is ready to accept more data. Likewise, a single status bit in a different port could specify whether a keystroke is available at the keyboard data port. The CPU can test these bits prior to writing a character to the printer, or reading a key from the keyboard.

Using status bits to indicate that a device is ready to accept or transmit data is known as handshaking . It gets this name because the protocol is similar to two people agreeing on some method of transfer by a handshake.

To demonstrate how handshaking works, consider the following short 80x86 assembly language program segment. This code fragment will continuously loop while the HO bit of the printer status register (at input port $379) contains zero and will exit once the HO bit is set (indicating that the printer is ready to accept data):

 mov( 9, dx );     // Initialize DX with the address of the status port.  repeat      in( dx, al );    // Get the parallel port status into the AL register.      and( , al );  // Clear Z flag if the HO bit is set.  until( @nz );        // Repeat until the HO bit contains a one.  // Okay to write another byte to the printer data port here.