SUMMARY
1. The 8086–Core2 microprocessors have two basic types of I/O instructions: IN and OUT. The IN instruction inputs data from an external I/O device into either the AL (8-bit) or AX (16-bit) register. The IN instruction is available as a fixed port instruction, a variable port instruction, or a string instruction (80286–Pentium 4) INSB or INSW. The OUT instruction outputs data from AL or AX to an external I/O device and is available as a fixed, variable, or string instruction OUTSB or OUTSW. The fixed port instruction uses an 8-bit I/O port address, while the variable and string I/O instructions use a 16-bit port number found in the DX register.
2. Isolated I/O, sometimes called direct I/O, uses a separate map for the I/O space, freeing the entire memory for use by the program. Isolated I/O uses the IN and OUT instructions to transfer data between the I/O device and the microprocessor. The control structure of the isolated I/O map uses IORC (I/O read control) and IOWC (I/O write control), plus the bank selection signals BHE and BLE (A0 on the 8086 and 80286), to effect the I/O transfer. The early 8086/8088 used the M/IO (IO/M) signal with RD and WR to generate the I/O control signals.
3. Memory-mapped I/O uses a portion of the memory space for I/O transfers. This reduces the amount of memory available, but it negates the need to use the IORC and IOWC signals for I/O transfers. In addition, any instruction that addresses a memory location using any addressing mode can be used to transfer data between the microprocessor and the I/O device using memory-mapped I/O.
4. All input devices are buffered so that the I/O data are connected only to the data bus during the execution of the IN instruction. The buffer is either built into a programmable peripheral or located separately.
5. All output devices use a latch to capture output data during the execution of the OUT instruction. This is necessary because data appear on the data bus for less than 100 ns for an OUT instruction, and most output devices require the data for a longer time. In many cases, the latch is built into the peripheral.
6. Handshaking or polling is the act of two independent devices synchronizing with a few control lines. For example, the computer asks a printer if it is busy by inputting the BUSY signal from the printer. If it isn’t busy, the computer outputs data to the printer and informs the printer that data are available with a data strobe (DS) signal. This communication between the computer and the printer is a handshake or a poll.
7. Interfaces are required for most switch-based input devices and for most output devices that are not TTL-compatible.
8. The I/O port number appears on address bus connections A7–A0 for a fixed port I/O instruction and on A15–A0 for a variable port I/O instruction (note that A15–A8 contains zeros for an 8-bit port). In both cases, address bits above A15 are undefined.
9. Because the 8086/80286/80386SX microprocessors contain a 16-bit data bus and the I/O addresses reference byte-sized I/O locations, the I/O space is also organized in banks, as is the memory system. In order to interface an 8-bit I/O device to the 16-bit data bus, we often require separate write strobes (an upper and a lower) for I/O write operations. Likewise, the 80486 and Pentium–Core2 also have I/O arranged in banks.
10. The I/O port decoder is much like the memory address decoder, except instead of decoding the entire address, the I/O port decoder decodes only a l6-bit address for variable port instructions and often an 8-bit port number for fixed I/O instructions.
11. The 82C55 is a programmable peripheral interface (PIA) that has 24 I/O pins that are programmable in two groups of 12 pins each (group A and group B). The 82C55 operates in three modes: simple I/O (mode 0), strobed I/O (mode 1), and bidirectional I/O (mode 2). When the 82C55 is interfaced to the 8086 operating at 8 MHz, we insert two wait states because the speed of the microprocessor is faster than the 82C55 can handle.
12. The LCD display device requires a fair amount of software, but it displays ASCII-coded information.
13. The 8254 is a programmable interval timer that contains three l6-bit counters that count in binary or binary-coded decimal (BCD). Each counter is independent and operates in six different modes: (1) events counter, (2) retriggerable, monostable multivibrator, (3) pulse generator, (4) square-wave generator, (5) software-triggered pulse generator, and (6) hardware- triggered pulse generator.
14. The 16550 is a programmable communications interface, capable of receiving and transmitting asynchronous serial data.
15. The DAC0830 is an 8-bit digital-to-analog converter that converts a digital signal to an analog voltage within 1.0 μs.
16. The ADC0804 is an 8-bit analog-to-digital converter that converts an analog signal into a digital signal within 100 μs.