第11章 基本接口.pptVIP

CHAPTER 11 Basic I/O Interface(基本I/O接口)(P. 383) 11-1 INTRODUCTION TO I/O INTERFACE(I/O接口简介)(P. 383) I/O Instructions(I/O指令)(P. 384) The instruction set contains one byte of instruction that transfers information to an I/O device (OUT) and another to read information from an I/O device. I/O address is often called a port number or simply port. Intel calls the 8-bit form (p8) a fixed address because it is stored with the instruction, usually in a ROM. The 8-bit fixed port number (p8) appears on address bus connections A7-A0 with bits A15-A8 equal to. The address connections above A15 are undefined for an I/O instruction. So the fixed I/O instructions address the first 256 I/O port addresses (0000H-00FFH). The 16-bit I/O address in DX is called a variable address because it is stored in DX and then used to address the I/O device. The 16-bit variable port number (DX) appears on address bus connections A15-A0. So the variable I/O instructions address any I/O port address from 0000H-FFFFH. Isolated And Memory-Mapped I/O (独立编址I/O与存储器映像I/O)(P. 385) There are two different methods of interfacing 1/O to the microprocessor. Isolated I/O: the I/O ports are isolated from the memory system in a separate I/O address space. An advantage of isolated I/O is because the ports are separate, the user can expand the memory to its full size without using any of memory space for I/O devices. A disadvantage is that the data transferred between I/O and the microprocessor must be accessed by the IN and OUT instructions. Another disadvantage is that separate control signals for the I/O space are developed (using M/IO and WR or RD), which indicate that an I/O read (IORC) or an I/O write (IOWC) operation. Memory-Mapped I/O: a memory-mapped I/O device is treated as a memory location in the memory map. The main advantage of memory-mapped I/O is that any memory transfer instruction can be used to access the I/O device. Another advantage is that the IORC and IOWC signals have no funct