8.1 I/O Addressing

The 80386 allows input/output to be performed in either of two ways:

By means of a separate I/O address space (using specific I/O instructions)
By means of memory-mapped I/O (using general-purpose operand manipulation instructions).

8.1.1 I/O Address Space

The 80386 provides a separate I/O address space, distinct from physical memory, that can be used to address the input/output ports that are used for external 16 devices. The I/O address space consists of 2^(16) (64K) individually addressable 8-bit ports; any two consecutive 8-bit ports can be treated as a 16-bit port; and four consecutive 8-bit ports can be treated as a 32-bit port. Thus, the I/O address space can accommodate up to 64K 8-bit ports, up to 32K 16-bit ports, or up to 16K 32-bit ports.

The program can specify the address of the port in two ways. Using an immediate byte constant, the program can specify:

256 8-bit ports numbered 0 through 255.
128 16-bit ports numbered 0, 2, 4, . . . , 252, 254.
64 32-bit ports numbered 0, 4, 8, . . . , 248, 252.

Using a value in DX, the program can specify:

8-bit ports numbered 0 through 65535
16-bit ports numbered 0, 2, 4, . . . , 65532, 65534
32-bit ports numbered 0, 4, 8, . . . , 65528, 65532

The 80386 can transfer 32, 16, or 8 bits at a time to a device located in the I/O space. Like doublewords in memory, 32-bit ports should be aligned at addresses evenly divisible by four so that the 32 bits can be transferred in a single bus access. Like words in memory, 16-bit ports should be aligned at even-numbered addresses so that the 16 bits can be transferred in a single bus access. An 8-bit port may be located at either an even or odd address.

The instructions IN and OUT move data between a register and a port in the I/O address space. The instructions INS and OUTS move strings of data between the memory address space and ports in the I/O address space.

8.1.2 Memory-Mapped I/O

I/O devices also may be placed in the 80386 memory address space. As long as the devices respond like memory components, they are indistinguishable to the processor.

Memory-mapped I/O provides additional programming flexibility. Any instruction that references memory may be used to access an I/O port located in the memory space. For example, the MOV instruction can transfer data between any register and a port; and the AND, OR, and TEST instructions may be used to manipulate bits in the internal registers of a device (see Figure 8-1 ). Memory-mapped I/O performed via the full instruction set maintains the full complement of addressing modes for selecting the desired I/O device (e.g., direct address, indirect address, base register, index register, scaling).

Memory-mapped I/O, like any other memory reference, is subject to access protection and control when executing in protected mode. Refer to Chapter 6 for a discussion of memory protection.

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