Intel 80286, 80287 manual Chapter Extended Instruction SET, Block 1/0 Instructions

CHAPTER 4

EXTENDED INSTRUCTION SET

The instructions described in this chapter extend the capabilities of the base architecture instruction set described in Chapter 3. These extensions consist of new instructions and variations of some instruc- tions that are not strictly part of the base architecture (in other words, not included on the 8086 and 8088). These instructions are also available on the 80186 and 80188. The instruction variations, described in Chapter 3, include the immediate forms of the PUSH and MUL instructions, PUSHA, paPA, and the privilege level restrictions on POPF.

New instructions described in this chapter include the string input and output instructions (INS and OUTS), the ENTER procedure and LEAVE procedure instructions, and the check index BOUND instruction.

4.1 BLOCK 1/0 INSTRUCTIONS

REP, the Repeat prefix, modifies INS and OUTS (the string I/O instructions) to provide a means of transferring blocks of data between an I/O port and Memory. These block I/O instructions are string primitives. They simplify programming and increase the speed of data transfer by eliminating the need to use a separate LOOP instruction or an intermediate register to hold the data.

INS and OUTS are trusted instructions. To use trusted instructions, a program must execute at a privilege level at least as privileged as that specified by the 2-bit IOPL flag (CPL ::s IOPL). Any attempt by a less-privileged program to use a trusted instruction results in a protection exception. See Chapter 7 for information on protection concepts.

One of two possible opcodes represents each string primitive depending on whether it operates on byte strings or word strings. After each transfer, the memory address in SI or DI is updated by 1 for byte values and by 2 for word values. The value in the DF field determines if SI or DI is to be auto incre- mented (DF=O) or auto decremented (DF= 1).

INS and OUTS use DX to specify I/O ports numbered 0 through 65,535 or 16-bit ports using only even port addresses in the same range.

INS (Input String from Port) transfers a byte or a word string element from an input port to memory. If a program specifies INSB, the processor transfers 8 bits from the selected port to the memory location indicated by ES:DI. Alternately, if a program specifies INSW, the processor transfers 16 bits from the port to the memory location indicated by ES:DI. The destination segment register choice (ES) cannot be changed for the INS instruction.

Combined with the REP prefix, INS moves a block of information from an input port to a series of consecutive memory locations.

Example: REP INSB. The processor repeatedly transfers 8 bits to the memory location indicated by ES:DI from the port selected by the 16-bit port number contained in DX. Following each byte transfer, the CPU decrements CX. The instruction terminates the block transfer when CX=O. After decrementing CX, the processor increments DI by one if DF=O. It decre- ments DI by one if DF= 1.

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