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BASIC INSTRUCTION SET
Example: STI. Sets
IF=
1,
which enables the processing of maskable external interrupts.
Example: CLI.
Sets
IF=O
to disable maskable interrupt processing.
HLT
(Halt) causes the processor
to
suspend processing operations pending an interrupt or a system
reset. This trusted instruction provides an alternative
to
an endless software loop
in
situations where a
program must wait for an interrupt. The return address saved after the interrupt
will
point to the
instruction immediately following HLT. This instruction may
be
executed only when CPL =
O.
Example:
HL
T
LOCK (Assert Bus Lock)
is
a i-byte prefix code that causes the processor
to
assert the bus LOCK
signal during execution of the instruction that
follows.
LOCK does not affect any flags. LOCK may
be
used only when CPL
:5
IOPL. A protection exception
will
occur if LOCK
is
used
when
CPL >
10PL.
3.11.3
Input and Output Instructions
These trusted instructions provide access to the processor's
I/O
ports
to
transfer data
to
and from
peripheral devices. In Protected Mode, these instructions may be executed only when
CPL
:5
IOPL.
IN
(Input/rom Port) transfers a byte or a word from an input port to AL or AX.
If
a program specifies
AL with the
IN
instruction, the processor transfers 8 bits from the selected port
to
AL. Alternately, if
a program specifies AX with the
IN
instruction, the processor transfers
16
bits from the port to AX.
The program can specify the number of the port in
two
ways.
Using an immediate byte constant, the
program
can
specify 256 8-bit ports
numbered
0 through 255 or 128 16-bit ports
numbered
0,2,4,
...
,252,254. Using the current value contained
in
DX, the program can specify 8-bit ports numbered
o through 65,535, or 16-bit ports using even-numbered ports in the same range.
Example:
IN
AL,
BYTEYORT_NUMBER. Transfers 8 bits
to
AL from the port identified
by
the immediate
constant
BYTEYORT
_NUMBER.
OUT (Output to Port) transfers a byte or a word to an output port from AL or AX. The program can
specify the number of the port using the
same methods of the
IN
instruction.
Example:
OUT AX, DX. Transfers
16
bits from AX
to
the port identified
by
the Hi-bit number
contained in DX.
INS
and OUTS (Input String and Output String) cause block input or output operations
using
a Repeat
prefix.
See Chapter 4 for more information
on
INS
and OUTS.
3.12
PROCESSOR EXTENSION INSTRUCTIONS
Processor Extension provides an extension
to
the instruction set of the base architecture (e.g., 80287).
The
NPX
extends the instruction set of the CPU-based architecture to support high-precision integer
and floating-point calculations. This extended instruction set includes arithmetic, comparison, transcen-
dental, and data transfer instructions. The
NPX
also contains a set of useful constants
to
enhance the
speed of numeric calculations.
3-29