22-34 Vol. 3B
ARCHITECTURE COMPATIBILITY
Specifically, the store buffers are flushed before the IN instruction is executed. No reads (as a result of cache miss)
are reordered around previously generated writes sitting in the store buffers. The implication of this is that the
store buffers will be flushed or emptied before a subsequent bus cycle is run on the external bus.
On both the Intel486 and Pentium processors, under certain conditions, a memory read will go onto the external
bus before the pending memory writes in the buffer even though the writes occurred earlier in the program execu-
tion. A memory read will only be reordered in front of all writes pending in the buffers if all writes pending in the
buffers are cache hits and the read is a cache miss. Under these conditions, the Intel486 and Pentium processors
will not read from an external memory location that needs to be updated by one of the pending writes.
During a locked bus cycle, the Intel486 processor will always access external memory, it will never look for the
location in the on-chip cache. All data pending in the Intel486 processor's store buffers will be written to memory
before a locked cycle is allowed to proceed to the external bus. Thus, the locked bus cycle can be used for elimi-
nating the possibility of reordering read cycles on the Intel486 processor. The Pentium processor does check its
cache on a read-modify-write access and, if the cache line has been modified, writes the contents back to memory
before locking the bus. The P6 family processors write to their cache on a read-modify-write operation (if the
access does not split across a cache line) and does not write back to system memory. If the access does split across
a cache line, it locks the bus and accesses system memory.
I/O reads are never reordered in front of buffered memory writes on an IA-32 processor. This ensures an update of
all memory locations before reading the status from an I/O device.
22.35 BUS
LOCKING
The Intel 286 processor performs the bus locking differently than the Intel P6 family, Pentium, Intel486, and
Intel386 processors. Programs that use forms of memory locking specific to the Intel 286 processor may not run
properly when run on later processors.
A locked instruction is guaranteed to lock only the area of memory defined by the destination operand, but may
lock a larger memory area. For example, typical 8086 and Intel 286 configurations lock the entire physical memory
space. Programmers should not depend on this.
On the Intel 286 processor, the LOCK prefix is sensitive to IOPL. If the CPL is greater than the IOPL, a general-
protection exception (#GP) is generated. On the Intel386 DX, Intel486, and Pentium, and P6 family processors, no
check against IOPL is performed.
The Pentium processor automatically asserts the LOCK# signal when acknowledging external interrupts. After
signaling an interrupt request, an external interrupt controller may use the data bus to send the interrupt vector to
the processor. After receiving the interrupt request signal, the processor asserts LOCK# to insure that no other
data appears on the data bus until the interrupt vector is received. This bus locking does not occur on the P6 family
processors.
22.36 BUS
HOLD
Unlike the 8086 and Intel 286 processors, but like the Intel386 and Intel486 processors, the P6 family and Pentium
processors respond to requests for control of the bus from other potential bus masters, such as DMA controllers,
between transfers of parts of an unaligned operand, such as two words which form a doubleword. Unlike the
Intel386 processor, the P6 family, Pentium and Intel486 processors respond to bus hold during reset initialization.
22.37 MODEL-SPECIFIC EXTENSIONS TO THE IA-32
Certain extensions to the IA-32 are specific to a processor or family of IA-32 processors and may not be imple-
mented or implemented in the same way in future processors. The following sections describe these model-specific
extensions. The CPUID instruction indicates the availability of some of the model-specific features.