Intel IA-32 - Manual (Page 382)

9-8 Vol. 3A
PROCESSOR MANAGEMENT AND INITIALIZATION
To emulate floating-point instructions, the EM, MP, and NE flag in control register CR0 should
be set as shown in Table 9-3.
Regardless of the value of the EM bit, the Intel486 SX processor generates a device-not-avail-
able exception (#NM) upon encountering any floating-point instruction.
9.3 CACHE ENABLING
The IA-32 processors (beginning with the Intel486 processor) contain internal instruction and
data caches. These caches are enabled by clearing the CD and NW flags in control register CR0.
(They are set during a hardware reset.) Because all internal cache lines are invalid following
reset initialization, it is not necessary to invalidate the cache before enabling caching. Any
external caches may require initialization and invalidation using a system-specific initialization
and invalidation code sequence.
Depending on the hardware and operating system or executive requirements, additional config-
uration of the processor’s caching facilities will probably be required. Beginning with the
Intel486 processor, page-level caching can be controlled with the PCD and PWT flags in page-
directory and page-table entries. Beginning with the P6 family processors, the memory type
range registers (MTRRs) control the caching characteristics of the regions of physical memory.
(For the Intel486 and Pentium processors, external hardware can be used to control the caching
characteristics of regions of physical memory.) See Chapter 10, “Memory Cache Control,” for
detailed information on configuration of the caching facilities in the Pentium 4, Intel Xeon, and
P6 family processors and system memory.
Table 9-3. Software Emulation Settings of EM, MP, and NE Flags
CR0 Bit Value
EM 1
MP 0
NE 1