2-18 Vol. 3A

SYSTEM ARCHITECTURE OVERVIEW
When loading a control register, reserved bits should always be set to the values previously read.The flags in control registers are:PG Paging (bit 31 of CR0) — Enables paging when set; disables paging when clear.When paging is disabled, all linear addresses are treated as physical addresses. The PGflag has no effect if the PE flag (bit 0 of register CR0) is not also set; setting the PGflag when the PE flag is clear causes a general-protection exception (#GP). See also:Section 3.6, “Paging (Virtual Memory) Overview.”On IA-32 processors that support Intel® EM64T, enabling and disabling IA-32e modeoperation also requires modifying CR0.PG.CD Cache Disable (bit 30 of CR0) — When the CD and NW flags are clear, caching ofmemory locations for the whole of physical memory in the processor’s internal (andexternal) caches is enabled. When the CD flag is set, caching is restricted as describedin Table 10-5. To prevent the processor from accessing and updating its caches, the CDflag must be set and the caches must be invalidated so that no cache hits can occur.See also: Section 10.5.3, “Preventing Caching,” and Section 10.5, “Cache Control.”

Figure 2-6. Control Registers

CR1

W
P
A
M

Page-Directory Base

V
M
E
P
S
E
T
S
D
D
E
P
V
I
P
G
E
M
C
E
P
A
E
P
C
E
N
W
P
GC
D
P
W
T
P
C
D

Page-Fault Linear Address

P
E
E
MM
P
T
S
N
EE
T

CR2

CR0

CR4

Reserved

CR3

Reserved (set to 0)

31 29
30 28 1918 17 16 15 6543210
31(63) 0
31(63) 0
31(63) 1211 54320
31(63) 9876543210

(PDBR)

10
OSFXSR
OSXMMEXCPT