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Intel386
TM
SX MICROPROCESSOR
Segment Registers:
Six 16-bit special purpose reg-
isters select, at any given time, the segments of
memory that are immediately addressable for code,
stack, and data.
Flags and Instruction Pointer Registers:
The two
32-bit special purpose registers in figure 2.1 record
or control certain aspects of the Intel386 SX Micro-
processor state. The EFLAGS register includes
status and control bits that are used to reflect the
outcome of many instructions and modify the se-
mantics of some instructions. The Instruction Point-
er, called EIP, is 32 bits wide. The Instruction Pointer
controls instruction fetching and the processor auto-
matically increments it after executing an instruction.
Control Registers:
The four 32-bit control register
are used to control the global nature of the Intel386
SX Microprocessor. The CR0 register contains bits
that set the different processor modes (Protected,
Real, Paging and Coprocessor Emulation). CR2 and
CR3 registers are used in the paging operation.
System Address Registers:
These four special
registers reference the tables or segments support-
ed by the 80286/Intel386 SX/Intel386 DX CPU’s
protection model. These tables or segments are:
GDTR (Global Descriptor Table Register),
IDTR (Interrupt Descriptor Table Register),
LDTR (Local Descriptor Table Register),
TR (Task State Segment Register).
Debug Registers:
The six programmer accessible
debug registers provide on-chip support for debug-
ging. The use of the debug registers is described in
Section 2.10
Debugging Support
.
Test Registers:
Two registers are used to control
the testing of the RAM/CAM (Content Addressable
Memories) in the Translation Lookaside Buffer por-
tion of the Intel386 SX Microprocessor. Their use is
discussed in
Testability
.
240187–3
Figure 2.2. Status and Control Register Bit Functions
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