Specifications
Intel
®
Quark SoC X1000 Core
October 2013 Developer’s Manual
Order Number: 329679-001US 63
System Register Organization—Intel
®
Quark Core
4.7.1 FPU Register Usage
In addition to the differences listed in Table 21, Table 22 summarizes the differences for
the on-chip FPU.
4.8 Reserved Bits and Software Compatibility
In many register and memory layout descriptions, certain bits are marked as reserved.
When bits are marked as reserved, it is essential for compatibility with future
processors that software treat these bits as having a future, though unknown, effect.
The behavior of reserved bits should be regarded as not only undefined, but
unpredictable.
Follow these guidelines in dealing with reserved bits:
• Do not depend on the states of any reserved bits when testing the values of
registers that contain such bits. Mask out the reserved bits when testing.
• Do not depend on the states of any reserved bits when storing to memory or
another register.
Table 21. Register Usage
Register
Use in
Real Mode
Use in
Protected Mode
Use in
Virtual 8086 Mode
Load Store Load Store Load Store
General Registers Yes Yes Yes Yes Yes Yes
Segment Register Yes Yes Yes Yes Yes Yes
Flag Register Yes Yes Yes Yes IOPL
(1) IOPL
Control Registers Yes Yes PL = 0(2) PL = 0 No Yes
GDTR Yes Yes PL = 0 Yes No Yes
IDTR Yes Yes PL = 0 Yes No Yes
LDTR No No PL = 0 Yes No No
TR No No PL = 0 Yes No No
Debug Registers Yes Yes PL = 0 PL = 0 No No
Test Registers Yes Yes PL = 0 PL = 0 No No
Notes:
1. IOPL: The PUSHF and POPF instructions are made I/O Privilege Level sensitive in Virtual 8086 Mode.
2. PL = 0: The registers can be accessed only when the current privilege level is zero.
Table 22. FPU Register Usage Differences
Register
Use in
Real Mode
Use in
Protected Mode
Use in
Virtual 8086 Mode
Load Store Load Store Load Store
FPU Data Registers Yes Yes Yes Yes Yes Yes
FPU Control Registers Yes Yes Yes Yes Yes Yes
FPU Status Registers Yes Yes Yes Yes Yes Yes
FPU Instruction Pointer Yes Yes Yes Yes Yes Yes
FPU Data Pointer Yes Yes Yes Yes Yes Yes