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Model 7 and Model 8/[7:0] Registers
Embedded AMD-K6™ Processors BIOS Design Guide
23913A/0—November 2000
Preliminary Information
SYSCALL/SYSRET Target Address Register (STAR)
Models 8, 9, and D implement the STAR register. This register
contains the target EIP address used by the SYSCALL
instruction and the 16-bit code and stack segment selector
bases used by the SYSCALL and SYSRET instructions.
Figure 4 shows the format of the STAR register, and Table 11
defines the function of each field of the STAR register. The
STAR register is MSR C000_0081h.
For more information about SYSCALL/SYSRET, see the
SYSCALL and SYSRET Instruction Specification Application Note,
order# 21086.
Figure 4. SYSCALL/SYSRET Target Address Register (STAR) (Models 8, 9, and D)
Table 11. SYSCALL/SYSRET Target Address Register (STAR) Definition (Models 8, 9, and D)
Bit
Description
R/W
Function
63–48
SYSRET CS and SS Selector Base
R/W
During the SYSRET instruction, this field is copied into the CS
register and the contents of this field, plus 1000b, are copied
into the SS register.
47–32
SYSCALL CS and SS Selector Base
R/W
During the SYSCALL instruction, this field is copied into the
CS register and the contents of this field, plus 1000b, are
copied into the SS register.
31–0
Target EIP Address
R/W
During the SYSCALL instruction, this address is copied into
the EIP and points to the new starting address.
31
0
63
Target EIP Address
32
47
48
SYSCALL CS Selector and SS
Selector Base
SYSRET CS Selector and SS
Selector Base