AMD AMD-K6-2/450 Design Guide - Page 34

Preliminary Information Embedded AMD-K6™ Processors BIOS Design Guide 23913A/0-November 2000 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. 63 48 47 32 31 SYSRET CS Selector and SS Selector Base SYSCALL CS Selector and SS Selector Base 0 Target EIP Address 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 63-48 SYSRET CS and SS Selector Base 47-32 SYSCALL CS and SS Selector Base 31-0 Target EIP Address R/W Function During the SYSRET instruction, this field is copied into the CS R/W register and the contents of this field, plus 1000b, are copied into the SS register. During the SYSCALL instruction, this field is copied into the R/W CS register and the contents of this field, plus 1000b, are copied into the SS register. R/W During the SYSCALL instruction, this address is copied into the EIP and points to the new starting address. 22 Model 7 and Model 8/[7:0] Registers