 |
|
| If you can't view the Datasheet, Please click here to try to view without PDF Reader . |
|
|
|
| If you can't view the Datasheet, Please click here to try to view without PDF Reader . |
|
|
| Datasheet File OCR Text: |
| (R) SYSCALL and SYSRET Instruction Specification Application Note Publication # 21086 Rev: C Issue Date: May 1998 Amendment/0 This document contains information on a product under development at Advanced Micro Devices (AMD). The information is intended to help you evaluate this product. AMD reserves the right to change or discontinue work on this proposed product without notice. (c) 1998 Advanced Micro Devices, Inc. All rights reserved. Advanced Micro Devices, Inc. ("AMD") reserves the right to make changes in its products without notice in order to improve design or performance characteristics. The information in this publication is believed to be accurate at the time of publication, but AMD makes no representations or warranties with respect to the accuracy or completeness of the contents of this publication or the information contained herein, and reserves the right to make changes at any time, without notice. AMD disclaims responsibility for any consequences resulting from the use of the information included in this publication. This publication neither states nor implies any representations or warranties of any kind, including but not limited to, any implied warranty of merchantability or fitness for a particular purpose. AMD products are not authorized for use as critical components in life support devices or systems without AMD's written approval. AMD assumes no liability whatsoever for claims associated with the sale or use (including the use of engineering samples) of AMD products, except as provided in AMD's Terms and Conditions of Sale for such products. Trademarks AMD, the AMD logo, and combinations thereof are trademarks of Advanced Micro Devices, Inc. Microsoft, Windows, and Windows NT are registered trademarks of Microsoft Corporation. Other product names used in this publication are for identification purposes only and may be trademarks of their respective companies. 21086C/0--May 1998 SYSCALL and SYSRET Instruction Specification Contents Revision History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v Audience . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Feature Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 New Method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 SYSCALL/SYSRET Target Address Register (STAR-MSR C000_0081h) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 SYSCALL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Related Instructions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 SYSRET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Related Instructions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Contents iii SYSCALL and SYSRET Instruction Specification 21086C/0--May 1998 iv Contents 21086C/0--May 1998 SYSCALL and SYSRET Instruction Specification Revision History Date Sept 1997 May 1998 Rev B C Initial published release. Revised code sample on testing for SYSCALL/SYSRET support on page 2. Description Revision History v SYSCALL and SYSRET Instruction Specification 21086C/0--May 1998 vi Revision History 21086C/0--May 1998 SYSCALL and SYSRET Instruction Specification Application Note SYSCALL and SYSRET Instruction Specification Audience Th e SYS CALL a nd SYS R ET ins t ruc t io n spe ci f ica t io n application note is intended for operating system designers. Overview SYSCALL and SYSRET are instructions used for low-latency system calls and returns in operating systems with a flat memory model and no segmentation. These instructions have been optimized by reducing the number of checks and memory references that are normally made so that a call or return takes less than one-fourth the number of internal clock cycles when compared to the current CALL/RET instruction method. Feature Detection To use the SYSCALL/SYSRET instructions, the programmer must determine if the processor supports them. The CPUID instruction gives programmers the ability to determine the presence of these instructions on the processor. Software must Audience 1 SYSCALL and SYSRET Instruction Specification 21086C/0--May 1998 first test to see if the CPUID instruction is supported. For a detailed description of the CPUID instruction, see the AMD Processor Recognition Application Note, order# 20734. The presence of the CPUID instruction is indicated by the ID bit (21) in the EFLAGS register. If this bit is writeable, the CPUID instruction is supported. The following code sample shows how to test for the presence of the CPUID instruction. pushfd pop eax mov ebx, eax xor eax, 00200000h push eax popfd pushfd pop eax cmp eax, ebx jz NO_CPUID ; ; ; ; ; ; ; ; ; ; save EFLAGS store EFLAGS in EAX save in EBX for later testing toggle bit 21 put to stack save changed EAX to EFLAGS push EFLAGS to TOS store EFLAGS in EAX see if bit 21 has changed if no change, no CPUID I f the proce sso r su ppor ts the CP UI D in structio n, the programmer must execute the standard function, EAX=0. The CPUID function returns a 12-character string that identifies the processor's vendor. For AMD processors, standard function 0 returns a vendor string of "AuthenticAMD". This string requires the software to follow the AMD definitions for subsequent CPUID functions and the values returned for those functions. The next step for the programmer is to determine if the SYSCALL/SYSRET instructions are supported. Extended function 8000_0001h of the CPUID instruction provides this information. Function 8000_0001h (EAX=8000_0001h) of the AMD CPUID instruction returns the feature bits in the EDX re g i s t e r. I f b i t 1 1 i n t h e E D X re g i s t e r i s s e t t o 1 , SYSCALL/SYSRET is supported. The following code sample shows how to test for SYSCALL/SYSRET support. mov eax, 80000001h ; setup function 8000_0001h CPUID ; call the function test edx, 800h ; test bit 11 jnz YES_SYSCALL/SYSRET 2 Feature Detection 21086C/0--May 1998 SYSCALL and SYSRET Instruction Specification Background The x86 operating system designers use both segmentation and paging to implement various memory models in their designs. Memory can be protected or unprotected, and segmentation and paging can be implemented with attributes such as supervisor, user, read, write, and execute. Segment descriptors provide the necessary memory protection and privilege checking of segment accesses. By setting the fields within the segment descriptors appropriately, operating systems can enforce access restrictions as needed. One disadvantage of segment-based protection and privilege checking is the overhead associated with the loading of new segments (and their descriptors) into segment registers. Even with pure 32-bit code, this overhead still occurs when switching between ring or privilege levels, and Code Segment (CS) and Stack Segment (SS) are reloaded with different segment descriptors. To initiate a call to the operating system, an application transfers control to the OS through gate descriptors (task, interrupt, trap, or call gates). Control transfer is done by using either a CALL instruction or a software interrupt. (The Windows(R) 95 operating system uses CALL gates while Windows NT(R) uses software interrupts.) Setting up these control gates (as well as the later return via a RET or IRET instruction) is slowed down by the segmentation-related overhead. For example, a CALL to a call gate must initiate the following checks to ensure protection and stability: s s s s The CS selector must not be a NULL selector. If the selector is NULL, a General Protection Fault occurs. The Type field must define the selected descriptor as a code segment descriptor. If the descriptor is not a code segment descriptor, a General Protection Fault occurs. The CS selector must be able to index within the limits of the descriptor table. If the index is not within these limits, a General Protection Fault occurs. The Descriptor Privilege Level of the call gate must be greater than or equal to the Requestor Privilege level. If it is not, a General Protection Fault occurs. Background 3 SYSCALL and SYSRET Instruction Specification 21086C/0--May 1998 s s s s s s s s The Descriptor Privilege Level of the call gate must be greater than or equal to the Current Privilege Level. If it is not, a General Protection Fault occurs. The Stack Segment selector must not be a NULL selector. If the selector is NULL, an Invalid Task State Segment Exception occurs. The Stack Segment selector must be able to index within the limits of the descriptor table. If the index is not within these limits, an Invalid Task State Segment Exception occurs. The Descriptor Privilege Level of the Stack Segment must be equal to the Descriptor Privilege Level of the Code Segment. If it is not, an Invalid Task State Segment Exception occurs. The Requestor Privilege Level of the Stack Segment must be equal to the Descriptor Privilege Level of the Code Segment. If it is not, an Invalid Task State Segment Exception occurs. The stack created by the call must be large enough to accommodate a predetermined number of bytes. If not, a Stack Fault Exception occurs. The Stack Segment must be present. If not, a Stack Fault Exception occurs. The descriptor of the Stack Segment must define the data segment as writeable. If not, an Invalid Task State Segment Exception occurs. This list is an example of some of the checks that must occur. In addition, there are other checks that involve the Task State Segment. New Method The SYSCALL and SYSRET instructions are ideally suited for an operating system with a flat memory model (such as Windows NT) where segmentation is disregarded by making all segments 4 Gbytes (base=0 and limit =4 Gbytes). By making the assumption that the operating system design is flat, calls and returns can be greatly simplified. Such an assumption also allows for greater security and reliability by 4 New Method 21086C/0--May 1998 SYSCALL and SYSRET Instruction Specification forcing all applications to use a common and consistent view of the memory model. This simplification comes from eliminating unneeded checks as well as loading pre-determined values into the on-chip Code Segment and Stack Segment descriptor caches. It is assumed that the base, limit, and attributes of the Code Segment will remain the same for all processes and for the operating system, and that only the current privilege level for the selector of the calling process should be changed from a current privilege level of three to a new privilege level of zero. It is also assumed (but not checked) that the privilege level of the selectors for the CALL and RET target CS segments are set to zero and three, respectively (for SYSCALL --code segment selector requested privilege level =0 and for SYSRET -- code segment selector requested privilege level=3). The SYSCALL and SYSRET instructions should be used in environments where neither the operating system nor the applications alter the Code Segment or Stack Segment in a manner different than what is assumed by the instructions. The SYSCALL instruction sets the base, limit, and attributes of both CS and SS to fixed values on entry. The SYSRET instruction sets CS to the same fixed values upon exiting. In a flat memory model, only the privilege level of the segments need to change. No memory accesses to descriptors are required. It is the responsibility of the operating system to keep the descriptors in memory that correspond to the CS and SS selectors loaded by the SYSCALL and SYSRET instructions consistent with the segment base, limit, and attribute values forced by these instructions. New Method 5 SYSCALL and SYSRET Instruction Specification 21086C/0--May 1998 SYSCALL/SYSRET Target Address Register (STAR-MSR C000_0081h) The SYSCALL/SYSRET Target Address Register (STAR) 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 1 shows the format of the STAR register, and Table 1 defines the function of each bit of the STAR register. 63 SYSRET CS Selector and SS Selector Base 48 47 SYSCALL CS Selector and SS Selector Base 32 31 Target EIP Address 0 Figure 1. SYSCALL/SYSRET Target Address Register (STAR) Table 1. Bit 63-48 SYSCALL/SYSRET Target Address Register (STAR) Definition Description SYSRET CS and SS Selector Base R/W R/W Function 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. 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. During the SYSCALL instruction, this address is copied into the EIP and points to the new starting address 47-32 31-0 SYSCALL CS and SS Selector Base Target EIP Address R/W R/W 6 SYSCALL/SYSRET Target Address Register (STAR-MSR C000_0081h) 21086C/0--May 1998 SYSCALL and SYSRET Instruction Specification SYSCALL mnemonic SYSCALL Privilege: Registers Affected: Flags Affected: Machine State Affected: Exceptions Generated: Exception Invalid opcode (6) opcode 0F 05h description Call operating system None ECX, EIP, CS, SS IF, VM CPL, CS (base, limit, attr), SS (base, limit, attr) Virtual 8086 Protected Description X X The System Call Extension bit (SCE) of the Extended Feature Enable Register (EFER) is set to 0. (The EFER register is MSR C000_0080h.) Real X The SYSCALL instruction provides a fast method for transferring control to a fixed entry point in an operating system. The EIP register is copied into the ECX register. Bits [31-0] of the 64-bit SYSCALL/SYSRET Target Address Register (STAR) are copied into the EIP register. (The STAR register is Model-Specific Register C000_0081h.) The IF and VM flags are set to 0 in order to disable interrupts and force the processor out of Virtual-8086 mode. New selectors are loaded without any checking performed as follows: s s Bits [47-32] of the STAR register specify the selector that is copied into the CS register. Bits [47-32]+1000b of the STAR register specify the selector that is copied into the SS register. Note: This effectively increments the index field of the CS selector such that the resultant SS selector points to the next descriptor in a descriptor table, after the CS descriptor. The CS and SS registers should not be modified by the operating system between the execution of the SYSCALL instruction and its corresponding SYSRET instruction. The CPL is set to 0 regardless of the value of bits [33-32] of the STAR register. There are no permission checks based on the CPL, Real mode, or Virtual-8086 mode. SYSCALL 7 SYSCALL and SYSRET Instruction Specification 21086C/0--May 1998 The following descriptors are loaded to specify fixed, 4-Gbyte flat segments as follows: s s s s The CS_base and the SS_base are both set to zero. The CS_limit and the SS_limit are both set to 4 Gbyte. The CS segment attributes are set to Read-only. The SS segment attributes are set to Read/Write and Expand-Up. The operating system must ensure that the descriptors corresponding to the selectors in the STAR register are consistent with the corresponding on-chip values (such as, base, limit, and attributes) that are set by the SYSCALL and SYSRET instructions. Related Instructions See the SYSRET instruction. 8 SYSCALL 21086C/0--May 1998 SYSCALL and SYSRET Instruction Specification SYSRET mnemonic SYSRET Privilege: Registers Affected: Flags Affected: Machine State Affected: Exceptions Generated: Exception Invalid opcode (6) General protection (13) opcode 0F 07h description Return from operating system None EIP, SS, CS IF CPL, CS (base, limit, attr) Virtual 8086 Protected Description X X X X The System Call Extension bit (SCE) of the Extended Feature Enable Register (EFER) is set to 0. (The EFER register is MSR C000_0080h.) The CPL is not equal to 0. Real X X The SYSRET instruction is the return instruction used in conjunction with the SYSCALL instruction to provide fast entry/exit to an operating system. The ECX register, which points to the next sequential instruction after the corresponding SYSCALL instruction, is copied into the EIP register. The IF flag is set to 1 in order to enable interrupts. New selectors are loaded without any checking performed as follows: s s Bits [63-48] of the STAR register specify the selector that is copied into the CS register. Bits [63-48]+1000b of the STAR register specify the selector that is copied into the SS register. Note: This effectively increments the index field of the CS selector such that the resultant SS selector points to the next descriptor in a descriptor table, after the CS descriptor. Bits [1-0] of the SS register are set to 11b (RPL of 3) regardless of the value of bits [49-48] of the STAR register. s The CS and SS registers must not be modified by the operating system between the execution of the SYSCALL instruction and its corresponding SYSRET instruction. The CPL is set to 3 regardless of the value of bits [49-48] of the STAR register. If the CPL is not equal to 0 when the SYSRET instruction is executed, a general protection fault exception is generated with an error code of 0. SYSRET 9 SYSCALL and SYSRET Instruction Specification 21086C/0--May 1998 A new descriptor is loaded for CS to specify a fixed 4-Gbyte flat segment as follows: s s s The CS_base is set to zero. The CS_limit is set to 4 Gbyte. The CS segment attributes are set to Read-only. The operating system must ensure that the descriptors corresponding to the selectors in the STAR register are consistent with the corresponding on-chip values (such as, base, limit, and attributes) that are set by the SYSCALL and SYSRET instructions. Related Instructions See the SYSCALL instruction. 10 SYSRET |
Price & Availability of 21086
 |
|
|
|
|
All Rights Reserved © IC-ON-LINE 2003 - 2022 |
| [Add Bookmark] [Contact Us] [Link exchange] [Privacy policy] |
|
Mirror Sites : [www.datasheet.hk]
[www.maxim4u.com] [www.ic-on-line.cn]
[www.ic-on-line.com] [www.ic-on-line.net]
[www.alldatasheet.com.cn]
[www.gdcy.com]
[www.gdcy.net] |